Impact of Mulches on Landscape Plants and the Environment — A Review

Abstract

Mulches provide aesthetic, economic and environmental benefits to urban landscapes. Mulching is especially useful in the establishment of trees in landscapes that receive minimal care, such as restoration sites. In general, mulches improve soil health, creating healthy populations of plants and associated animals. These biodiverse, stable landscapes are more resistant to stress, are more aesthetically pleasing, require fewer applications of pesticides and fertilizers, and are ultimately more sustainable than those without mulch cover. All mulches are not created equally, however, and this review compares the costs and benefits of landscape mulches as reported in the scientific literature. It also presents real and perceived problems associated with various landscape mulches.

Full text

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Impact of Mulches on Landscape Plants and the
Environment — A Review1
Linda Chalker-Scott2
Washington State University, Puyallup Research and Extension Center
7612 Pioneer Way E., Puyallup, WA 98371
Abstract
Mulches provide aesthetic, economic and environmental benefits to urban landscapes. Mulching is especially useful in the establishment
of trees in landscapes that receive minimal care, such as restoration sites. In general, mulches improve soil health, creating healthy
populations of plants and associated animals. These biodiverse, stable landscapes are more resistant to stress, are more aesthetically
pleasing, require fewer applications of pesticides and fertilizers, and are ultimately more sustainable than those without mulch cover.
All mulches are not created equally, however, and this review compares the costs and benefits of landscape mulches as reported in the
scientific literature. It also presents real and perceived problems associated with various landscape mulches.
Key words: aesthetics, economics, inorganic mulch, living mulch, mulch management, organic mulch, pesticide reduction, plant
establishment, soil protection, urban landscapes.
Significance to the Nursery Industry
There is a vast array of mulch materials available for land-
scape use, benefiting plants and soils through weed suppres-
sion, evaporation reduction, and other environmental modi-
fications. Given the available choices, it can be difficult to
determine which mulch materials are best suited for a par-
ticular landscape. The purpose of this review is to provide a
comprehensive analysis of the scientific research on the ben-
efits and drawbacks of mulches used in ornamental and ur-
ban landscapes. The article will be of particular value to in-
dustry professionals who sell, apply, or manage mulch mate-
rials, allowing them to make informed recommendations spe-
cific to their customers’ landscaping needs.
Introduction
The term ‘mulch’ is derived from the Germanic word
‘molsh’, which means soft. Though not all mulches are soft,
for many the word connotes the soft, spongy layer found in
forest ecosystems. Mulches are defined as materials that are
applied to, or grow upon, the soil surface, as opposed to ma-
terials that are incorporated into the soil profile (amendments).
Therefore, any material laid or grown over the soil surface
can be considered a mulch, though some materials are more
beneficial than others.
Mulching received serious study as an environmental modi-
fication in forest, agriculture, and landscape applications be-
ginning in the late 1930s, perhaps spurred by earlier studies
examining the negative impacts of grasses on tree growth
(9). ‘Deep, permanent mulches’ were recommended for
shrubs and trees as early as 1941 (101) as a way of prevent-
ing drought stress (also 62, 131); likewise, prevention of
freeze damage and frost heave were documented benefits (27,
72). In a comparative study, leaf material used as a mulch
was found to be more effective in terms of water conserva-
tion than the same leaf material incorporated into the soil
(116).
The increase in mulch research in the mid-1900s was
closely tied to interest in reusing agricultural and forestry
byproducts including wood pulp and shredded paper (22).
More recently, other recyclables such as arborist trimmings,
yard waste, and agricultural crop byproducts have been in-
corporated as mulching materials. Though landscape mulches
were first reviewed in 1957 (2), there have been no analyses
summarizing the scientific research on landscape mulches.
Given the substantial use of mulch materials in the manage-
ment of urban or ornamental landscapes, such a summary is
long overdue.
Comparative Benefits of Mulches
Improved soil moisture. Exposed to heat, wind, and com-
pacting forces, bare soil loses water through evaporation and
is less able to absorb rainfall or irrigation as it becomes in-
creasingly compressed. Weeds can increase evapotranspira-
tion of soil moisture by 25% in a summer day (54). In con-
trast, mulches will increase soil water by increasing percola-
tion and retention, reducing evaporation, and reducing weeds.
An early study (105) demonstrated that a layer of straw only
3.8 cm (1.5 in) thick reduced evaporation by about 35% com-
pared to bare soil. Later, Kacinski (62) demonstrated that
most mulched soil has greater water retention than bare soil,
with the exception of competitive living mulches such as turf.
What is less consistent is how different mulch types influ-
ence water movement. For instance, black plastic generally
inhibits water movement (7, 12) between the soil and the
above-ground environment, thus limiting recharge. Soil wa-
ter recharge is dependent upon infiltration, which in turn is
influenced by surface permeability. Activities and products
that compact soils and/or create hydrophobic conditions will
limit recharge while increasing runoff and erosion. Plastics,
geotextiles, fine-textured organic mulches, sheet mulches,
and mulches with waxy components are poor choices in this
regard. Therefore, though these mulches may initially increase
soil water retention since evaporation is reduced (68), over
the long term they will create soils that are unnaturally dry.
Viewpoint
1Received for publication May 12, 2007; in revised form July 24, 2007.
Acknowledgements: I am most grateful for the thoughtful, constructive com-
ments provided by two anonymous reviewers and for Dr. Fretz’s assistance
in preparing this Viewpoint article.
2Associate Professor. Email: <lindacs@wsu.edu>.
J. Environ. Hort. 25(4):239–249. December 2007

240 J. Environ. Hort. 25(4):239–249. December 2007
In contrast, there is a wide variety of mulching materials
that do not limit soil water infiltration and retention: their
one similarity is that they are all permeable materials. Most
comparative studies among mulch types indicate that organic
mulches conserve water more effectively than inorganic (6,
60); organic and inorganic are better conservers than syn-
thetic (5, 73) and all are better than bare soil (6, 60, 65, 68,
76, 107, 112, 134). Mulches with demonstrated ability to re-
tain water include gravel and stone (6, 60, 65, 112, 133),
livestock manure (15), and a vast array of plant materials.
These consist of rapid decomposers such as grass clippings
(31, 107), leaves (116, 134) and local crop residues (68, 76,
92, 108, 127); moderate decomposers including hay and straw
(73, 93, 103, 108, 133), coir pith (6, 53, 76), and jute (8); and
slowly decomposing timber residues (46) including sawdust
(5, 66), and barks and chips from both hard- and softwoods
(31, 43, 60, 65, 68, 73, 96, 99, 131, 134). Cover crops are
generally less effective than either organic or inorganic
mulches (31, 68, 96, 107, 133) as they must compete with
other landscape plant materials for water.
From a practical viewpoint, an appropriate mulch will sig-
nificantly reduce the amount of irrigation needed for all land-
scapes, and in some cases can eliminate it altogether (99). In
addition to protecting soil reserves, coarse organic mulches
will hold water much like a sponge, thereby capturing rain-
fall and irrigation water for later release and preventing run-
off. An early study demonstrated that 1.5 cm (0.6 inch) of
straw mulch reduced water runoff by 43% (10); mowed sod
and bark were likewise found to reduce runoff (87). Less
runoff and improved retention will translate to reduced needs
for supplemental irrigation. In addition, mulch protection
from drought stress can also protect trees and shrubs from
subsequent environmental stresses such as cold injury (118).
Reduced soil erosion and compaction. Mulch will protect
soils from wind, water, and traffic-induced erosion and com-
paction, all of which contribute directly to root stress and
poor plant health. Though living mulches are often the most
effective in this regard, holding the soil matrix together even
on the steepest slopes, they may not be the best practical or
economic choice. Grass sowing, for instance, can reduce ero-
sion but often increases runoff compared to other mulch
choices (104, 126) as did barley (Hordeum vulgare) (109).
Adding even a thin organic mulch will protect soils: Borst
and Woodburn (10) found that a 1.5 cm (0.6 in) layer of straw
mulch reduced soil erosion by 86%. Straw from rice (Oryza
sativa) and other grains continues to be commonly used (107)
and in some cases can outperform living mulches such as
legumes and grasses (107, 126). Straw mulch in combina-
tion with an erosion net was found to decrease erosion by
95% over bare soil treatment in a forest plantation (84). Fallen
pine (Pinus spp.) needles resulting from beetle attack helped
prevent soil erosion (80), and logging debris was used to in-
tercept water and reduce overland flow (104). These studies
all underscore the importance of leaving fallen vegetation
on forest sites. [It is important to note that mulches cannot be
use to ‘stabilize’ slopes but only reduce soil loss. Slope sta-
bilization requires an engineering solution, not a horticul-
tural one.]
Compaction is a common aliment of urban soils; while
the impacts of foot and vehicular traffic are self-evident, it’s
less obvious that rainfall will compact unprotected soils.
Adding organic mulch such as bark (96) or jute (8) disperses
the direct impact of water droplets, feet, and tires, thus re-
storing soil aggregation and porosity. It is better to apply
mulch before compaction occurs rather than after the fact.
Research has demonstrated that proactive mulching will pro-
tect soil integrity, while the same mulch applied after com-
paction could not reverse bulk density changes even after
two years (30).
Maintenance of optimal soil temperatures. Mulches pro-
tect soils from extreme temperatures in that soils can be kept
cooler in hot conditions (37, 43, 74) and warmer in cold con-
ditions (66). Temperature extremes will kill fine roots and
while rarely killing established plantings, they can induce a
chronic stress as the plant expends energy to generate new
fine roots. Temperature modification is especially important
near the soil surface, where fine roots can be killed by freez-
ing and frost heaving (49). Hot or cold surface soils can kill
new transplants that have not had time to generate a large
root mass and establish into deeper, more moderate, surround-
ing soils.
This moderating effect is especially important during the
winter and/or in alpine and arctic regions, where warmer soil
can enhance root growth and thus establishment of desirable
plants. Cold hardiness of the plants, however, is not lessened
by this treatment (140). Protection of sensitive root systems
from freezing has the added benefit of preventing opportu-
nistic root rots from attacking stressed seedlings (128). In
summer months or in hotter geographical regions, organic
mulches have been shown to lower soil temperature nearly
10C (50F) compared to unprotected soil (76).
Coarse mulches are more temperature moderating than
finely textured mulches of the same general category; for
example, the soil under cobbles is cooler than that under
gravel, and the soil under leaf mulch is cooler than that un-
der compost (127). Likewise, thicker applications of organic
mulches are more temperature moderating than thin applica-
tions (131). Once again, coarse mulches are better in this
regard (57) as thick layers of finely textured mulches can
inhibit both water and gas transfer.
Among mulch categories, living (133) and organic (6, 60,
76, 89, 133) mulches are more temperature moderating than
inorganic mulches. Chunky inorganic mulches such as gravel
and lava rock are more effective temperature moderators (6,
60, 89) than solid inorganic surfaces such as concrete. Syn-
thetic mulches including asphalt, fabrics and plastics are poor-
est in this regard (73, 89, 133), routinely raising the underly-
ing soil temperature as deep as 30 cm (12 in) below the sur-
face (33). For some special applications (such as soil solar-
ization to kill pests [82]), this might be desirable, but not for
general landscape or garden maintenance. Black plastic
mulches can either raise (73) or lower soil temperatures (130),
probably depending on how much light is absorbed by the
plastic and whether heat is retained or reflected. Clear plas-
tic mulches routinely raise soil temperatures since radiation
(including infrared wavelengths) is transmitted through the
plastic and heat is retained.
While the impacts of mulches on soil temperature have
been well documented, there is also an effect of mulch type
on surface temperature. There are far fewer studies on this
phenomenon, but it is clear that some mulches heat the soil
as a function of solar radiation absorption more than bare
soils and living mulches (89). The increased surface tem-
perature due to pine bark mulch caused nearby leaves to lose

241
more water (143), though at greater distances [e.g. 1 m (3.3
ft)] there is no mulch effect upon air temperature (144).
Living mulches such as turf release water vapor through
evapotranspiration and reduce surface temperatures by evapo-
rative cooling (89), though they use more soil water than
non-living mulches. Interestingly, the soil temperature be-
neath turf was shown to be higher than that below mulch
(23), perhaps because soil beneath turf was drier and thus
less protected against high temperatures. Heat-reflecting
mulches are sometimes useful, especially in improving fruit
maturation (48); for most urban or managed landscapes, how-
ever, this is probably not a priority.
Increased soil nutrition. Living and organic mulches can
increase, decrease, or have no effect upon nutrient levels de-
pending upon mulch type, soil chemistry, and particular nu-
trients of interest. As living and organic mulches decompose
under appropriate water and temperature levels, nutrients are
released into the soil and become available for root uptake or
microbial use. Generally, green and animal manures used as
mulch supply nutrients at higher rates than other mulch
choices (such as straw, bark and wood chips) and often per-
form better than inorganic fertilizers (3, 31, 100, 116). While
immediately available nutrients are sometimes desirable for
a landscape, it is important to note that overapplication of
these materials can lead to excess mineral availability, caus-
ing damage to plants, soil organisms, and nearby watersheds.
Therefore, nutrient-rich mulches should be applied sparingly
and may be most effective as part of a mulch layer.
While living mulches often compete for nutrients as well
as water, this characteristic can be valuable on landscapes
where fertility is too high. Fast-growing plant materials will
reduce soil nutrient levels (94) as can microbial activity in
low-fertility organic mulches. This has been helpful in resto-
ration of ecosystems with naturally low fertility, allowing
native plants to compete more effectively with invasive spe-
cies (145). Low nutrient mulches such as uncomposted bark
or straw were found to decrease nitrogen levels of soil water
while not impacting plant nutrition (95), thus reducing wa-
tershed pollution.
While mulches with relatively high nitrogen content often
result in higher yields (127), low nitrogen mulches can also
increase soil fertility and plant nutrition. For example, straw
(125), sawdust (5) and bark (99) mulches have been shown
to increase nutrient levels in soil and/or foliage. Likewise,
mulches of husks were most effective in increasing available
soil nutrients compared to grasses and leaf-litter (114), which
presumably would have higher nitrogen levels.
Reduction of salt and pesticide contamination. Many land-
scapes experience salinity stress besides those found near
marine coastlines. Arid landscapes in particular are often
highly saline as evaporating water leaves behind salt crusts.
Irrigation water in arid environments and improperly treated
greywater (domestic, non-sewage waste water) can also con-
tain high levels of salts from fertilizers, detergents and other
chemical sources. Container plants that are over-fertilized
will likewise experience increasing levels of salts.
Because mulches reduce evaporation, more water is left
in the soil and salts are diluted. Furthermore, organic mulches
can reduce the effect of salt toxicity on plant growth (3, 69,
142) or actively accelerate soil desalinization (29). Plastic
mulches are not effective in this regard (123), probably be-
cause they are not able to bind ions as organic materials can.
Organic mulches can also help degrade pesticides and other
contaminants (45, 117), presumably by providing increasing
microbial populations that degrade pesticides.
Increased binding of heavy metals. Organic as well as liv-
ing mulches can be effective in removing heavy metals from
landscape and garden soils. Common urban contaminants
such as lead and cadmium can be removed from the soil so-
lution by mulched leaves of eucalyptus (Eucalyptus spp.),
pine, poplar (Populus spp.), and arborvitae (Thuja spp.) (106).
Likewise, a mixture of compost and woodchips was found to
decontaminate forest soils by complexing copper into a less
toxic form (63).
Improved plant establishment and growth. Mulches are
used globally to enhance establishment of many woody and
herbaceous species. There are hundreds of controlled studies
demonstrating that mulches improve seed germination and
seedling survival, enhance root establishment and transplant
survival, and increase overall plant performance when com-
pared to unmulched controls. Practically, this translates to
healthier trees and shrubs requiring less maintenance and
chemical application.
A. Improved seed germination and seedling survival.
Kacinski (62) demonstrated that mulching planting pits with
manure or sawdust improved oak (Quercus spp.) seedling
survival compared to unmulched pits and surface plantings.
Since then, numerous studies have found mulch to enhance
seed germination and seedling survival.
Seedling emergence and survival presents a management
conundrum: we want to encourage desirable plants yet pre-
vent weeds from establishing. Unfortunately, mulches do not
distinguish between weeds and desirable plants. For this rea-
son, many mulches are not appropriate for annual flower beds
and vegetable gardens. On the other hand, these same mulches
are excellent choices for repelling weed colonization.
Success in this respect may be determined by mulch depth
and/or seedling maturity. Deeper mulches are associated with
improved weed control and are not the best choices for areas
that are to be seeded rather than planted, especially if the
species of interest have small seeds. Broadcast seeding on a
restoration site was successful when a thin mulch layer was
applied post-seeding, but significantly reduced when the
mulch depth was doubled or when mulch was applied before
seeding (103). Organic mulch may be a better choice for seed
germination than gravel (90), which in deeper layers can pre-
vent seedling emergence (137). Once seedlings have emerged,
mulches are associated with improved seedling performance
under both nursery (88, 128) and field (92) conditions.
B. Enhanced root establishment and transplant survival.
Numerous studies have demonstrated that improved water
retention and reduced weed growth are correlated with in-
creased root growth and exploration by desirable plants (39,
138). Mulches allow roots of trees and shrubs to extend and
establish far beyond the trunk compared to bare soil (17, 134)
and thus become increasingly stabilized.
Mulch choice is important in determining how well roots
will explore the underlying soil. Root development and den-
sity was greatest under organic mulches compared to that
under plastic (39), bare soil (134) or living mulches (50). In
contrast, sheet and film mulches that act as barriers to water
and air movement will encourage root growth on top of the
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242 J. Environ. Hort. 25(4):239–249. December 2007
mulch (4), which can eventually injure desirable plants when
and if the sheet mulch is removed. In another study (58),
plastic mulches used with a fertilizer treatment led to in-
creased mortality of transplanted materials.
Roots tend to grow into organic mulch layers (134–135),
but by and large these are fine roots whose presence is tran-
sient. Generally, these roots exploit water and nutrient re-
sources in mulch until conditions become unfavorable (e.g.
when much begins to dry in the summer). These roots die
back and new feeder roots appear where resources are more
available. In any case, it does not appear to injure the plant to
have roots exploring the mulch layer. However, roots will
also colonize landscape fabrics and if these materials are even-
tually removed they could cause extensive damage to fine
root systems. This is one reason not to use landscape fabrics
around woody plants.
If roots can establish successfully, then plant survival is
more likely. Thus, use of landscape mulches have been shown
to decrease mortality of new transplants even in harsh envi-
ronments such as mine tailings (91, 139), saline soils (3, 123)
and subarctic systems (58). Enhanced survival through mulch-
ing has been seen in nursery and field production (76, 79,
92), silvipasture systems (108), forest plantations (31, 46,
59), and restoration sites (18, 145). Competitive cover crops
such as turf will increase mortality of transplants (31).
C. Increased overall plant growth performance. As early
as 1942, researchers found that mulched trees grew 67% bet-
ter than those grown on bare soil (56). Many others since
then have shown similar improvements in growth of trees,
shrubs, and other plant materials in field and nursery condi-
tions (6, 15, 19, 42–43, 50, 99, 112, 114, 116). Specifically,
increases in plant height (6, 18, 73, 76, 91, 102, 108, 115),
stem or trunk diameter (6, 31, 50, 91, 107, 108, 115), leaf
size and/or number (26, 31, 76, 91, 99), and flower, fruit
and/or seed production (99, 127, 131) have all been reported
as a result of mulching with appropriate materials.
The best mulches for overall plant performance are or-
ganic materials, consistently rated as the best or second best
in comparative field trials. Tested mulches include rapid de-
composers such as grass clippings, leaves, and compost (50,
99, 107, 115, 127), moderate decomposers including paper
(102), hay and straw (26, 73, 108, 131), and other crop resi-
dues (6, 76, 115) and slow decomposers, especially bark and
woody chips (31, 50, 99, 102). The exceptions to this trend
are almost exclusively found in annual crop production re-
search, where slow decomposers such as bark can create nu-
tritional problems for fast-growing species with limited root
mass (26).
Gravel and stone are generally not as effective as organic
mulches (6, 108, 112, 115) in optimizing plant performance.
Sheet mulches can also produce disappointing results (73,
112). Not surprisingly, competitive ground covers such as
turf grasses result in reduced growth (107, 131) even com-
pared to bare soil conditions (31, 65).
Reduction of disease. Physically, mulches will reduce
splashing of rain or irrigation water, which can carry spores
of disease organisms up to the stems or leaves of susceptible
species. Additionally, the populations of beneficial microbes
that colonize many mulch materials can reduce soil patho-
gens either through direct competition for resources or through
chemical inhibition. Regardless of the mechanism involved,
disease reduction is an important benefit of many mulches.
Researchers have long suspected that mulches play com-
plex roles in disease prevention and recovery. Spaulding and
Hansbrough (119) found the ‘retention of a normal layer of
fallen dead needles under the trees … will help the trees to
resume normal growth’ after suffering needle blight. Part of
the complexity is that mulches can act indirectly and/or di-
rectly to prevent disease establishment. This relationship was
explored by Downer et al. (32) who identified both short-
and long-term effects of mulching on the incidence of
Phytophthora root rot. Indirect effects are both short- and
long-term and include increased soil moisture, soil tempera-
ture moderation, improved soil nutrition, and improved soil
aggregation and drainage. Thus, mulches maintain an opti-
mal soil environment, which in turn supports healthy plants
that are less susceptible to opportunistic pathogens (130).
Mulches can combat disease organisms directly as well.
Researchers have found that Western red cedar (Thuja plicata)
heartwood contains thujaplicin, a water-soluble tropolone not
only inhibitory to various bacteria and fungi, but with anti-
tumor activity as well. This antimicrobial activity is prob-
ably responsible for the rot-resistant nature of cedar wood. In
addition to plant-derived antibiotics, healthy organic mulches
may also contain a variety of soil microbes that can exert
biological control over pathogens, either through resource
competition or enzymatic degradation (24). Many microbes
produce cellulase enzymes that attack the cell wall of patho-
gens such as cinnamon fungus (Phytophthora cinnamomi)
(32). Mulching soils to encourage populations of indigenous,
beneficial soil microbes will increase the effectiveness of
biological control in managing disease (38). This may ex-
plain why organic mulches such as straw (11) and wood chips
(26) are more effective in suppressing disease than landscape
fabric and black polyethylene, respectively.
Some mulches, however, can increase the incidence of dis-
ease by exacerbating already poor soil conditions (e.g. using
a plastic mulch). Sawdust was implicated as a likely medium
for shoestring root rot (Armillaria mellea) when used as a
garden mulch in 1948 (77). Bacterial soft rot (Erwinia
carotovora) was significantly greater in plants grown with a
black polyethylene mulch than with bark or wood chips (26).
Therefore, selecting an appropriate mulch is crucial as part
of an IPM program.
Reduction of weeds. Mulching as a means for landscape
weed control is highly effective, though the mechanism(s)
responsible for control are not completely understood for all
mulch types. Mulches can effectively be used in nursery pro-
duction as well as in the field; Wilen et al. (136) found a 92%
reduction in weeds of container plants that were mulched
rather than left bare. Nearly all mulches reduce light, which
will stress existing weeds and prevent the germination of
many weed species, especially those with small seeds. A com-
parison of 15 mulch types showed that all significantly re-
duced weed growth as compared to bare soil, but there were
no differences between types tested (122). The physical bar-
rier created by other mulches can prevent weeds from emerg-
ing, though this effect is temporary and disappears as mulches
decompose. Certain organic mulches, especially wood chips,
may control weeds chemically through the leaching of al-
lelopathic chemicals naturally occurring in the wood. Addi-
tionally, the protected soil habitat created by the use of
mulches can increase beneficial organisms that prey upon
weeds or eat their seeds.

243
Living mulches can reduce weed problems through both
competition for resources and allelopathy. Ideally, cover crops
and ground covers suppress weed seed germination and es-
tablishment while having little effect on desirable plants. This
ideal is realized in situations where ground covers occupy a
different niche than the desirable plants (e.g. trees and large
shrubs whose roots are typically deeper than ground covers)
(52). If ground covers are too much like other plants in the
landscape, such as low-growing herbaceous perennials, then
they may compete more directly for limited resources like
water, nutrients and sunlight.
Although they can be highly effective in immediately elimi-
nating weeds, plastic films and landscape fabrics should not
be used as a long-term approach of weed control in land-
scapes. White and green plastics do not eliminate photosyn-
thetic radiation, thus allowing weeds to continue to grow un-
derneath; darker mulch colors will eliminate these wave-
lengths and prevent weed growth (57). Regardless of mulch
color, eventually, weeds will colonize soil above these
mulches and some weeds can pierce and grow through plas-
tic films (57). Replacement of plastics and fabrics is not only
time-consuming and expensive but also damages the roots
of desirable plants that invariably will grow through and over
these mulches (4).
In general, inorganic and organic mulches are most effec-
tive in weed control when applied at sufficient depth (51)
and are least susceptible to compaction (37). Inorganic
mulches such as gravel will prevent weed growth if the lay-
ers are at least 4 cm (1.5 in) deep (137). Because inorganic
mulches do create otherwise optimal conditions (i.e. adequate
soil moisture and moderated temperature), the absence of
light in these deeper mulches is probably responsible for the
lack of germination of weed seeds that require light for ger-
mination.
Organic mulches are variable in their weed-controlling
abilities. Nutrient-rich, finely textured materials like com-
post are not satisfactory mulches for weed control (75, 79,
95). Instead, they act as a fertile base and potential seed bank
for establishment of new weeds or enhancement of peren-
nial weeds. Weed seeds that settle on top of organic mulches
are more likely to germinate, especially if the mulch layers
are thin (meaning seedling roots can more quickly reach the
underlying soil. Applying two, rather than one organic mulch
layer results in significantly less seed germination (103).
Organic mulches that are coarse, applied in thicker lay-
ers, and/or less nutrient-rich are more effective in control-
ling weeds — sometimes even better than herbicides (18,
44). Locally-derived residues from crops (26, 76, 93, 95)
and forest products (19, 26, 37, 46, 57, 95) have all proved
effective in reducing weed success in a variety of agricul-
tural and landscape situations, especially in uncomposted
form (95). While some studies recommend the use of saw-
dust for weed control on forest lands (5, 131) or in container
production (90), others have noted that thick layers of saw-
dust can be impermeable to gas and water movement (120).
This material might best be used in situations where soils
are less frequently compacted by vehicular or foot traffic:
not urban landscapes.
Reduced pesticide use. Mulches reduce plant stress and
susceptibility to pests. This important function means that
plants will be more resistant to weed invasion and opportu-
nistic pests and pathogens, which leads to reduced use of
herbicides, insecticides, and fungicides. Reduction in unnec-
essary chemicals not only saves money but also preserves
the health of beneficial insects, bacteria, fungi, and other soil
organisms that might otherwise be negatively affected.
Aesthetic improvement. Mulches can be beautiful as well
as functional; though this is not a scientifically measurable
aspect, the fact remains that aesthetics will influence mulch
choices and usage. Many mulches, such as ground covers or
tumbled glass, can add to a landscape’s design elements while
protecting soil. Visually distinctive mulches can be used to
control foot traffic by directing pedestrians through a land-
scape, which both protects sensitive root zones and adds a
design element. Some mulches add other sensory elements
in addition to visual interest: smooth rock and soft ground
covers invite touching, while fragrant ground covers and fresh
organic mulch add enjoyable scents to the landscape. The
aesthetic appeal of mulches is critical to their acceptance by
consumers, who may otherwise perceive mulches as ‘messy’
and prefer the appearance of bare soil.
Economic value. For any landscape management practice
to become widely adopted its economic viability must be
established. Many decades of research has demonstrated that
mulching improves crop production. Far fewer studies have
addressed the economic impacts in urban landscapes and so
it is difficult to make detailed economic arguments based
solely on tangible costs and benefits.
Over 40 years ago Hunt (59) found that ‘the increase in
survival of mulched plants more than compensated for the
extra cost.’ Likewise, Brantseg (14) found that retaining log-
ging slash increased increment size of new tree seedlings
and thus the economic value of replanted pine forests. Not
all mulches result in defensible cost:benefit ratios. Paper, plas-
tic discs and black polyethylene mulches all failed to im-
prove survival and growth for several tree species, causing
the researcher to recommend against their use for economic
reasons (70). Furthermore, the synthetic mats and films tend
to be the most expensive choice (111, 141).
Cost savings have been more specifically identified by oth-
ers and include reduced use of pesticides (21, 46) or other
weed control methods (46). Utilizing locally-produced woody
debris as a production mulch was described as ‘a useful and
affordable tool’ with low external input for restoring dam-
aged land, improving crop tree growth, and increasing farmer
income (91). In a more urban application, brush mulch was
found to be ‘applied easily and economically’ during reveg-
etation of roadsides (104).
Locally available materials continue to be good economic
choices. An early study (131) recommended timber harvest
residue as a mulch over peat materials based on both cost
and performance. Another study which compared several dif-
ferent mulches found unprocessed bark from regional saw-
mills to have the greatest cost:benefit ratio as long as the
cost did not exceed $2.75 per m3 ($2.10 per yd3) (102). In
tropical regions, paddy straw mulch is routinely used and
one study found it to have the highest benefit:cost ratio (93).
Mulch Problems — Real and Perceived
Acidification. Organic mulches such as wood chips and
bark are thought by some to be soil acidifiers. No scientific
research supports this, and in fact studies refute this percep-
tion. One study found neither pine bark nor pine needles had
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244 J. Environ. Hort. 25(4):239–249. December 2007
any affect on soil pH (51). A second report (60) found bare
soil to be more acidic than soil covered by inorganic mulch,
and that shredded bark and wood chips were least acidifying
of all treatments. Similarly, a year-long study found that the
soils under organic mulches were either more alkaline or not
affected by mulch treatment (100).
It’s likely that in artificial conditions, such as nursery pro-
duction, that woody materials do have an acidifying effect
when they are used as part of a potting medium. Release of
phenolic acids is one stage of the decomposition of woody
material, and if this material comprises the bulk of medium
then acidification is likely to occur. In a field situation, how-
ever, where the woody material is used as a mulch (and not
worked into the soil), any acidification will be localized within
the mulch layer and have little effect on the vast underlying
soil environment below. Thus, soil acidification due to mulch-
ing with woody plant material is unlikely to occur under real
world conditions.
Allelopathy. Allelopathy is the inhibition of seed germina-
tion and growth of plants through the release of chemicals
and apparently plays a large part in the weed-controlling be-
havior of many organic and living mulches. A few growth-
inhibiting substances have been isolated and identified, in-
cluding the classic example of juglone (and juglonic acid)
which is produced in all parts of black walnut (Juglans ni-
gra). Juglone can injure or kill seedlings and shallowly rooted
plants, though it apparently has little effect on established
plants (54). In laboratory tests, allelopathic activity of a com-
pound is usually confirmed by inhibition of seed germina-
tion (34) rather than how it affects mature plant materials.
Other well-studied, plant-derived natural pesticides, such as
those within the Thuja species described earlier, have no dem-
onstrated negative effect upon plant materials.
Seeds and seedlings, whether weeds or desirable species,
are most sensitive to mulch suppression as they do not have
the extensive root systems of established plants. Mulches
made of pine, eucalyptus, and acacia (Acacia spp.) were able
to suppress germination of several common weed species as
were water extracts of these materials, supporting an allelo-
pathic function (110). Grasses may be less affected than di-
cot weed species (110), and this may help explain the appar-
ently contradictory evidence demonstrating that eucalyptus
leaf mulch has no effect on rice seed germination (Oryza is a
monocot genus) (71).
It is unlikely that any properly applied landscape mulch
will have allelopathic effects on established landscape plants,
but is most likely to injure newly planted or shallowly rooted
plants in the landscape. For such plantings, a short period of
composting and correct application of woody mulch will pre-
vent damage.
Competition. As mentioned earlier, living mulches can be
competitive with landscape plants for water, nutrients, and
space. Bedford and Pickering (1919) were perhaps the first
to document both the interference of grasses with tree growth,
as well as its subsequent recovery upon grass removal (9).
Turf and other grasses are very competitive (9, 47), espe-
cially during plant establishment (65, 139). Thus, turf grass
must be kept away from newly installed shrubs and trees and
can easily be replaced with an organic mulch. These ‘tree
skirts’ allow rapid root establishment without competition
from turf roots (Fig. 1).
Chemical contamination. As with composts, woody mulch
quality is influenced by the source of materials. Mulches cre-
ated from branches and tree trimmings often contain a diver-
sity of leaves, wood, and bark, which contributes to a highly
functional mulch. In contrast, woody mulch made from wood
recovered from construction and demolition debris can con-
tain pressure-treated lumber. In one Florida study, 18 of 22
samples collected from debris processing facilities contained
arsenic (from chromated copper arsenate-treated wood) at
concentrations greater than the state’s allowable levels (129).
Similarly, mill wastes that contained formaldehyde and other
wood processing residues reduced survival of tree seedlings
when used as a mulch (85).
Disease. Mulches made from diseased plant materials can
contain those pathogens. For this reason, many mulches are
composted (55) or otherwise treated at temperatures that kill
the pathogens along with other harmless or beneficial organ-
isms. Therefore, many commercially available organic
mulches are relatively sterile.
While mulches made from diseased wood can contain vi-
able populations of pathogens such as honey locust canker
(Thyronectria austro-americana) (64), few examples of dis-
ease transference exist in the literature. Wood chips made
from infected maple trees and used as a mulch failed to spread
Verticillium spp. to healthy trees (28). Likewise, a wood chip
mulch containing the shoestring root rot pathogen did not
exhibit disease transmission, perhaps because the organism
Fig. 1. Tree skirt of coarse arborist wood chips creates a competitor-
free zone to allow root establishment in turf.

245
dried out rapidly (98). Finally, a six-year study of diseased
woody mulches found no transmission of either butt rot
(Armillaria gallica) or canker (Botryosphaeria ribis) patho-
gens to common landscape tree species (61). The only evi-
dence of disease transmission occurred when foliage from
Austrian pines (Pinus nigra) infected with tip blight
(Sphaeropsis sapinea) was used as a mulch around healthy
saplings of the same species (61). No trees from other spe-
cies were affected, and the author acknowledges that Aus-
trian pines are particularly susceptible to this pathogen. It is
not surprising that so few examples of mulched-mediated
disease transmission have been documented. The pathogen
of interest must be present in such a way as to fit the epide-
miology of the disease cycle; simply existing in a mulch
source is not enough.
While disease transmission from mulch to tree is unlikely,
there is greater probability of infection if backfill soil is
amended with wood chips. A researcher working with rhodo-
dendron (Rhododendron spp.) lost plant material to
Phytophthora root rot after amending the soil with 33%
composted wood chips (97). A similar transmittal of verticil-
lium wilt (Verticillium dahliae) was seen when infected wood
chips were used as part of a potting mix (41). Not only is this
a poor practice for installing woody plants, it also casts doubt
on the efficacy of composting to eliminate pathogens.
Many landscape pathogens are both opportunistic and per-
vasive in the soil environment. Armillaria spp., for instance,
are widespread in many soils as a decomposer but can be-
come pathogenic under unhealthy soil conditions. Healthy
soil communities, on the other hand, have diverse fungal and
bacterial species, many of which are symbiotic partners of
plant root systems. These beneficial species can outcompete
pathogens as long as soil conditions remain optimal for root
growth. When soils become compacted and anaerobic, plants
decline and become susceptible to opportunistic pathogenic
microbes — always present but inactive in healthy soils.
Given the distance between wood chip mulch and plant
roots, it’s doubtful that pathogens would travel far under
healthy soil conditions. Fresh wood chips have been used as
a mulch in other long-term studies without any report of dis-
ease transmission (43, 95). It does, however, point out the
importance of keeping wood chip mulches away from the
trunks of trees and shrubs as moist trunk conditions are at
risk of pathogen infection. In addition, only unprocessed
wood should be used in making wood chips. Mulches de-
rived from shipping pallets and other wood packing materi-
als, especially if uncomposted, could introduce exotic plant
pathogens (64).
Though they do not qualify as disease organisms, other
fungal species should be mentioned as possible nuisances in
woody mulches. The artillery fungus (Sphaerobolus stellatus)
can be found on landscape mulches where it can propel sticky
spore masses onto the sides of nearby light-colored cars and
houses (13, 55). Spent mushroom compost has recently been
identified as an antagonist to this fungus (25) and might be a
wise choice as a mulch component in affected landscapes.
The colorfully named ‘dog vomit fungus’ (Fuligo septica)
— actually a slime mold — creates a bright yellow mass on
woody mulches. This is not a pathogenic species but may be
of questionable aesthetic value.
Flammability. Though there are documented incidences of
spontaneous combustion of yard wastes (16), in general wood-
based mulches are not flammable. A recent comparison of 13
landscape mulches (121) found rubber mulch to be the most
flammable, followed by fine textured organic mulches (dried
pine needles, straw, shredded bark), coarse textured organic
mulches (chipped wood, bark nuggets, cocoa shells), mulches
with higher water content (composted yard waste and sod),
and finally brick chips (which never ignited). These com-
parisons should be carefully considered when mulching in
regions where there is significant fire danger.
Nitrogen deficiency. A common misconception about
woody mulches is that they impose a nutrient deficiency upon
plant materials. This is based on the fact that woody mulches
have a high C:N ratio and nitrogen will be ‘tied up’ by mi-
crobes during the decomposition process. Furthermore,
woody materials that are used as amendments incorporated
into soil or potting mixes will create zones of nitrogen defi-
ciency, which is visualized by spindly, chlorotic growth of
plants in these zones.
Experimental research reveals that neither nitrogen im-
mobilization nor growth suppression occurs as a result of
using woody materials for mulch (51, 100). To the contrary,
many studies have demonstrated that woody mulch materi-
als actually increase nutrient levels in soils and/or associated
plant foliage (5, 99, 114, 125). A zone of nitrogen deficiency
exists at the mulch/soil interface (Chalker-Scott, unpublished
data), possibly inhibiting weed seed germination while hav-
ing no influence upon established plant roots below the soil
surface. For this reason, it is inadvisable to use high C:N
mulches in annual beds or vegetable gardens where the plants
of interest do not have deep root systems.
Pests. Many organic mulches, especially those based on
wood products, have an undeserved reputation as ‘pest mag-
nets.’ In fact, many of these wood-based mulches are not
attractive to pest insects but are actually insect repellent.
Salvia spp., pine needles, and cedar shavings were found to
repel fire ants (Solenopsis invicta) under laboratory condi-
tions (1). Thuja species have developed a number of chemi-
cal weapons against pests including thujone, one of several
essential oils found in arborvitae foliage and that of other
non-Thuja species. Best known for its ability to repel clothes
moths, thujone and other foliar terpenes also repel, inhibit,
or kill cockroaches (Blattodea), termites (Isoptera), carpet
beetles (Dermestidae), Argentine ants (Iridomyrmex humilis),
and odorous house ants (Tapinoma sessile).
A common concern is whether wood-based mulches are
attractive to termites. There have been specific studies tar-
geting this question with sometimes surprising results. One
recent study (74) compared subterranean termite
(Reticulitermes virginicus) activity underneath both organic
(bark and wood) and inorganic (gravel) mulches. The great-
est termite activity was found beneath the gravel mulch. Not
only were the wood and bark mulches unappealing to ter-
mites, but when fed a diet of these materials in the lab they
suffered increased mortality. These results are partially ex-
plained by an earlier study (35), which found that termites
preferred mulches with higher nitrogen and phosphorus con-
tent. Martin and Poultney (76) confirm this in a study dem-
onstrating termite partiality for banana mulch, a relatively
nutrient-rich material. Therefore, in regions where subterra-
nean termites are potential pests, organic mulches should be
selected that are low in nutrients.
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246 J. Environ. Hort. 25(4):239–249. December 2007
Research upon the ability of mulch to exacerbate or con-
trol other pest insects and nematodes is sparser and more
variable, so it is not easy to draw generalizations. Dry-sur-
faced mulches, such as gravel or wood chips, are recom-
mended as deterrents to ticks (Ixodes scapularis) that carry
Lyme disease (78); other insects (beneficial or otherwise)
may likewise be discouraged. Black plastic and landscape
fabric may reduce certain pest species in the short term (33,
83), but their long-term negative impacts on soil and plant
health are so significant that their perpetual use in a land-
scape cannot be recommended. In general, any mulch which
increases the general health of the soil environment will un-
doubtedly enhance the diversity of beneficial microbes and
insects in the landscape (124).
Mulches are variable in their ability to attract or repel mam-
malian pests as well. Some materials may naturally repel her-
bivorous mammals by virtue of their thorns or odors (20) or
texture (125), while others may attract undesirable pests, es-
pecially rodents, who can use dense ground covers or sheet-
type mulches for shelter (40, 86, 113).
Weed contamination. Mulches lacking pedigrees can be
carriers of weed seeds and other undesirable plant parts. While
controlled research on this problem is lacking, anecdotal evi-
dence suggests that improperly treated crop residues and com-
posts as well as bark mulches are often carriers of weed seed.
Woody mulches may contain invasive species associated with
tree materials that are chipped. Many of these species, such
as English ivy (Hedera helix), can easily grow from seed or
regenerate vegetatively, thus colonizing landscapes. If it is
not possible to document the mulch source, it might be pru-
dent to use it on a small area of the landscape and monitor it
for problems before using it more widely.
A successful mulch must be deep enough to suppress weeds
and promote healthy soils and plants: research has demon-
strated that weed control is directly linked to mulch depth
(81), as is enhanced plant performance (42). As mentioned
earlier, coarse materials are more effective in this regard as
their depth will not have the negative impacts found with
fine-textured materials.
A review of the research on coarse organic mulches and
weed control reveals that shallow mulch layers will promote
weed growth (67) and/or require additional weed control mea-
sures (144). Coarse organic mulches applied at depths of 7.6
cm (3 in) or less are most likely to fall into this category. On
the other hand, mulches applied at 10 cm (4 in) (36) or 15 cm
(6 in) (57) were effective in weed control.
Other considerations. In addition to the objective science
that should be used to guide landscape management deci-
sions, there are other factors to consider. Perhaps most im-
portant are those that affect the sustainability of not only the
local landscape but global ecosystems as well. While this
literature review has focused directly on plant and soil crite-
ria associated with landscape mulches, other important en-
vironmental issues may well play a role in the ultimate choice
of landscape mulch materials. Such issues as environmental
degradation associated with acquisition of some mulching
materials, use of invasive species as ground covers, nutrient
overload from overuse of organic mulches, and toxic
leachates from some synthetic mulches, must also be con-
sidered.
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