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112 Tree Planters’ Notes
Abstract
This paper is based on questions from an audi-
ence participation discussion with the author S.
Grossnickle during the Joint Annual Meeting of the
Forest Nursery Association of British Columbia
and the Western Forest and Conservation Nursery
Association (Sidney, BC, September 30-October
2, 2019). The ve question topics presented herein
were, by consensus, the most discussed questions
presented by the audience of nursery practitioners
and foresters. Topics explored in this paper relate to
nursery hardening practices, irrigation management
to promote stress resistance, cultural strategies to
promote vigorous root growth, storage practices for
hot-lifted seedlings, and storage length for overwin-
ter stored seedlings. The following answers to these
specic topics are the authors’ combined views on
these nursery cultural practices.
Introduction
Nursery cultural practices have a direct impact on
seedling quality and subsequent eld performance
(Dumroese et al. 2016, Grossnickle 2012, Grossnickle
and MacDonald 2018a, Mattsson 1997, Sutton 1979).
Culturing seedlings requires specialized knowledge
and skill to produce adequate quantities of high-qual-
ity plants from appropriate genetic seed sources in
a timely manner. This process starts with a partner-
ship between the client and the nursery manager to
determine plant specications that are matched to
the outplanting site (Dumroese et al. 2016). These
plant specications include species, seed source, and
stocktype, as well as particular morphological and
physiological characteristics that will maximize the
seedling potential to survive and thrive after outplant-
ing (Haase 2008).
For any given seedling crop, it is important to dene
and rene the path required to go from start to nish.
Plants are biological organisms and must be treated as
such; they are not widgets in a factory. Morphology
is relatively easy to see and measure, and most target
specications are based on these measures. Nonethe-
less, physiological function must also be considered
because seedling physiological responses to the envi-
ronment determine their survival and morphological
development (Grossnickle 2000).
Plants’ physiological function is ever changing and
responding to their external environment. Thus,
constant monitoring of seedling development in the
nursery is essential, especially for identifying and
addressing any problems (Duryea 1985, Grossnickle
and MacDonald 2018b). Throughout the process,
growers must manage risks to maximize yield and
performance. Without good quality upon leaving the
nursery, seedlings with the best genetics cannot do
well in the eld.
This paper explores ve questions about seedling
ecophysiology and nursery culturing raised during the
2019 Joint Annual Meeting of the Forest Nursery As-
sociation of British Columbia and the Western Forest
and Conservation Nursery Association.
Question 1 – What Are the Best
Cultural Hardening Practices To
Maintain Physiological Quality?
Cultural hardening practices that improve seedling
“physiological quality” have long been considered
important for increasing survival and growth potential
after eld planting for both bareroot (Wakeley 1948,
1954) and container-grown (Landis et al. 2010, Lav-
ender and Cleary 1974, Tinus 1974) seedlings. This
is because hardened seedlings usually have quality
Seedling Ecophysiology: Five Questions To Explore in
the Nursery for Optimizing Subsequent Field Success
Steven C. Grossnickle, Steven B. Kiiskila, and Diane L. Haase
Consultant, NurseryToForest Solutions, North Saanich, BC; Crop Manager, Arbutus Grove Nursery, Victoria, BC;
Western Nursery Specialist, U.S. Department of Agriculture, Forest Service, Portland, OR
Volume 63, Number 2 (Fall 2020) 113
attributes necessary to become established after plant-
ing on restoration sites (Grossnickle 2012, Grossnickle
and MacDonald 2018a). As nursery-grown seedlings
reach a desired morphological size, cultural practices
to modify daylength, temperature, watering, and fertil-
ization can be applied to harden seedlings (Landis et al.
1999, Landis 2013, Tinus and McDonald 1979).
Stress resistance is not considered to be related to plant
age (e.g., freezing resistance [Sakai and Larcher 1987];
drought resistance [Teskey et al. 1984]), but rather to
its morphological, physiological, and phenological state
(Fuchigami et al. 1982, Lavender 1985). Changes in
phenological and physiological parameters are known
to occur in parallel (Fuchigami et al. 1982, Fuchigami
and Nee 1987, Lang et al. 1985) with stress resistance
varying seasonally with plant development (Bigras
1996, Burr 1990, Grossnickle 2000) in temperate
zone tree species (gure 1). In addition, root growth is
related to seasonal shoot dormancy patterns, decreasing
as shoot endodormancy (regulated by internal factors)
intensies in the fall and increasing as seedlings move
toward ecodormancy (regulated by environmental
factors) (Ritchie and Dunlap 1980, Ritchie and Tanaka
1990). This knowledge of plant acclimation in relation
to the phenological state can be used for scheduling
hardening practices during the last stages of a nursery
cultural program, thereby improving seedling quality
and enhancing subsequent eld performance (Landis et
al. 2010, Lavender and Cleary 1974, Tinus 1974).
Acclimation of seedlings is based on the concept of
“slowly increasing stresses to induce physiological
adjustments in plants” (Kozlowski and Pallardy 2002);
thus, cultural practices that enhance stress tolerance or
avoidance can help seedlings develop morphological
and physiological protection from potentially limiting
eld site conditions (Landis et al. 1999, Lavender and
Cleary 1974, Tinus 1974, Wakeley 1954). The follow-
ing sections describe cultural practices of modied
daylength (photoperiod), temperature, and fertilization
to promote seedling hardening while maintaining
Figure 1. Seedlings have distinct phenological cycles which can vary somewhat based on species, geographic seed source, and weather patterns. (a) Bud
dormancy (measured as days to budbreak) is high in the fall and declines through the winter and early spring, while stress resistance and cold hardiness peak in
winter. (b) Root and shoot growth follow different patterns. (a - adapted from Landis et al. 2010; b – adapted from Landis et al. 1999)
114 Tree Planters’ Notes
physiological quality. A detailed discussion on water-
ing as a seedling hardening cultural practice is de-
scribed in the answer to Question 2.
Daylength
After the summer solstice, daylength shortens, pro-
moting development of endodormancy. With north-
ern-latitude tree species, the end of shoot elongation
and development of terminal buds is considered to
be the rst stage of fall acclimation to low tempera-
tures (Weiser 1970) and an overall increase in stress
resistance (Levitt 1980). Seedlings normally enter
the rst stage of fall acclimation to low temperatures
(Grossnickle 2000, Lang et al. 1985, Levitt 1980,
Weiser 1970) and develop increased drought resis-
tance (Abrams 1988, Teskey et al. 1984) in the latter
half of summer, when shoot elongation has ended
and terminal buds are developing (Burr 1990). As
seedlings develop a “hard bud,” they are considered
endodormant and will not break bud even if they are
exposed to optimal environmental conditions (see
Temperature section). In this state, they continue to
grow roots, though root growth is declining (Ritchie
and Dunlap 1980, Ritchie and Tanaka 1990).
Because temperate tree species respond to seasonal
decreases in daylength, short-day treatments have
been developed in northern-latitude container nurser-
ies to induce shoot growth cessation and bud forma-
tion (Landis et al. 1999, Tinus and McDonald 1979).
The typical short-day treatment for spring-planted
seedlings is initiated in August with an 8- to 10-h day
and 14- to 16-h night treatment for 10 to 12 days, with
variations depending on species and genetic sources
(Grossnickle 2000, Landis et al. 1999). Seedlings are
then placed under a cultural regime to maintain budset
(i.e., moderate water stress, shortened photoperiod,
and low N fertilization; Landis et al. 1999, Lavender
and Cleary 1974). During hardening, the reduction
of N fertilization is an optional practice that brings N
levels below their optimum range, with N levels re-
turned to their optimum range when limiting seasonal
environmental conditions ensure seedlings remain
endodormant and will not reush. These practices are
then maintained until they are lifted for storage in late
fall or early winter. For summer-planted (Grossnickle
and Folk 2003, Luoranen et al. 2006) and fall-planted
(Luoranen and Rikala 2015; MacDonald and Ow-
ens 2006, 2010) seedlings, short-day treatment (as
dened above) is initiated approximately 2 months
before seedlings are lifted and shipped to the eld to
allow for a 5- to 6-week exposure to seasonal short-
ening photoperiods and ambient temperatures. This
approach recognizes the annual seedling phenological
and physiological cycles (gure 1), and utilizes them
to promote budset development, dormancy, freezing
tolerance, and drought resistance (Colombo et al.
2001, Grossnickle 2000, Landis et al. 2010), thereby
producing hardened seedlings. The advantage of us-
ing photoperiod manipulation is that it allows for the
application of a uniform cultural treatment over the
entire crop (Landis et al. 1999).
Temperature
As seedlings are exposed to cold fall temperatures
and accumulate chilling hours, they move through
the endodormancy phase, with maximum days to
budbreak in late summer and early fall decreasing
through the fall and into winter (Burr 1990, Fuch-
igami et al. 1982) and increasing stress resistance
(Grossnickle 2000) peaking in winter (gure 1).
When seedlings complete the endodormancy phase
and move into the ecodormancy phase, root growth
potential increases (Burr 1990, Ritchie and Tanaka
1990) and seedlings only remain inactive as long as
environmental conditions are unfavorable for growth
(Burr 1990, Fuchigami et al. 1982, Lang et al. 1985).
Chilling hours, rather than calendar date, are used by
nursery practitioners to track fall acclimation be-
cause temperate conifers require a period of chilling
to move through endodormancy and become ready
for overwinter storage. Chilling hours are quantied
based on specic temperature ranges. For example,
in the Pacic Northwest and Canada, chilling hours
are often recorded from 0 to 4.4 °C (32 to 40 °F)
(Timmis et al. 1994, van den Driessche 1977), or to
10 °C (50 °F) (Burdett and Simpson 1984, Ritchie et
al. 1985), while in the southern United States, chilling
hours are typically reported within the range of 0 to
8 °C (32 to 46.5 °F) (Carlson 1985, Garber 1983). In
some instances, temperatures above or below a cer-
tain level are given partial or negative chilling hours
(Haase et al. 2016, Harrington et al. 2010). Chill days
(O’Reilly et al. 1999), degree-hardening-days (Landis
et al. 2010), or hardening degree days (Carles et al.
2012) are sometimes reported when hourly data are
not available. As chilling hours increase, the days to
Volume 63, Number 2 (Fall 2020) 115
budbreak decrease and stress resistance increases for
a wide range of temperate tree species (Grossnickle
and South 2014) (gure 1).
Fertilization
Reduction, reformulation, or withdrawal of fertilizer
toward the end of the growing season is an effective
means to slow growth and induce bud formation
(Landis et al. 1999, Tinus and McDonald 1979). This
practice is sometimes done in concert with short-day
treatments at container nurseries. Typically, N fertil-
ization is reduced by 50 to 90 percent from rates used
during the rapid growth phase of seedling development
(Landis et al. 1989). Fall fertilization regimes, applied
after the hardening fertilization treatment, have been
developed to result in optimum nutrient levels available
for growth after outplanting (Dumroese 2003, Hawkins
2011, Landis 1985), while fall nutrient loading after the
completion of budset is designed to increase seedling
nutrient reserves to luxury consumption levels, thus in-
creasing eld performance potential (Dumroese 2003,
Grossnickle 2012, Grossnickle and MacDonald 2018a,
Hawkins 2011, Timmer 1997).
Question 2 – How Can Irrigation
Management Be Used To Promote
Stress Resistance?
Modifying irrigation practices to create water stress
events at the end of the growing season affects plant
development and can be used to increase stress resis-
tance and hardening. These water stress events result in
“physiological adjustments” in plants (Kozlowski and
Pallardy 2002), increased drought resistance (Teskey
et al. 1984), and induction of bud formation (Calme´ et
al. 1993, Lavender and Cleary 1974, Macey and Arnott
1986, Timmis and Tanaka 1976, Young and Hanover
1978). Drought resistance is a combination of drought
avoidance and drought tolerance (Abrams 1988, Tes-
key et al. 1984). Drought avoidance (i.e., postponement
of plant dehydration through reduction in water loss)
includes cuticular development (Grossnickle 2000),
stomatal sensitivity (Folk and Grossnickle 1997, Tim-
mis 1980), morphological balance (Mexal and Landis
1990, Thompson 1985), increased water absorption by
roots (Carlson and Miller 1990), and improved root
growth capacity (van den Driessche 1991). Drought
tolerance (i.e., capacity to undergo dehydration with-
out irreversible injury) includes osmotic and cell wall
elasticity adjustment (Joly 1985, Lopushinsky 1990,
Ritchie 1984, Timmis 1980) and chloroplast drought
resistance (Timmis 1980).
Exposing seedlings to water stress, in combination with
reduced photoperiod and fertilization, is used to harden
seedlings (Landis et al. 1999). Successful implementa-
tion of this cultural practice requires an understanding
of necessary water stress levels for the development of
seedling drought resistance. For example, loblolly pine
(
Pinus taeda
L.) seedlings developed drought re-
sistance during a 5-week reduced irrigation regime
(gure 2a) with a 50-percent increase in drought
avoidance (cuticular transpiration declined from 3.8
to 2.3 percent water loss h-1 after stomatal closure)
and a 100-percent increase in drought tolerance (os-
motic potential at turgor loss point that declined from
Figure 2. (a) Mid-day shoot water potential of loblolly pine (Pinus taeda L.)
seedlings changes in relation to the water content container capacity percent-
age (CC%). The arrows along the X-axis are hardening targets to progressively
lower the CC% to 40 percent over a series of weeks. (b) Drought resistance
is measured by drought avoidance (cuticular transpiration that declined from
3.8- to 2.3-percent water loss per hour after stomatal closure) and drought
tolerance (osmotic potential at turgor loss point that declined from -1.0 to
-2.0 MPa) during nursery hardening (i.e., reduced fertilization and watering)
(Grossnickle unpublished).
116 Tree Planters’ Notes
-1.0 to -2.0 MPa) (gure 2b). Other studies have also
found that restricted watering hardens loblolly pine
seedlings (Bongarten and Teskey 1986, Hennessey and
Dougherty 1984, Seiler and Johnson 1985). As loblolly
pine seedlings proceed through this drought-harden-
ing event, their shoot and root systems stop growing,
needle cuticular development occurs resulting in tactile
changes from a feather-like to a stiff feel when moving
one’s hand across the foliage, needle color changes
from lush green to light green, and root suberization
occurs resulting in a color shift from white to brown
(gure 3). These visual cues allow the nursery prac-
titioner a means to track seedling changes during the
drought hardening process.
For a water-stress cultural practice to be successful,
one needs to increase water stress in a stepwise
Figure 3. Loblolly pine (Pinus taeda L.) seedling morphological development during drought hardening. Phase 0 (onset of hardening, week 0) is an actively growing
seedling with needles exhibiting a lush, green color, feather-like feel when moving one’s hand across the foliage, and more than 50 percent of the root system is
unsuberized with a white color. In Phase 1 (occurring by week 2), seedling needles start to lose their green luster and roots show initial stages of suberization on
the upper portions of the plug. Phase 2 (occurring by week 3 to 4) is characterized by light green needles that exhibit initial cuticle development and have a slightly
stiff feel; also, less than 25 percent of the root system shows an unsuberized white color. In Phase 3 (occurring by week 5), needles are light green and exhibit full
cuticle development with a stiff feel, plus 100 percent of the root system shows brown, suberized roots. (Photos by Steven C. Grossnickle)
Volume 63, Number 2 (Fall 2020) 117
progression as seedlings transition from the growing
phase into the hardening phase. For example, contain-
er-grown loblolly pine seedlings typically go through a
series of drying cycles (i.e., watered to saturation and
allowed to dry to a dened container weight) with an
initial dry down to 60-percent container capacity, fol-
lowed by progressively lower levels over 3 to 5 weeks
until reaching 40-percent container capacity and a
mid-day shoot water potential of -1.5 MPa (gure 2a).
These drying cycles are intended to expose seedlings
to drought stress that comes near, but does not exceed,
the shoot wilting point (Landis et al. 1999). A standard
operational monitoring practice for certain species is to
wait until 10 percent (Kiiskila, personal communica-
tion), 25 percent (Grossnickle et al. 1991), or even up
to 40 percent (Grossnickle, personal communication)
of the crop has shoot tip wilting before rewatering.
A minimum predawn water potential of -1.0 MPa
(Lavender and Cleary 1974) or daytime readings
between -1.2 and -1.5 MPa (Cleary 1978), or even
as low as -1.5 to -1.7 MPa (Landis et al. 1999, Tinus
1982) over a series of stress events was sufcient to
terminate shoot growth and develop stress resistance in
conifer species. When seedlings are fully hardened, the
crop will not show shoot system wilt during a drought
event (Grossnickle, personal communication). If water
stress is too severe or too rapid during these drying
cycles, it impedes the physiological development of
drought resistance (Cleary 1978). Avoiding rapid de-
velopment of water stress is critical to ensure this is an
effective cultural practice.
Vapor pressure decit is another environmental variable
related to the plant-water balance and can be used to
harden seedlings. Seedlings harden with exposure to the
combination of lower available soil water and higher
vapor pressure decit (Larcher 1995). These conditions
will cause moderate plant water stress and reduced pho-
tosynthesis (Grossnickle 2000, Kozlowski et al. 1991)
which can slow or stop seedling growth (Grossnickle
2000, Kozlowski 1982) and help harden seedlings for
reforestation site conditions (Landis et al. 1999).
The use of water stress is not always successful in
hardening seedlings within an operational nursery
environment (Landis et al. 1989). First, there is dif-
culty in implementing a uniform drought treatment
due to differences in irrigation coverage and variation
in individual seedling water use. Second, when stan-
dard peat-based growing media dry, they can become
hydrophobic, making it difcult to rewet and thereby
causing uneven exposure to the drying regime. To
avoid or overcome media becoming hydrophobic, it is
important to overwater after a drought-stress treat-
ment to ensure all cavities are fully saturated (Kiiski-
la, personal communication). Third, species differ in
development of drought resistance (Abrams 1988),
making it difcult to apply water stress as a univer-
sal hardening treatment across all species. Thus, it is
important to monitor water stress treatments to ensure
they are applied uniformly and result in successful
hardening.
Question 3 – What Nursery Cultural
Strategies Promote Vigorous Root
Growth?
A well-developed, functional root system is critical
for outplanting success (Grossnickle 2005, 2012;
Grossnickle and MacDonald 2018a). Quality root
systems readily uptake water and nutrients and give
structural support to the seedling. Measures of root
quality include mass, shoot-to-root ratio, form, length,
brosity, root growth potential, and nutrient/carbohy-
drate content (Davis and Jacobs 2005, Haase 2011a).
Although the root system is not easily observed com-
pared with the shoot due to its belowground nature,
attention to root morphology and physiology in the
nursery are imperative to help ensure good eld per-
formance. When working with growers, Landis (2008)
often referred to seedlings as a “root crop” to empha-
size the importance of good-quality root systems.
For the most part, nursery strategies for developing
vigorous seedling root systems are inextricably linked
with strategies for promoting overall plant quality. For
instance, root vigor is tied to the transfer of photosyn-
thates from the shoots (Binder et al. 1990, Philipson
1988, van den Driessche 1987). To achieve target
specications, the grower must consider the phenolog-
ical cycle for the species and seed source (gure 1),
along with environmental patterns at the nursery. As
such, growing regimes must be tailored to stocktype
(i.e., container type, size, depth, and density, seedling
age, and outplanting season) and its associated target
specications for the outplanting site conditions. For
example, some species (e.g., pine) are strongly taproot-
ed and tend to not generate lateral roots in the upper
part of the root system. In a nursery setting, however,
development of lateral roots and numerous root tips is
118 Tree Planters’ Notes
a primary goal for ensuring root egress and vigor after
outplanting (gure 4). In studies with overwintered
spruce (
Picea
spp.) seedlings, root hydraulic conduc-
tivity increased with new root growth because newly
developed roots have low root resistance and high
water uptake capability during the rst few weeks after
thawing (Colombo and Asselstine 1989, Grossnickle
1988). Thus, alleviation of planting stress depends on
the number of new roots a seedling develops just after
planting (Grossnickle 2005).
To encourage a quality seedling with well-devel-
oped roots, the grower must sow seed into a well-
drained container growing medium (or bareroot
seedbed) with adequate aeration (Landis et al. 1990)
during temperature and moisture conditions suitable
for germination and rapid root elongation. Irriga-
tion is one of the most useful culturing tools in any
nursery and can make all the difference between the
production of high-quality or low-quality plants.
Irrigation based on the plant’s transpirational de-
mands, target water content, and seedling growth
phase is far more effective and efcient than irriga-
tion on a set schedule (Dumroese et al. 2015). The
best irrigation programs always involve watering to
saturation and then allowing a dry down sufcient
to ensure good root aeration. High irrigation levels
tend to result in higher shoot-to-root ratio (Moser et
al. 2014) and proliferation of pathogens and other
pests (Dumroese and Haase 2018). Similarly, exces-
sive fertilizer, especially nitrogen, promotes exces-
sive shoot growth and an unbalanced shoot-to-root
ratio (Landis et al. 1989).
Proper timing of nutrient and water deprivation to
induce budset and hardening correlates with the
push to generate stem diameter and root growth in
the fall before temperatures drop and all growth
ceases. This phase is critical for achieving target
height-to-diameter and shoot-to-root ratios. Quali-
ty container-grown seedlings have root plugs with
good integrity such that the plug is readily extract-
able and stays together during, lifting, handling,
storage, and planting. Root development should
be adequate to ll the plug and hold the growing
medium, but care must be taken to not create a
rootbound condition (South and Mitchell 2006).
After outplanting, rootbound seedlings may have
poor root egress, root deformation, slowed growth,
instability, and/or reduced survival. This issue can
be avoided with careful attention to sow date, con-
tainer size, irrigation, and fertilization.
Root pruning is another tool to manipulate root
architecture and function. For container seed-
ling production, the use of containers with cop-
per-coated walls chemically prunes elongating
roots and increases the proliferation of a brous
root system within the plug (Sword-Sayer et al.
2009, Tsakaldimi and Ganatsas 2006). For bareroot
seedling production, nursery growers prune roots
horizontally (i.e., undercutting or wrenching) or
vertically (sidecutting) (Landis 2008, Riley and
Steinfeld 2005). When applied and timed properly,
bareroot root culturing results in a more compact,
brous root system at the time of lifting for both
conifer (Dierauf et al. 1995) and hardwood (Schultz
and Thompson 1997) seedlings. This practice is also
used to create a mild stress event to control height
growth (Buse and Day 1989), induce bud formation
(van Dorsser and Rook 1972), and mitigate soil
compaction (Miller et al. 1985).
Question 4 – When and How Long
Can Storage Be Used For “Hot-Lifted”
Seedlings?
Hot-lifted seedlings used for summer or fall plant-
ing have usually developed a “hard bud” that will
not break even if the seedlings are exposed to
optimal environmental conditions (MacDonald and
Owens 2006, 2010), although they are still grow-
ing roots (Ritchie and Dunlap 1980, Ritchie and
Figure 4. Good quality seedlings have vigorous roots that egress rapidly after
outplanting, such as the Douglas-r container seedling. (Photo by Diane L.
Haase 2013)
Volume 63, Number 2 (Fall 2020) 119
Tanaka 1990) and developing drought resistance
(Abrams 1988, Teskey et al. 1984) and freezing
tolerance (Weiser 1970). Thus, hot-lift seedlings are
still physiologically active at planting and require
unique handling procedures.
In Western Canada and the United States, hot-lift-
ed seedlings are commonly planted in two distinct
periods: the rst being late June through July (sum-
mer planting), and the second being mid-August
through early October (fall planting). Seedlings for
both summer and fall planting programs are sub-
ject to the same cultural hardening practices at the
nursery (see Question 1) and are in a similar phe-
nological state at the time of planting. Thus, physi-
ological hardiness is similar between summer- and
fall-planted seedlings and any eld performance dif-
ferences are generally associated with environmen-
tal conditions during and after planting (Pikkarainen
et al. 2020).
Handling and storage practices can affect quality
of hot-lifted seedlings (Binder and Fielder 1995,
DeYoe 1986, Landis et al. 2010). In particular, tem-
perature conditions will inuence maintenance res-
piration; each 10 °C (18 °F) increase approximately
doubles the respiration rate (Kramer and Kozlowski
1979). The temperatures inside closed boxes can
quickly increase, causing hot-lift seedlings to use
more of their stored carbohydrates (Landis et al.
2010). Thus, hot-planted seedlings must be kept
cool to maintain their vigor. After harvest, hot-lifted
seedlings should be kept in a nursery cooler and/
or refrigerated trailer at 2 to 10 °C (35 to 50 °F)
prior to shipment, with 2° C (35 °F) being the ideal
short-term storage temperature (Grossnickle per-
sonal communication). Depending on seed source,
species, and nursery hardening regime, seedlings
in both summer and fall planting programs develop
some degree of cold hardiness after budset (Bigras
et al. 2001) and can easily withstand cold storage
temperature conditions.
Properly hardened summer/fall planted seedlings
can be safely cold stored for approximately 4 weeks
prior to outplanting without any chilling require-
ment (Jackson et al. 2012). Cultural practices to
induce hardening (i.e., water stress and low N)
resulted in container-grown loblolly pine seedlings
being able to withstand 4 to 6 weeks of cold storage
without prior chilling hours (Grossnickle and South
2014). While it is possible to safely hold hot-lifted
seedlings for a maximum of 4-weeks, it is contin-
gent on maintaining a 2 °C (35 °F) storage tempera-
ture; safe storage duration decreases with increasing
storage temperature (Paterson et al. 2001).
Shipping hot-lifted seedlings to the outplanting site
occurs in refrigerated trailers with temperatures
below 10 °C (50 °F) (Dunsworth 1997, Stjernberg
1997). Upon arrival at the planting site, seedlings
may be kept in a refrigerated trailer (gure 5) or
transferred to a eld cache in a shady location and/
or under a suspended tarp with boxes opened to
prevent heat buildup (Kiiskila 1999, Landis et al.
2010). During summer and fall months, moisture
stress might occur; therefore, seedlings need to
be monitored and irrigated if required (Landis et
al. 2010). Under these conditions, only enough seed-
lings for 1 day of planting should be transported
to the site. Alternatively, hot-lifted seedlings have
been stored at 4 to 21 °C (40 to 70 °F) in refriger-
ated trailers on the planting site for up to a week
(Dumroese and Barnett 2004), although lower
temperatures between 2 to 4 °C (36 to 39 °F) are
recommended (Landis et al. 2010). The full storage
duration for hot-planted seedlings includes time
spent in the nursery’s cold storage facility and time
spent in storage away from the nursery (e.g., in
refrigerated trucks or other off-site holding areas).
The combined length and care for all of these han-
dling and storage steps is critical to ensure quality
seedlings are outplanted.
Figure 5. Refrigerated trailers are required for transporting large quantities of
hot-lift seedlings from the nursery and are ideal for short-term cool seedling
storage prior to planting. (Photo by Steven B. Kiiskila 2010)
120 Tree Planters’ Notes
Question 5 – How Long Can Seedlings
Be Overwintered in Refrigerated
Storage?
Overwintered spring plant seedlings are harvested
in the fall once dormant and most commonly held in
refrigerated storage until shortly before planting. Re-
frigerated storage is differentiated by temperature into
cooler (1 to 2 °C) or freezer (–2 to –4 °C) storage, with
the storage practice dependent on species’ tolerance to
freezing temperatures, available facilities, and expected
storage duration (Grossnickle and South 2014, Landis
et al. 2010).
Properly hardened and dormant seedlings (see Question
1) can be lifted in late fall and early winter and stored
well into the spring for planting (Camm et al. 1994,
Ritchie 1987). A dark and cold or frozen environment,
however, is an unnatural environment for seedlings.
The lack of light in storage prevents seedlings from
replenishing carbohydrates lost through respiration
(Ritchie 1987) and interrupts the seedling’s circadian
rhythm (Camm et al. 1994, Lavender 1985). Seedlings
lifted and stored correctly are rarely damaged by cold
or frozen storage, though some plant deterioration can
occur as storage time lengthens (McKay 1997). Both
cold and frozen storage conditions, when managed
properly, allow properly hardened seedlings to maintain
their physiological integrity required for good seedling
quality (Landis et al. 2010). This is critical because
quality seedlings typically have vigorous rooting at
planting, which is required to overcome planting stress
(Grossnickle 2005), thus increasing chances for suc-
cessful seedling establishment (Grossnickle 2012).
Cold storage is a cultural practice where seedlings are
held at 1 to 2 °C (35 to 36 °F) for no longer than 2
months (Landis et al. 2010, Ritchie 2004). Increasing
cold storage can result in decreases in days to bud break
(DBB), root growth potential (RGP), freezing tolerance,
and carbohydrates (Grossnickle and South 2014). Ex-
tended exposure to cold temperatures and high humid-
ity during cold storage creates conditions for storage
molds (Camm et al. 1994, Hocking 1971, Landis et al.
2010, Ritchie 2004). Treating seedlings with appropri-
ate fungicides prior to storage can improve seedling
storability (Barnett et al. 1988), though their benecial
effects diminish as cold storage lengths reach 2 or more
months (Grossnickle personal communication). Man-
agers holding seedlings in cold storage should monitor
stored seedlings regularly to detect problems.
Frozen storage is a cultural practice where seedlings
are held at -2 to -4 °C (25 to 28 °F). This below-freez-
ing storage temperature further slows physiological
changes in the seedlings, thereby allowing them to
be stored longer compared with cold storage. Frozen
storage temperatures should not drop below -5 °C (23
°F), however, because some species are susceptible
root damage at lower temperatures (Bigras et al. 2001,
Kooistra 2004). Seedlings harvested at the correct phe-
nological stage, and thus in a state of maximum stress
resistance, are usually freezer stored for 4 to 6 months
(Grossnickle 2000, Kooistra 2004), though seedlings
have been successfully freezer stored for up to 8
months (Helenius et al. 2005, Luoranen et al. 2012).
Once planted, seedlings are in a state of ecodormancy
whereby the chilling requirement has been met and
buds will break after exposure to favorable tempera-
tures and begin the yearly cycle of growth (Burr 1990,
Haase 2011b, Lavender 1985).
Two issues should be considered with regard to
freezer storage effects on seedling quality. First,
seedlings are still physiologically active (albeit at
a low level), which is reected in the decrease in
DBB, RGP, freezing tolerance, and carbohydrates
as the storage duration lengthens (Grossnickle and
South 2014, Landis et al 2010). Second, the low
humidity in freezer storage prevents storage molds
(Haase and Taylor 2012, Hansen 1990, Trotter et al.
1991) but can desiccate seedlings with excessive
storage duration, which may lead to reduced root
growth potential (Deans et al. 1990). Packaging
frozen-stored seedlings in a plastic bag or a po-
ly-lined paper bag inside a waxed box minimizes
seedling desiccation (Kooistra 2004), although
seedlings may still lose up to 10 percent water
content after 5 or more months in frozen storage
(Lefevre et al. 1991).
The thawing process prior to planting should also
be considered in conjunction with frozen storage
duration (gure 6). While frozen seedlings were
originally thawed slowly over a period of weeks, it
has been shown that slow thawing causes seedling
quality to decrease with increasing thawing duration
(Silim and Guy 1997), and rapid thawing within a
matter of days maintains seedling quality (Rose and
Haase 1997). Thus, thawing seedlings as quickly as
possible is now recommended (Landis et al. 2010).
Rapid thawing is also preferred because it prevents
Volume 63, Number 2 (Fall 2020) 121
storage molds (Rose and Haase 1997), minimizes
depletion of carbohydrates (Silim and Guy 1997),
and results in seedlings having later bud break and
greater frost hardiness at time of planting (Camm
et al. 1995). Seedlings can also be planted frozen
without thawing, but must be individually wrapped
at harvest such that seedling plugs can be separated
from one another while frozen (gure 7). Eliminat-
ing the thawing process requires more effort in the
nursery at lifting but offers operational exibility
during the busy planting window. Research to date
has shown no deleterious physiological effects
of planting frozen seedlings (Camm et al. 1995,
Kooistra and Baaker 2005), although limiting site
environmental conditions at the time of planting
such as dry cold soils can have a negative effect
(Helenius 2005).
Long-term frozen storage for “late” spring planting
may result in seedlings being initially out of sync
with the annual growth rhythms of the planting
site. That is, planted seedlings may have budbreak
patterns that do not reect that of natural seedlings
on the planting site (Grossnickle 2000). Seedlings
require sufcient time to complete the growth
processes initiated with budush and begin devel-
opment of hardiness before the onset of fall frosts.
The potential risk of fall frost damage to seedlings
at different planting dates can be estimated through
analysis of long-term climatic data (Hänninen et al.
2009). Delaying spring planting into early summer
increases the likelihood of bud break and shoot
elongation when the site environment has warm
temperatures, high vapor pressure decits, and dry
soils (Grossnickle 2005, Mitchell et al. 1990). As
such, the site may not be suitable for planting until
late summer/fall and may be more appropriate for
planting with hot-lifted seedlings (see Question 4).
Conclusions
Seedlings are not widgets; they are biological organ-
isms that respond to their surrounding environment.
Nursery cultural practices have a direct inuence
on the seedling environment, thereby inuencing
seedlings’ physiological function and subsequent
morphological development. This discussion shows
that all nursery cultural decisions, from hardening
practices, to strategies that promote vigorous root
growth, to storage practices affect seedling develop-
ment. Understanding how cultural practices affect
seedling performance will ensure that the nursery
practitioner develops sound practices that enhance
seedling quality and subsequent success of forest
restoration programs.
Address correspondence to—
Steve Grossnickle, Nursery
To
Forest Solutions,
1325 Readings Drive, North Saanich, BC, Canada,
V8L 5K7; email: sgrossnickle@shaw.ca; phone:
250-655-9155.
Figure 6. Rapid thawing of frozen seedlings in closed boxes can be done by
spacing stacked boxes in a warm location without direct sunlight and rotating
the boxes top to bottom. (Photo by Steven B. Kiiskila 2009)
Figure 7. One method to enable separation of frozen seedlings from one
another without damaging their roots is to place poly wrap around each seed-
ling’s root plug when bundled after lifting. (Photo by Steven B. Kiiskila 2010)
122 Tree Planters’ Notes
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