Production Systems for Small Trees and Shrubs in New Hampshire
- Introduction
- Materials and Methods
- Results
- Conclusions and Discussion
- Photo Gallery
- References Cited
- Acknowledgements
- Author
Container production of trees and shrubs has shown a rapid increase
in the past two decades and has surpassed field production in many parts
of the country. Advantages of container production include more rapid
growth, more efficient use of land and labor, more control of the growing
environment, and extended planting and harvesting seasons. Plants are
easier to handle and ship, more attractive at retail outlets, and easy
to transplant. The entire root system is transplanted, unlike field dug
material where less than 10 percent of the root volume is included in
the harvested root ball (Gilman, 1988; Watson and Himelick, 1982).
Inputs for container production, however, are greater than for field
production. Pots, growing media, and the necessity for increased precision
in irrigation and fertilization practices all increase production costs
over field grown material. Other disadvantages of container growing include
the need for winter protection, the potential for root loss from high
summer temperatures, and frequent wind throw of plants in containers.
Formation of root defects in containers can contribute to tree decline
in the landscape (Watson and Himelick, 1997).
Pot-in-pot production systems were developed in the South for preventing
wind throw and moderating excessive summer media temperatures. Research
has shown biomass increases of 20 percent for above-ground portions and
50 percent increases in root mass of plants grown in this system, compared
to traditional container production (London et. al., 1998; Ruter 1998a,
1998b). Modifications of the pot-in-pot system include bag-in-pot and
the Above Ground System™ (AGS).
Little research has been done to test the suitability of pot-in-pot
production for northern climates, although several nurseries have installed
pot-in-pot production areas and grown crops successfully. The objectives
of this research were to compare growth of two woody species in field,
container, pot-in-pot, bag-in-pot and the AGS and monitor root zone temperatures
to answer the following questions:
- Is growth enhanced in pot-in-pot or other modified container systems?
- Are summer temperatures in northern New England a cause of root mortality, and do pot-in-pot or modified container systems alleviate this problem?
- How do winter root zone temperatures in pot-in-pot systems correlate with soil temperatures, and can plants be over-wintered in place?
Two-foot whips of Malus x ‘Donald Wyman’ were
planted on June 5, 2001, in a randomized complete block experiment with
ten replications of five treatments as described below. Lilacs (Syringa
vulgaris ‘Monge’) were planted in an adjacent production
area on July 12 from #1 (2.8 L) containers.
Pots for the container, pot-in-pot, bag-in-pot and AGS systems were
all #7 (24.6 L) containers (Nursery Supplies, Inc. Hagerstown , MD).
The interior surface of containers for the pot-in-pot and AGS liner pots,
and the container treatment, were all painted with latex paint containing
copper hydroxide (Spinout®; Griffin Chemical Corp., Valdosta, GA
) to prevent root circling. The growing medium was a commercial pine
bark and peat nursery mix (Fafard Inc., Agawam, MA ). All plants were
irrigated as needed with microsprayers (Netafim® 3 gph, Netafim Irrigation,
Inc., Altamonte Springs, FL 32714 ) and fertilized with Nutricote® (Chisso-Asahi
Fert. Co. Ltd., Tokyo, Japan ) Type 100 18-6-12 with minors (85 g/plant)
after planting. In the second season, Nutricote® Type 40 16-6-12
(20 g/plant) was applied in early May, followed by Nutricote® Type
100 18-6-12 (85 g/plant) in June.
Growth measurements (height and caliper) for Malus were taken
in July and November of each growing season. The annual growth of the
central leader and total length of each lateral branch was also measured
at the end of each season. In the fall of 2002, after two complete growing
seasons, the crabapple trees were harvested. Rootballs from five plants
of each treatment were washed with a high-pressure hose. Plants were
cut up and the root and shoot portions were bagged separately, dried
and weighed.
Lilacs, however, were not judged to be of sufficient size afer two growing
seasons and so were overwintered and maintained in the treatments for
a third season. Harvest data for lilacs has not yet been completed.
Data recorders (Hobo® H8 Outdoor/Industrial logger; Onset Computer,
Pocasset , MA ) were installed to record air, soil, and media temperatures
every 15-30 minutes, year round, in one representative pot per treatment
for each species. Temperature sensors were placed in the southwest pot
quadrant, 4" deep and 1" inside the pot wall.
Field: Whips were transplanted directly into a well-drained sandy loam
soil which had been cover-cropped the previous season. Harvesting of
field-grown trees was accomplished with a manual tree digger (Tree Toad® Tree
Transplanters, Long Lake, MN ) to obtain a consistent 24" root ball
on each tree.
Container: the container treatment was a typical nursery production
system with containers set on the surface of the ground. After experiencing
windthrow problems during the second growing season, each container was
staked with a single reebar rod to prevent further blow over. Container
plants were overwintered between October 18, 2001 and May 1, 2002 in
an unheated poly house covered with white plastic.
Pot-in-pot (PiP): socket pots were installed in the field by hand-digging
holes, setting the pots, and backfilling soil around the pot to within
three inches of the top. The liner pots containing the plants were inserted
into the socket pots. Squares of copper-treated geotextile fabric (Tex-R® Insert,
Texel Inc., Quebec, Canada ) were placed between the inner and outer
pots to prevent rooting out through the drainage holes. Plants were overwintered
in place.
Bag-in-pot (BiP): plants were planted into copper-treated, rounded
geotextile bags (Tex-R® Agroliner™, Texel Inc., Quebec, Canada)
and placed into containers. They were overwintered in an unheated poly
house covered with white plastic.
Above Ground System™ (AGS) (Nursery Supplies Inc., Chambersburg
PA): This system consists of a tapered socket pot with a broad base for
stability, into which the equivalent size container plant is inserted.
Air space between the inner and outer pots provided some shade and possible
insulation. The entire double container was overwintered in an unheated
poly house covered with white plastic.
Growth Response
During the first growing season, all the trees increased rapidly in height,
caliper (Fig. 1) and total shoot growth
(Fig. 2) except trees in
the field grown treatment, which were significantly slower in growth.
Differences between other treatments were not significant, although
the trend was toward most rapid growth initially in the pot-in-pot treatment.
During the second growing season, the field grown trees put on very
rapid growth and caught up with the other treatments so that by mid-season
there were no longer any significant differences in size or caliper due
to treatment.
Shoot, fruit, and total plant dry weights were not significantly different
at the end of the trial. Dry root weights, however, were greatest for
pot-in-pot and AGS grown trees, and these treatments were significantly
greater than the harvested root weight of field grown trees (Fig.
3).
Root to shoot ratio also was significantly less for field grown trees
than for container or pot-in-pot trees.
Data discussed here are for the Malus trial only, since the Syringa trial
has not yet been completed. No visual differences in growth of Syringa were
observed until May, 2003 when five of the ten pot-in-pot replicates budded
out slowly and displayed very weak vegetative growth and virtually no
flowers. Four of the five subsequently died. Likewise, the field grown
lilacs aborted most of their flower buds, presumably due to cold, but
vegetative growth was strong and no plants died. All of the plants overwintered
in the poly house survived, flowered, and put on strong spring growth.
Temperatures in unprotected containers exceeded 100º F on over
45 days in 2001 and 65 days in 2002. The highest air temperature was
102º F, recorded on July 3, 2002 . Media temperatures reached a
high of 118º in containers, 111º in BiP, 99 in PiP, and 90º in
the soil (Fig. 4; complete AGS data
not available.) Visual observation of the root systems confirmed that
roots were killed due to lethal temperatures on the west and southwest
sides of the containers (Photo 2). Very
little insulation was provided by the bag-in-pot treatment; the AGS did
provide several degrees of temperature moderation on hot days and therefore
there was less root mortality.
The copper hydroxide paint used on all containers failed to prevent
extensive root circling of Malus; however, the agrotextile
bag was very effective in this regard. Root balls from the BiP system
were smaller than the other containers due to the slightly smaller soil
volumes within the agrotextile bag.
The winter of 2001-02 was relatively mild but root systems had little
protective snow cover. Minimum and mean media temperatures in the pot-in-pot
treatments closely reflected soil temperatures (Figure
5 and Figure 6). The minimum temperature
recorded in both treatments was 20º F. Media temperatures in containers,
BiP and AGS treatments inside the poly house often fluctuated by 30º F
in a day during the late winter months, but the temperatures were very
similar in all three treatments. The minimum temperatures recorded for
these three treatments were 21º, 22º, and 25º F for container,
BiP, and AGS respectively.
Is growth enhanced in pot-in-pot or other modified container systems?
Growth of Malus and Syringa showed no distinct advantage
associated with any production system in comparison to the others, except
that field planted trees/shrubs had a reduced growth rate during the
first season. This disadvantage, however, was compensated for by a faster
growth rate during the second season. The enhancement of growth reported
in the South for pot-in-pot grown trees and shrubs was not observed in
this northern New England trial.
Are summer temperatures in northern New England a cause of root
mortality, and do pot-in-pot or modified container systems alleviate
this problem?
It is clear that summer media temperatures in containers frequently
reach the lethal range of 100-120º F and do cause root death in
the southwest pot quadrant. The effects of high temperatures along the
pot wall can be mitigated slightly by use of copper hydroxide which promotes
root development within the root ball and reduces the root mass at the
circumference, where temperatures are highest. However, in this trial
copper hydroxide was not very effective at reducing root circling of
the aggressive Malus root stock in above ground containers.
The pot-in-pot system, as expected, was buffered by the surrounding
soil to prevent media temperature from reaching lethal levels. Maximum
PiP temps in summer, however, were several degrees higher than soil temperatures
at the same depth (Fig. 4). The AGS
system experienced high temperature spikes during the summer, although
maximum temperatures were on the order of 10º-12º F cooler
than containers. The BiP system only provided 5º
-10º F of moderation. The supra optimal temperatures in containers
were reached earlier in the day, had higher peaks, and lasted for longer
durations than any other treatments. These results and temperature ranges
are not unlike those reported by London et. al. in South Carolina or
Ruter (1993) in Georgia .
How do winter root zone temperatures in pot-in-pot systems correlate
with soil temperatures, and can plants be over-wintered in place?
Root zone temperatures of pot-in-pot plants during the winter appear
to be very similar to those experienced by field-grown plants, due to
the buffering capacity of the surrounding soil. Therefore there is no
need to provide additional winter protection to pot-in-pot plants, as
long as they are cold hardy for the local climate.
The AGS provides a moderate degree of temperature modification in summer,
but is not adequate for protecting the root zone from freeze damage;
therefore, the plants must be overwintered in a protected environment
like other container grown plants. The bag-in-pot system, used here in
an above-ground container, was effective in preventing root circling
but offers only minimal temperature buffering. Planting in the geotextile
bag then placing the bag in a socket pot below ground, however, could
combine the benefits of BiP and PiP.
Although the growth enhancement associated with pot-in-pot systems
in other climates was not observed here, pot-in-pot can still be a useful
production system for northern nurseries. The benefits of root zone temperature
moderation, although not reflected in significantly greater root dry
weights in the PiP and AGS systems, could confer an advantage when transplanted
to the landscape. The labor savings associated with not having to stack
and cover large container material for overwintering is definitely a
plus. Likewise, the pot-in-pot system prevents wind throw and associated
potential mechanical damage and labor costs for uprighting the plants
on a frequent basis. Compared to field production, the ease of harvesting
and handling the plants and having fresh landscape material available
all season is advantageous; however, some growers and landscapers prefer
field grown material over that produced in any container system.
Photo gallery of crabapples in
production systems trials.
Gilman, E.F. 1988. Tree root spread in relation to branch dripline and
harvestable rootball. HortScience 23(2):351-353.
London , J., R.T. Fernandez, R.E. Young and G.D. Christenbury. 1998.
Comparing above-ground and in-ground pot-in-pot container systems. SNA
Res. Conf. 43:71-75.
Ruter, J.M. 1993. Growth and landscape performance of three landscape
plants produced in conventional and pot-in-pot production systems. J.
Environ. Hort. 11(3):124-27.
Ruter, J.M. 1998a. Fertilizer rate and pot-in-pot production increase
growth of Heritage river birch. J. Environ. Hort. 16(3):135-138.
Ruter, J.M. 1998b. Pot in pot production and cyclic irrigation influence
growth and irrigation efficiency of ‘Okame’ cherries. J.
Environ. Hort. 16(3):159-62.
Watson, G.W. and E.B. Himelick. 1982. Root distribution of nursery trees
and its relationship to transplanting success. J. Arboric. 8(9):225-229.
Watson, G.W. and E.B. Himelick. 1997. Principles and Practice of Planting
Trees and Shrubs. Intl. Soc. Arboric. , Savoy IL.
Funding was provided by the New Hampshire Horticulture Endowment, New England Grows, UNH Agricultural Experiment Station, and UNH Cooperative Extension. Geotextile products were donated by Texel, Inc., Quebec, Canada.
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Catherine Neal, Extension Professor and Ornamentals Specialist
UNH Cooperative Extension
113 Spaulding Hall, 38 College Rd., Durham NH 03824
Tel. 603-862-3208
Email Cathy.neal@unh.edu
January, 2004
