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University of Florida Central Science
Institute of Food & Agricultural Science library
CENTRAL FLORIDA RESEARCH & EDUCATION CENTER
Sanford, Florida JAN 1990
Research Report SAN-91-04 ill4~trb$Op0O
POST-DIGGING ACCLIMATIZATION OF TREES PRODUCED IN FABRIC CONTAINERS
R. C. Beeson, Jr.1 & E. F. Gilman2
Since the 1985 freeze, a substantial increase in the number of farms
producing large ornamental trees has occurred. The majority of these new
farms replaced old citrus orchards. Because of the deep sand characteristics
and available information, many of the trees planted on these new farms were
in fabric containers. One of the major drawbacks to the fabric container
technology has been often severe losses (10 to 50%) of Quercus virginiana
(live oak) during the post-digging hardening-off period. While for most other
species, near perfect survival rates _are standard, nearly all farms have
difficulty with this species. Most farms use some method of overhead misting
in hardening-off their trees. Yet, some farms seem able to forego this and
still maintain reasonable success rates; especially for species other than
The results presented below are from experiments designed to understand
the physical and physiological factors that affect survival during hardening-
off. These experiments were conducted with live oak, due to its almost
universal reputation as the most difficult to successfully transplant. There
were 3 objectives of these experiments. First, we wanted to relate the degree
of water stress during hardening-off to survival and to the root system
contained inside the bag. Second, we tried to develop a procedure that would
insure tree survival while minimizing the amount of overhead misting required.
And third, we evaluated pre-digging root pruning as a method to improve post-
MATERIALS AND METHODS
Three and 4-in caliper live oaks grown in 18 or 24-in fabric containers,
respectively, were dug in late afternoon and maintained under overhead misting
throughout the night. Early the following morning they were transported to
the research station in Sanford in open trucks covered with shade cloth. It
took 3 hours to load and transport the trees. Following unloading, trees
were immediately placed under constant overhead mist until 8 P.M. and supplied
with irrigation to the football using micro emitters (Robert's emitter -
green; 15 min on/30 min off). All roots were removed from the exterior of the
'Ornamental Horticulturist, University of Florida, IFAS, Central Florida
Research and Education Center, Sanford, FL 32771.
2Ornamental Horticulturist, University of Florida, IFAS, Department of
Environmental Horticulture, Gainesville, FL 32611.
bags, but the bags were not removed from the rootballs. Treatments were
initiated the following day with 8 trees per treatment.
In Experiment 1 (Exp. 1), trees produced in 24-in fabric containers and
transported to Sanford on May 23, 1990, were divided into 3 treatments.
Treatments consisted of: 1) trees initially receiving continuous overhead
misting during the day (continuous misting), 2) trees initially receiving
overhead mist 15 min on/15 min off during the day (intermittent misting), and
3) trees receiving football irrigation only footballl only). All trees
received football irrigation 15 min on/15 min off from 0630 until 2000 with an
additional 15 min at 2200. The overhead misting was reduced gradually over a
3 week period (Table 1). During this time, tree water stress was measured
periodically every 2 hr from dawn to dusk for each tree. Once the overhead
misting was stopped, 3 dead or dying and 4 randomly selected healthy trees
were scarified and roots inside the fabric container sorted and quantified.
In Experiment 2 (Exp. 2), eight 3 to 3.5-in caliper trees in 18-in
fabric containers were root pruned inside the bag in early April. Root
pruning consisted of driving a 6-in wide spade through the bottom of the bag
in 3 places evenly spaced around the inside perimeter of the fabric container.
Approximately 35% of the circumference was pruned. In late June, these along
with 8 control trees were dug and transported as before. All trees were
placed under overhead misting which was gradually reduced to zero over a 3-wk
period (Table 1). After hardening-off, the roots were sorted and quantified
on 3 surviving root-pruned trees and unpruned control trees and 3 dead trees.
Experiment 1 Overhead misting treatments
Surviving trees receiving only football irrigation rapidly lost water,
becoming highly water stressed often by 1100 the first week of hardening-off.
Water stress increased rapidly on these trees, resulting in stomatal closure
most of the day. This suggests that little photosynthesis occurred in these
trees throughout most of the hardening-off period. Photosynthesis is required
for maximum root regeneration. Photosynthesis was further reduced by leaf
drop which began within 3 days after treatments were started.
Water stress of surviving trees given overhead misting, whether
initially continuous or intermittent, remained low to moderate until overhead
misting was reduced to 15 min on/45 min off. When overhead misting was
reduced to 15 min on/30 min off, the degree of water stress increased
significantly over that measured on Day 1. Yet, the resulting stress was not
enough to affect photosynthesis or shoot growth. This was true for both the
continuous and intermittent treatments. At this time water stress remained
well below the point of stomata closure, suggesting trees were
photosynthesizing at maximum rates. Little leaf drop was evident during this
Once the overhead misting was reduced to 15 min on/45 min off (13 days
after digging; Table 1), water stress levels were comparable to those
receiving irrigation on the football only. This was exhibited as an increase
in leaf drop. However, once overhead misting was stopped (Day 21), sufficient
leaves had fallen from surviving trees so that water stress was reduced to
moderate levels. The degree of stress-suggests that moderate rates of
photosynthesis may have been maintained throughout the day.
Survival rates were similar among treatments. Only one tree receiving
the rootball-only irrigation died. Two trees initially given the intermittent
misting treatment suffered severe water stress and leaf drop, and were for our
purposes considered dying at the end of the experiment (3 weeks). Differences
in the degree of water stress between these two and the surviving trees in
this treatment were not apparent until several days after overhead misting had
been reduced to 15 min on/ 30 min off.
Experiment 2 Effect of root pruning
The trees used in Exp. 2 far exceeded the recommended caliper for
production in 18-in bags. Earlier, root pruning inside the bag in October was
found to increase survival and lower post-digging water stress for oversized
live oaks grown in 24-in bags (Beeson, unpublished data). In this study, all
8 of the root pruned trees survived while only 4 of the 8 control trees were
alive after hardening-off. Surviving trees did not undergo high water stress
until overhead misting was reduced to 15 min on/45 min off. Again stress
levels reached those associated with stomatal closure. Stress levels of all
surviving trees were similar and independent of root pruning. However, that
appears to be a trend for lower water stress during mid-afternoon from trees
receiving the root pruning.
Severe water stress and death were directly associated with less fine
roots in the football. The number and size of large roots penetrating the
fabric container could not be used as a predictor of tree survival and was
only loosely associated with water stress. Differences were not significant
between the weight of fine roots in root-pruned and surviving unpruned control
trees. However, all dead trees were not root pruned and had significantly
less fine root mass.
From these experiments, we conclude several things about hardening-off
oaks grown in fabric containers. If the root system has many fine roots
inside the bag, overhead misting during hardening off does not appear
necessary for tree survival, if sufficient water is applied frequently to the
football. However, the use of overhead misting when root systems are weak
seems to make the difference between survival and death. Overhead misting
permits time for the tree to gradually shed leaves in excess of the root
system capacity and appears to permit time for fine root elongation in the bag
after digging. If the root system inside the bag is poor (i.e. few fine
roots), only extended overhead misting (possibly 5 to 7 weeks) may prevent
tree death. Unfortunately, one cannot reliably predict the quality of the
root system at digging. Relative root system quality may be determined non-
destructively only after hardening-off is completed by the relative amount of
leaves remaining on the tree.
Continuous overhead misting appears to be unnecessary. Trees remained
just as healthy and survival was no different from those receiving the
intermittent overhead misting treatment. These experiments hint at the
possibility of further reductions in initial overhead misting, to 15 min on/30
min off, without imposing undo stress on the trees, so long as sufficient
football irrigation is maintained. It may also be possible to decrease the 3-
wk hardening-off period to 2 weeks by more severe reductions in overhead
misting after the first week.
The benefits of root pruning in preparation for digging overgrown live
oaks in fabric containers are clear. Root pruning appears to decrease the
water stress experienced by the trees during hardening-off and was found to
improve survival rates. When compared to surviving unpruned trees, root
pruning was not found to statistically increase the amount of fine root mass.
However, when compared to all unpruned trees (dead and surviving), root
pruning significantly improved the fine root mass. Root pruning more than 40
to 50% of the circumference of the fabric container is not recommended in that
excessive root pruning would be equivalent to digging the tree without
providing the extra irrigation required. Also, excessively root pruned trees
can blow over with high winds.
Table 1. Duration and cycling of overhead misting following digging of
live oaks. Schedule for overhead misting from Experiments 1 and 2. Changes
in the time of day or number of minutes of irrigation are given at the date at
which they were changed. For example in Exp. 1, ten days after digging
overhead misting of the continuous misting treatment was reduced from 15 min
on/15 min off to 15 min on/30 min off. The number of hours the overhead was
used was also reduced from 8 A.M. until 8 P.M. down to 8 A.M. until 6 P.M.
Experiment 1 Experiment 2
Days after Overhead misting Rootball Overhead Rootball
digging Continuous Intermittent irrigation misting irrigation
1 0700-2000Y 0700-2000 0630-2100 0630-2100 0630-2030
2 60/OX 15/15 30/30 15/15 15/45
4 0700-2000 0700-2000 0730-1930
5 45/15 15/15 15/30
7 0800-2000 0800-2000 0815-1815
8 15/15 15/30 15/30
10 0800-1800 0800-1800 0830-1830
11 15/30 15/30 15/45
13 0800-1800 0800-1800 1030-1730 0730-1930
14 14/45 15/45 15/45 15/45
16 1030-1630 1030-1630 0730-2030 1230-1530
17 15/45 15/45 15/45 15/45
19 1230-1630 1230-1630 0830-2030 Overhead misting
20 15/45 15/45 15/45 terminated
22 Overhead misting terminated
ZWater supplied to the rootballs only using Robert's emitters. Rootball
irrigation as described was supplied to both overhead mist treatments and as
a separate treatment.
YDuration of irrigation system was operating based on a 24-hr clock.
XNumber of minutes on/number of minutes off.