ORNAMENTALS
NORTHWEST
ARCHIVES
Jan.-Feb.-Mar
1982
Vol.6, Issue 1
Pages 10-12
James L. Green (Extension Ornamentals
Specialist) and C.J. Weiser (Head,
Horticulture Department), Oregon State
University. Corvallis. Oregon 97331.
TIMING OF FALL DIGGING OF BAREROOT DECIDUOUS TREES - Where are we in
Solving the Problems?
Why Dig Early?
Digging before the arrival of winter rains reduces costs. Efficiency of field digging operations is
as much as fifteen times greater before the rains set in (Hotze, 1976).
Nursery trees and shrubs grow exceptionally well in the Pacific Northwest as a result of mild
climate, long growing season, and ample moisture. Some species of deciduous trees can easily
produce six feet of new growth in a single season (Nissila, 1977). This is a mixed blessing for
nurserymen because some species keep growing late into the autumn, are slow to mature
vegetatively, and are slow to drop their leaves. This causes costly delays in digging the crop after
winter rains begin, or equally serious dieback and death overwinter if they are dug early in the
fall prior to attaining vegetative maturity.
What Is "Vegetative Maturity"?
Vegetative maturity is a distinct stage of plant development that coincides with the onset of
winter dormancy (Nissila, 1977). This stage can be detected experimentally by removing the
leaves from a plant and observing how its buds respond. If the leaves are artificially removed
from an immature plant, the dormant buds in the axils of leaves rapidly resume growth -within a
few days. If leaves are removed from a mature plant, its axillary dormant buds remain dormant.
Vegetative maturity and natural defoliation do not necessarily coincide. Vegetative maturity may
proceed defoliation by several weeks in some species, but in other species vegetative maturity
may occur after the onset of defoliation.
Problems with Digging Trees Prior to Vegetative Maturity:
-Immature plants dug too early can suffer branch dieback and death during storage overwinter.
-Digging trees that are still in leaf is impractical because leaf transpiration might result in water
stress.
-Disease and insect problems may be enhanced in bulk storage of leafy plants.
-Intact leaves cause increased weight and bulk during handling.
Since a plant that is vegetatively mature is visually indistinguishable from one that is not, timing
of cultural practices would have to be based on the use of some physiological index of maturity.
A SIMPLE INDICATOR OF VEGETATIVE MATURITY (indicator of onset of winter
dormancy) WOULD HELP NURSERYMEN TO MORE CONFIDENTLY TIME THEIR
CULTURAL PRACTICES INVOLVING DEFOLIATION, HARVEST AND STORAGE.
Is There A Reliable, Rapidly Measurable Indicator of Vegetative Maturity?
Dr. Les Fuchigami and his graduate students have tested several maturity measuring tests, but
have yet to find a suitable technique.
Results of research attempts to identify reliable indicators of vegetative maturity that could be
rapidly measured are summarized from research reports:
-Xylem Water Potential: "...variability within samples was so large as to preclude the use of
xylem water potential as a reliable index of vegetative maturity (Nissila and Fuchigami, 1978).
-Measurements of Electrical Resistance: "Unfortunately, the correlations with accepted
measures of dormancy have never proven strong enough to provide a rapid and satisfactory
prediction of dormancy (Timmis, etal., 1981).
-Measurement of Electrical Square Waves: "Square waves might be expected to provide a
more sensitive indication of dormancy (than does electrical resistance) ... no such benefit is
apparent... the square wave predicted neither the onset of dormancy nor its release.(Timmis, etal.,
1981).
The inability to date to identify reliable, rapidly measurable indicators of vegetative
maturity means the nurseryman does not yet have a simple predictor of dormancy that is
needed to establish earliest possible time that plants can be dug and defoliated (chemically
or manually).
To summarize: A few days one way or another in digging time can make a big difference in
harvest cost and plant quality or survival - and the maturity status of the plant seems to be the
key factor in determining how plants will respond. Use of vegetative maturity to time cultural
procedure will undoubtedly be a powerful management tool for nurserymen in the future, but its
present application is complicated by two factors:
1. Plants, even of a given cultivar, do not mature at the same time each year. Year-to-year
differences in temperature and other environmental and cultural factors often change the
timing of maturity development by a week or more. You cannot tell by looking at a plant
whether it is mature.
2. The first problem could be overcome if there was a quick, simple method to assess
maturity. But, as of now there is no simple and reliable method for nurserymen to rapidly
assess the maturity status of their crops. Removing leaves from a plant and observing bud
response after several days works, but it is labor intensive and too slow in providing an
answer to be commercially useful.
-Can the Timing of Maturity be Regulated?
In their annual life cycle plants must progress through a series of developmental stages, one of
which is vegetative maturity which occurs prior to the onset of winter dormancy and prior to the
onset of cold acclimation. Can development of vegetative maturity be accelerated in the field to
facilitate safe early digging? OR, can plant development be managed in post-production
environments (i.e. storage) to ensure that plants dug prior to achievement of vegetative maturity
progress through development of vegetative maturity and sequential developmental stages prior
to shipment to markets?
-Can chemical defoliation solve the problem? Unfortunately, chemical defoliation does not
hasten vegetative maturity development in plants, and immature plants are often susceptible to
damage by chemical defoliant treatments. Mature plants are much less susceptible to defoliants
or other chemical or environmental stresses such as herbicides, water stress, low temperature,
etc. This means that chemical defoliants are useful and effective for removing leaves from mature plants -but not from immature plants.
Premature manual or chemical defoliation can be damaging. Plants must be vegetatively mature
prior to defoliation or stem tip dieback, overwintering damage, and/or poor performance the
subsequent year may result (Hotze, 1976). Many chemical defoliants have been tried, but the
amount of defoliation and injury vary with the time of application, variety, location and
concentration-these procedures have not worked well (Nissila, 1977). Deciduous plants
defoliated too early, before vegetative maturity, develop stem dieback. "Vegetative maturity"
coincides with the onset of winter dormancy (Nissila, 1977).
Steavenson pointed out the problems associated with digging and bareroot handling of deciduous
trees at Forrest Keeling Nursery in Missouri (Steavenson, 1982). "Although a number of
defoliants have been placed on the market and various leaf-stripping devices have been
introduced, we have no interest in them. We want our plants to become dormant naturally before
harvesting. The period between thorough dormancy and ground freeze-up is indeed limited. We
have to go flat out to complete our fall harvest in this brief period."
Again, the need for a safe reliable indicator of vegetative maturity so plants could be dug at the
earliest possible date, OR programmed postproduction environments to safely develop vegetative
maturity and defoliation of the early-dug leafy tree is obvious.
-Various cultural practices may hasten vegetative maturity, -possibly at the expense of
growth and/or general plant quality?
1) Withholding water and/or undercutting to promote defoliation in advance of harvest
has been used by evergreen nurseries and rose growers, but it has not been used widely
by growers of deciduous trees. (Nissila, 1977). Recall that defoliation and vegetative
maturity do not necessarily coincide.
2) "Where early fall digging is important, low nitrogen fertility could prove beneficial in
the year of digging" (Gilliam, etal, 1980). They also reported that maples receiving 150
ppm and 300 ppm nitrogen as ammonium nitrate produced a 2nd flush of new growth
(primarily on lateral branches)while those receiving 50 ppm N did not. Plants grown at
300 ppm N retained their leaves about 3 weeks longer than plants grown at 50 ppm.
Again, recall that defoliation and vegetative maturity do not necessarily coincide.
3) Vegetative maturity coincides with the onset of winter dormancy and the onset of cold
acclimation. Raker and Dirr (1979) reported that ammonium nitrogen fertilization,
rather than the rate of nitrogen application, may delay cold acclimation. Three
container-grown ornamentals (viburnum, cotoneaster, forsythia) had more freezing injury
when grown with ammoniacal sources of nitrogen than when grown with nitrate sources.
-Can Vegetative Maturity Date Be Predicted with Computer Modeling and Simulation?
(See accompanying article by Kent Kobayashi, COMPUTER MODELING AND SIMULATION
OF PLANT DEVELOPMENT). -Can Trees Attain Vegetative Maturity in Programmed,
Controlled, PostProduction Environments? (See accompanying article by Jim Green,
PROGRAMMED PLANT DEVELOPMENT).
Literature Cited:
Gilliam, C.H., S.M. Still, S. Moor, and M.E. Watson. 1980. Effects of three nitrogen levels on
container-grown Acer rubrum. HortScience 15(5):641642.
Hotze, M.L. 1976. Indicators of deciduous shade tree development useful for timing chemical
defoliation and harvest. Ph.D. thesis, Oregon State University, Corvallis, Oregon.
Nissila, Peter Carl. 1977. Hardinessmaturity relations in red-osier dogwood (Cornus stolonifera
Michx). M.S. thesis, Oregon State University, Corvallis, Oregon.
Nissila, P.C. and L.H. Fuchigami. 1978. Xylem water potential and electrical impedance ratios as
measures of vegetative maturity in redosier dogwood (Cornus stolonifera Michx.). J. Amer. Soc.
Hort. Sci. 103(6):708-709.
Raker, Rodney J. and Michael A. Dirr.1979. Effect of nitrogen form and rate on appearance and
cold acclimation of three container-grown woody ornamentals. Scientia Horticulturae 10(3):231236.
Steavenson, Hugh. 1982. Overwintering, bare-root and balled dormant plants. American
Nurseryman, January 1, 1982, pp 53 -.
Timmis, K.A., L.H. Fuchigami, and R. Timmis. 1981. Measuring dormancy: the rise and fall of
square waves. HortScience 16(2):200-202.
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