Tree Services of Omaha - Omaha, Nebraska
Tree Growth Regulators
by David Steg on 10/17/14
Trees and shrubs often grow too large for the
available space in urban areas. In the past, costly
mechanical trimming was the sole method available to
arborists and utility foresters to reduce tree and shrub
size. Consequently, chemical growth retardants were
developed as an inexpensive approach to limit size
and the growth rate of trees and, at the same time, to
enhance their tolerance to the harsh environmental
conditions of urban areas.
History of Tree Growth Retardants (TGRs)
Utility arborists were the first among those caring
for trees to peer over the fence at agricultural and
horticultural fields and ponder the potential of
growth regulators used in those cropping systems as
a tool for tree maintenance. Mechanical trimming,
which was the sole means to combat the unrelenting
growth of trees into overhead electrical wires, was a
costly operation and a chemical alternative was very
attractive. Hence, the electric utility industry provided
funding in the late 1950s for research on chemical
control of tree growth following trimming for electric
line clearance. Results of that early research led to the
use of napthaleneacetic acid (NAA), a synthetic auxin,
painted onto the surface of pruning wounds. Although
effective in reducing the regrowth of branches, coating
each cut surface high in the crown of trees took a
lot of time and was not cost effective. Hence, in the
1970s new TGRs and more economical application
techniques were sought.
The first major breakthrough in the commercial
feasibility of TGRs on a large scale was the
formulation in the late 1970s of the cell elongation
inhibitors, paclobutrazol, uniconazole, and
flurprimidol for trunk injection. Due to their low water
solubility, it was considered necessary to dissolve
the new generation of growth retardants in either
methyl or isopropyl alcohol. The active ingredients
of these formulations were unquestionably effective
in reducing tree growth. After several years of use
throughout the United States in the 1980s, problems
associated with trunk injection begin to appear. Cracks
in the bark and cambium, weeping from injection
holes, and internal wood discoloration due to the
alcohol carriers led to disenchanted utility arborists
and their customers. A decline in use of TGRs
followed. Uniconazole was even removed from the
tree care market. However, in spite of these problems,
interest among utility arborists continued in a chemical
tool to reduce trimming frequency and the amount of
wood waste removed from trees.
Flurprimidol, sold as Cutless Tree Implants®, was
pressed into tablets for insertion into shallow holes
drilled in tree trunks. Concern about drilling holes
into trees and the apparent compartmentalization
around the tablets that prevented continued slow
release of flurprimidol into the transpiration stream
resulted in limited use of the implants. Hence,
flurprimidol was removed from the tool kit of arborists
about two years ago.
Today, only one growth retardant for use on trees
remains, paclobutrazol. Satisfactory performance
of paclobutrazol as a growth retardant, as well as
several benefits to tree health, revealed through recent
research that resulted in a rebound in use of this TGR
today by some electric utilities and spurred an active
expansion of the market to commercial landscapes and
general arboricultural tree care.
Growth Retardants: A Promising Tool for Managing Urban Trees
Treatment is Easy
Paclobutrazol, formulated as Cambistat 2SC® or
Profile 2SC®, is applied as a water suspension. Both
formulations are approved by the EPA for soil injection
or application as a basal drench. The dose rate, which
is species specific, is determined by measuring trunk
diameter. The water suspension of paclobutrazol can
either be injected at about 150 psi into the soil to a depth
of approximately 6 inches as close to the tree trunk as
possible (Fig. 1) or simply poured into a shallow trench
around the base of each tree (Fig. 2). The product label
Figure 2. Basal or soil drench method of applying
paclobutrazol.
Figure 1. Soil injection method for applying
paclobutrazol.
Figure 3. Terpenoid pathway for biosynthesis of
gibberellins, abscisic acid, phytol, and steroids, and
path for degradation of abscisic acid. Steps blocked by
paclobutrazol indicated with X X.
provides detailed information for proper application.
Treatments can be made anytime the soil is not frozen or
saturated with water.
Actually paclobutrazol and other growth retardants
with the same mode of action are currently used in the
nursery industry for production of compact and hardy
bedding plants and on golf courses to reduce growth
of turf and the frequency of mowing fairways. The
dose rate for turf is lower than that applied to trees.
Consequently, the grass in a narrow ring around the base
of paclobutrazol-treated trees may be notably shorter.
However, this could be a benefit because the serious
problem of mower and string trimmer damage to tree
trucks is less likely without the need to mow close to
trees. Since paclobutrazol in very immobile in soils,
there is no need for concern about over-regulation of turf
more than a few inches away from the treatment zone.
Mode of Action
Suppression of growth by paclobutrazol occurs
because the compound blocks three steps in the
terpenoid pathway for the production of the hormone
gibberellin by binding with and inhibiting the enzymes
that catalyze the metabolic reactions (Fig. 3). One of
the main roles of gibberellins in trees is the stimulation
of cell elongation. When gibberellin production is
inhibited, cell division still occurs, but the new cells do
not elongate. The result is shoots with the same numbers
of leaves and internodes compressed into a shorter
length. For many years this was considered to be the
sole response of trees to treatment with paclobutrazol.
FNR-252-W Growth Retardants: A Promising Tool for Managing Urban Trees
However, recent research has demonstrated that blocking
a portion of the so-called terpenoid pathway causes
shunting of the accumulated intermediary compounds
above the blockage. The consequence is increased
production of the hormone abscisic acid and the
chlorophyll component phytol, both beneficial to tree
growth and health (Fig. 3).
The unique structure of paclobutrazol that allows it
to bind to an iron atom in the enzymes essential for the
production of gibberellins also has the capacity to bind
to enzymes necessary for the production of steroids
in fungi as well as those that promote destruction
of abscisic acid (Fig. 3). The consequence is that
paclobutrazol treated trees have greater tolerance to
environmental stresses and resistance to fungal diseases.
Morphological modifications of leaves induced by
treatment with paclobutrazol such as smaller stomatal
pores, thicker leaves, and increased number and size
of surface appendages on leaves may provide physical
barriers to some fungal, bacterial, and insect infestations.
Growth Reduction
Shoot Growth
Although growth reduction is dose sensitive and varies
widely among species, all evergreen and hardwood
species, and even palms, respond in some degree to
treatment with paclobutrazol. Treated trees have more
compact crowns and somewhat smaller and darker green
leaves, but otherwise look normal. The amount of shoot
growth reduction ranges from a low of 10 percent to
a high of 90 percent, with average growth reduction
being 40 to 60 percent when recommended dose rates
are applied. As a consequence of the reduced growth in
height, there is a parallel reduction in biomass removed
when trees eventually require trimming.
Cambial Growth
Although the principal focus of research with
paclobutrazol has been on growth in length of shoots,
reduced growth in diameter of the trunk and branches
of woody plants also has been found. Expansion of cells
produced by the vascular cambium also depends on
gibberellins just like cells in stems and leaves. This could
have significance in urban areas for trees planted in wells,
above ground containers, and in the parkway between
sidewalk and curb. Up to 30 percent of trees planted in the
city cause sidewalk and curb damage due to expansion in
girth of the trunk and roots, requiring significant portions
of annual tree budgets for costly repairs. Suppression
of diameter growth of tree trunks and roots at least
forestalls costly damage and the creation of hazards.
Root Growth
Effects of paclobutrazol on root growth vary from
enhancement to inhibition and are far from being clearly
defined and understood. In almost all cases, however,
the response in paclobutrazol-treated trees is an increase
in root to shoot ratio. Gary Watson at the Morton
Arboretum conducted one of the few studies on large
mature trees exposed to paclobutrazol. Soil injection at
the base of white and pin oaks caused fine root densities
to be 60 or 80 percent higher, respectively, near the trunk
base. It is unclear whether the responses observed in
roots of treated trees are a direct effect of paclobutrazol
on root growth or an indirect effect resulting from
shoot growth modification and a shift in carbohydrate
allocation to the roots. Root response to paclobutrazol
is an important question because root growth and vigor
influence not only water uptake but many other aspects
of tree health.
Greener Leaves
Trees treated with paclobutrazol generally have leaves
with a rich green color suggesting higher chlorophyll
content (Fig. 4). There are two possible explanations for
this response. One is that the leaves of both treated and
untreated trees contain the same number of cells, but
because the cells in leaves of treated trees are smaller,
the chlorophyll is more concentrated in the reduced cell
volume. In addition, however, there is evidence that the
amount of chlorophyll is actually increased too because
Figure 4. Sugar maple leaves from trees untreated or
treated with paclobutrazol showing higher chlorophyll
content .
Growth Retardants: A Promising Tool for Managing Urban Trees
phytol, an essential part of the chlorophyll molecule is
produced via the same terpenoid pathway as gibberellins.
Paclobutrazol treatment, which blocks the production
of gibberellins, results in a shunting of the intermediate
compounds from gibberellin synthesis to the production
of even more phytol (Fig. 3). An analogy might be an
accident blocking the flow of traffic on a major highway
causing more drivers to divert to alternate routes.
Reduced Water Stress
In addition to interfering with gibberellin production,
paclobutrazol is known to affect the synthesis of the
hormone abscisic acid. Abscisic acid also is made via the
terpenoid pathway (Fig. 3). Unlike the inhibiting effect
on gibberellin synthesis, treatment with paclobutrazol
promotes the production of abscisic acid much like it
promotes the production of phytol. When gibberellin
synthesis is inhibited, more precursors in the terpenoid
pathway accumulate and are shunted to the production of
abscisic acid.
Paclobutrazol also interferes with the normal
breakdown of abscisic acid. The mode of action
involves another iron containing enzyme to which the
paclobutrazol will attach, preventing its activity. The
combined effect on both the production and breakdown
processes results in enhanced concentrations of abscisic
acid in leaves. One of the functions of abscisic acid is to
cause stomates to close, reducing water loss from leaves
through transpiration.
Improved water relations in trees could arise from
a combination of increased abscisic acid contents that
physiologically reduce stomatal opening, reduced
shoot growth resulting in less leaf and stem surface
area for transpiration, more fine roots to absorb water,
and structural changes in leaves that provide physical
barriers to moisture loss. Fig. 5 shows dramatic scanning
electron microscope images of thicker leaves and masses
of hairs on leaf surfaces of cherrybark oaks in response
to treatment with paclobutrazol.
The improvement of water relations in paclobutrazoltreated
trees is an important secondary benefit of using
a TGR.
Effects on Fungal Diseases
Protection from fungal diseases that attack urban
trees is now recognized as another secondary benefit of
using paclobutrazol. There are numerous observations
of reduced incidence of common fungal diseases such
as anthracnose following treatment with paclobutrazol.
Karel Jacobs at the Morton Arboretum has shown
paclobutrazol to significantly reduce the growth of eight
fungal pathogens in laboratory cultures. More and more
data from field trials is being published to substantiate
the fungistatic benefit of using paclobutrazol. Bruce
Fraedrich with Bartlett Tree Expert Company has
recently demonstrated that even bacterial leaf scorch is
markedly reduced in red oaks following a soil drench
application of paclobutrazol.
The fungistatic property of paclobutrazol is due to the
inhibition of steroid production in fungi, also via the
terpenoid pathway (Fig. 3). This is the same mode of
action that accounts for the fungistatic property of the
class of fungicides known as SBIs or steroid biosynthesis
inhibitors. Steroids are essential constituents of
membranes.
The increased resistance of paclobutrazol-treated
trees to bacteria is not thought to be a direct effect on
the pathogen, but rather due to alteration in leaf surface
Figure 5. Scanning electron micrographs of the lower
surface of leaves of cherrybark oak untreated or treated
by the soil injection method with paclobutrazol.
