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.