Tree Growth Retardation by Injection of Chemicals

Greenhouse studies

A survey of the growth regulation literature was conducted to identify the candidate chemicals for testing purposes. Since 1974, twelve chemicals and 25 tree species have been evaluated in the greenhouse (Table 1). The species were selected on the recommendations of electric utility company representatives based on their concern of “problem” trees in their area. Although each species was treated with a range of chemical concentrations, not every chemical was used in the treatment of every species. Thus, the treatment sequence began initially with many chemicals and fewer species, gradually involving fewer chemicals and a greater number of species (14).

One-to two-year-old dormant seedlings of the listed species (Table 1) were planted in 15-cm plastic pots filled with a mixture of peat, perlite, and soil (2:2:1). The potted seedlings were placed in a greenhouse, provided with supplemental illumination, watered three times a week, and fertilized once a week to insure optimum conditions for growth and development (14). When the seedlings achieved full leaf development, approximately one-third of the foliage from the top of each plant was removed. One day after trimming, the seedlings were treated with growth regulating chemicals. A range of different concentrations of each chemical was used to determine the relative effectiveness in retarding sprout regrowth. Prior to treatment, the plants were set up in a completely randomized experimental design with 10 trees per treatment. Techniques described by Gregory (17) were used to apply the chemical. Five ml of treatment solution was added to a serum cap reservoir surrounding the stem, and the chemical was introduced by wounding the stem with a scapel, thus allowing the solution to be taken up in the transpiration stream. Distilled water was used as a treatment solution for seedlings designated as controls.

Over an 8-week period following treatment, measurements of vertical height increase, number of sprouts, sprout length and foliar phytotoxicity were collected every 2 weeks. Phytotoxicity (Foliar index rating, Fl) was rated on a subjective scale from 0 to 5. Trees with phytotoxicity ratings of 3 or above were considered unacceptable for landscape purposes (14).

Field evaluation of the trunk injection technique

Starting in 1973, series of growth regulating chemicals were evaluated as commercial formulations for controlling sprout growth in mature trees. The chemicals and species used in the field testing program are listed in Table 2. The field studies included use of 11 plant growth regulating chemicals and 15 tree species. Experiments on street trees were carried out with maleic hydrazide and dikegulac-sodium on 11 species at 17 locations in 12 states. Between 1973 and 1975 studies were conducted at the nursery in Delaware, OH, using the injection methods described by Himelick (9). From 1976, the field investigations were carried out employing the injection methods and equipment described by Brown (2). Using a cordless electric drill, 3 holes of 5.4 mm diameter were drilled 60 mm deep into the trunk 1 meter above the ground on all treated trees 40 cm DBH or less. For trees of large diameter, 6 holes of similar size were drilled. The number of injection sites was increased on the larger diameter trees to insure better initial distribution of the chemical. The holes were spaced equidistant around the trunk and drilled tangentially to intersect as many active xylem vessels as possible (14).

Each experiment consisted of ten trees, and treatments were assigned in a randomized complete block statistical design, with each treatment within a species having an equal distribution of DBH. For trees with a DBH of 40 cm or less, the volume (V) of chemical injected per tree was based on the square of the DBH using the formula: V = 40 × (DBH)²/161 ml. For trees larger than 40 cm DBH, V was computed on a linear relationship using the formula: V = 410 × DBH/40 ml. The linear relationship was used on larger diameter trees to prevent overdosing since crown size of large trees did not increase in proportion to trunk diameter (14). All trees were topped before treatment. Control trees, although topped, did not receive an injection treatment since earlier experiments showed no significant effects of water injection. Treatments were made between three-fourths and full leaf expansion during the period April-June, using the air-powered injection system at a pressure of 14 kg/cm²(200psi). Following injection, 3 randomly selected limb stubs on each tree in each treatment were selected for future evaluation and reference.

After leaf abscission, all sprout regrowth originating within 25 cm from the cut end of each previously marked limb was subsequently counted and measured.

From these measurements the number of sprouts, mean sprout length, and mean length of longest sprout were calculated for each tree (14). In most field studies, the condition of each tree and its foliar index rating were recorded at least once each year before fall coloration.

Greenhouse studies

Six chemicals (daminozide, dikegulac, MH, DOWCO 391, Krenite and NAA) were effective in retarding the regrowth of various species (Table 3). The degree of response varied among species due to inherent genetic factors. Table 3 summarizes the regrowth response of several greenhouse-grown species tested with six different plant growth regulating chemicals.

One chemical, ancymidol, was insoluble in water. To overcome this problem, a mixture of methanol and phosphate buffer was used to solubilize the chemical. The solvent system proved toxic, even in the absence of ancymidol, and thus no conclusions concerning the potential ability of this compound as a growth regulant can be drawn from these studies. These preliminary tests revealed that the other five chemicals were either ineffective in controlling regrowth or, if effective, they caused excessive phytotoxicity. Therefore, further evaluation of these chemicals was discontinued.

Field studies

Early studies with trees planted in field plots near Delaware, Ohio showed that three chemicals, daminozide, maleic hydrazide (MH) and dikegulac-sodium were effective in controlling regrowth. We later found that at high concentrations daminozide was difficult to inject due to its high viscosity. In addition, cost-benefit analysis showed that daminozide application would not provide any savings when compared to mechanical pruning. Therefore, dikegulac and maleic hydrazide were selected for extensive field testing throughout the United States. Most of the subsequent experiments were conducted with city street trees, growing under utility lines. Based on the regrowth data obtained in these experiments, an estimate was made of the pruning cycle extension of treated trees as compared to control trees (Tables 4 and 5).

The data showed that dikegulac generally is more effective than MH in extending the trimming cycle. An examination of the data in Table 5 leads us to conclude that geographical location and climate conditions play an important part in determining the growth response of trees to plantgrowth-regulator treatment. Developmental state of the tree at treatment time also may play a significant role in influencing the subsequent growth of plants. The trimming cycle of sycamore, injected in 1977 in Philadelphia, was extended by one growing season; however, the chemicals did not produce this response in trees treated in 1980 (Table 5). These results may be attributable to the different stages of tree development at time of injection. In 1977, the trees were at half-leaf development stage and in 1980 the leaves were fully developed at treatment time. An examination of the growth data reveals that with certain treatments, even though mean growth of treated trees was less than two-thirds of the control trees, statistical analysis indicated no significant differences (5). These findings indicate large tree-to-tree variability. Such variability becomes evident when one examines Fig. 1 a - 1 i. For experiments conducted between 1979 and 1981, the mean sprout length of untreated control trees was calculated and then assigned as 100% growth. Then the mean sprout length from each tree in each treatment (including controls) was divided into one of the following classes: 1) percent of all trees in each treatment showing a growth reduction of 20% or less when compared against mean sprout length assigned to untreated controls (100%); 2) percent of trees with growth reduction between 21 - 40% of controls; and 3) percent of trees showing growth reduction of 41% and greater.

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Fig. 1.

Regrowth response of sycamore, silver maple, bigleaf male, cottonwood, water oak, hackberry, and red maple to injected dikegulac or maleic hydrazide at different locations. The treatments were made in the spring of first year and regrowth measurements were taken at the end of the first and second growing season following injection.

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These graphs clearly illustrate the variability that exists among trees in a given treatment, including untreated controls. There may be several reasons for lack of uniform growth among trees: 1) inherent genetic differences in growth patterns among trees of the same species; 2) uneven distribution of chemical among limbs and trees so that certain limbs get more of the active ingredient than others; 3) since dosage is based upon DBH rather than canopy size, this may result in differences among trees of the same canopy size in the dose of growth regulator received per unit of tissue, thus contributing to tree-to-tree variability. Figure 1 also reveals that the chemicals are extremely effective in controlling growth during the first growing season following injection, however, in subsequent years their effectiveness is reduced and in several instances the tree apparently began to compensate for loss of growth during the first year.

Several factors, biotic and abiotic, must be taken into consideration in determining the proper concentration of chemical that would produce the desired results. Further research on interaction of these factors with plant growth regulators will be helpful in maximizing the potential of the injection technique for obtaining reliable and consistent growth reduction with minimal variability. The achievement of this objective will result in maximum benefit to the utility industry in their effort to control tree regrowth by commercial application of growth regulating chemicals.