Arborists and landscapers usually are concerned with trying to keep trees and shrubs up and growing. But often, situations arise when trees grow beyond where they’re desired, whether it is encroaching into power lines or blocking a desired view. Typically, the approach in these instances is to prune back the off ending trees or shrubs, often with heading cuts. Heading cuts can have negative impacts on plant form, and frequently, vigorous shoot regrowth is stimulated in response to pruning. Many arborists and landscapers are increasingly using tree growth regulators (TGRs) to control tree growth and reduce the need for pruning.
In this article, I’ll discuss some of the basics of TGRs – what they are and what they do – and some of the practical aspects of applying TGRs in an arboricultural context. My discussion is mainly focused on arborists and landscapers who are relatively new to TGRs, but it should also serve as a good review for those who have some experience with chemical growth control.
What are TGRs? Tree growth regulators or tree growth retardants are chemicals that control tree or shrub growth. TGRs are part of a larger group of compounds known as plant growth regulators (PGRs). Horticulturists have used PGRs for decades in a wide array of applications to control plant growth and development, particularly in fruit crops and floriculture production. Most PGRs are hormones that regulate plant growth and development or compounds that inhibit production of naturally occurring plant hormones.
Two of the most common horticultural applications of PGRs are to control flowering and fruit set in fruit crops and to control excessive shoot growth in annual bedding plants. For landscape trees, the earliest efforts at using growth regulators were directed at reducing shoot growth of trees near utility lines. Work in the late 1950s showed that naphthalene acetic acid (NAA) applied to cut branches after pruning could reduce resprouting. However, the practice was considered inefficient and time consuming. Utility-sponsored research in the 1970s focused on growth regulators that were trunk-injected or applied as bark treatments. The bark-banding treatments were applied with a carrier such as diesel or toluene and were discarded because of their unpleasant odor and potential environmental impact. Ultimately, paclobutrazol has emerged as the most effective and accepted growth retardant on the market for trees. Paclobutrazol products currently labeled and marketed for arboricultural application include Cambistat (Rainbow Treecare Scientific Advancements), Profile2SC (Se- PRO Corporation) and ShortStop (Plant Growth Management Systems).
How does paclobutrazol work?
Paclobutrazol reduces shoot growth by inhibiting synthesis of gibberellic acid (GA), which is a naturally occurring hormone that stimulates cell expansion. Most of the formulations of paclobutrazol currently available for arboricultural use are designed to be applied via soil drench or soil injection. The amount of paclobutrazol applied is determined based on the diameter at breast height (DBH) of the tree and usually ranges between 50 and 200 ml of dilute product per inch of DBH. For soil-drench treatments, applicators form a furrow in the soil around the base of the tree and fill this with an amount of product based on the product label (Figure 1).
Paclobutrazol also may be applied using a pressurized (50-200 psi) soil-injection system. Injection systems allow operators to apply metered amounts of product to the soil around a tree quickly and precisely (Figure 2).
Once in the soil, paclobutrazol is taken up by tree roots and translocated upward through the xylem to the growing points near the branch tips. Movement of paclobutrazol occurs primarily in the xylem, therefore uptake and movement will be most rapid under conditions that favor canopy transpiration, i.e., adequate soil moisture, good canopy development and warm, but not hot, temperatures. Paclobutrazol is persistent within trees and within soil, therefore the effects of a single application can last three years or longer. For example, in a study in Oklahoma, researchers monitored trees following the application of paclobutrazol by utility crews and found that shoot growth was controlled for five years. In another operational study, application of paclobutrazol reduced utility pruning and chipping time by 76% for red maple and 86% for silver maple over a 10-year period.
How effective is paclobutrazol at controlling growth?
Paclobutrazol has been shown to control shoot growth in more than 70 tree species. However, the degree to which paclobutrazol controls height growth is highly dependent on species. In a trial in Indiana and Louisiana, for example, application of paclobutrazol reduced shoot elongation of sweetgum trees by 96%. By comparison, paclobutrazol reduced growth of trees of other species by 15% to 55% in the same trial. Because of the dramatic difference in sensitivity of various species to paclobutrazol, applicators need to pay careful attention to rate schedules provided by product manufacturers, which provide rates based on species. Depending on the manufacturer, paclobutrazol products may group species into three to six rate categories. For example, trees that are relatively sensitive to paclobutrazol, such as sweetgum and redbud, only require 75 ml per inch of DBH, whereas less-sensitive trees such as conifers and tulip poplar may require up to 200 ml per inch of DBH (Table 1).
Tree condition also can impact application rates. Trees that are damaged or have dieback should be evaluated carefully before application. Manufacturers suggest reducing rates based on crown vigor. For example, if a tree canopy shows 30% dieback, the application rate calculated from the tree’s DBH should be reduced by 30%. Trees in sidewalk planters or other systems with confined roots may take up material more effectively than trees in nonconfined areas. Therefore, applicators should use their judgment and reduce rates accordingly.
Applicators should pay attention to slopes and surrounding vegetation, as paclobutrazol can affect nontarget plants if material runs off during application or if roots of nontarget plants occur near trees that are treated. Paclobutrazol is not labeled for food crops, so do not apply to fruit trees, nut trees or trees that may be tapped for syrup. Be sure to read and follow all label directions before applying any TGR.
Does paclobutrazol have other effects on trees?
As discussed above, the principle effect of paclobutrazol is reduced shoot elongation. In addition, researchers and practitioners have noted several secondary effects of paclobutrazol that may be desirable in urban forestry and landscape applications. A common observation following paclobutrazol application is that, in addition to shorter internodes, trees produce smaller, greener leaves (Figure 3). We observed this in a recent trial in which we treated Bradford pear trees in downtown Lansing, Michigan. Trees that were treated with paclobutrazol had less leaf area per leaf and had higher chlorophyll-content (SPAD index) values (Figures 4 & 5). This observation is consistent with observations from similar studies that show that paclobutrazol application increases leaf efficiency; that is, trees produce smaller leaves with higher chlorophyll content that are more efficient at photosynthesis.
Research also has shown that paclobutrazol application can increase drought tolerance. Improvements in drought tolerance could be due to one or more mechanisms. Research by Gary Watson at the Morton Arboretum in Lisle, Illinois, demonstrated that paclobutrazol increased fine-root production. This effect alone could increase the ability of trees to take up water, but it could further enhance drought tolerance when combined with reduced shoot and leaf growth resulting in a higher root/ shoot ratio. Moreover, paclobutrazol also has been linked to increased production of abscisic acid (ABA), which is an internal hormone in trees that reduces stomatal conductance and water loss through transpiration.
Paclobutrazol also may contribute to increased pest tolerance. Paclobutrazol is part of a group of compounds known as triazoles, many of which have fungicidal properties. In trees, paclobutrazol has fungistatic properties. This means it may not prevent or cure a disease by itself, but it may be able to slow or reduce disease progression. As with other secondary benefits of paclobutrazol, it is difficult to separate the fungistatic properties from its primary effect on plant growth. For example, reducing shoot and leaf growth can increase the proportion of photosynthates that trees may allocate to defensive compounds, so fungistatic effects of paclobutrazol are intertwined with improvements in the plant’s ability to increase its own defensive compounds.
The bottom line
The effectiveness of TGRs, particularly paclobutrazol, to control tree growth has been demonstrated through scores of research trials and practical experience over the past 30 to 40 years. Paclobutrazol can be especially effective in utility arboriculture by reducing regrowth after clearance pruning, thereby extending pruning cycles and reducing the risks and costs associated with tree trimming near utility lines. In landscape horticulture, paclobutrazol can slow the encroachment of established trees and shrubs toward buildings and into utility lines. However, it is important to note that chemical growth control should not be viewed as a substitute for sound tree selection and planting the right tree in the right place.
Numerous studies also have demonstrated that paclobutrazol can provide several benefits for trees beyond height growth control, such as improved drought and disease resistance. Again, these should be viewed as secondary benefits in addition to height control and should be used to augment, not replace, standard arboricultural care and integrated-pest-management practices.
Bert Cregg, Ph.D., is a professor in the Department of Forestry, College of Agriculture& Natural Resources, at Michigan State University, and an extension specialist at MSU. His research and extension programs focus on physiology and management of trees in landscapes, nurseries and Christmas-tree production.
This article was based on his presentation on the same subject at TCI EXPO in Charlotte, North Carolina, last fall. To listen to an audio recording of that presentation, go to this page in the digital version of this issue online, under the Publications tab, and click here.
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