Tree Growth Regulators Explained
Tree growth regulators slow a tree’s growth, which can benefit a specimen’s health and form. As a chemical application, homeowners often have questions around its safety and impacts, but arborists assure that, when applied properly by a professional, the treatment is safe and useful in various situations.
Here, we’ll explore what tree growth regulators are, how they work, and why their use sparks controversy. We’ll also share the defense and when it’s appropriate to have a certified arborist treat your tree with a growth regulator.
What Are Tree Growth Regulators?
Tree growth regulators are specific chemicals used to slow a tree's outward growth (we’ll explain more below), redirecting energy to the existing roots, trunk, and branches and resulting in denser, more compact growth.
Growth regulators are used for trees that are:
Close to or under utility lines.
In poor soils or difficult ground.
Damaged or in poor health.
In urban locations like Nashville, growth regulation treatments help trees survive difficult conditions:
Compacted or depleted soil.
Limited root space in grates, along sidewalks, or in other small or shared areas.
Pollution from cars and other urban sources.
Temperature extremes in the city that can cause wood cracking.
The use of these treatments helps manage tree size and growth habits that allow them to grow in challenging conditions, but some studies suggest there can also be general health benefits for the tree from changes in hormone balances and resource allocation, which may support a specimen’s ability to cope with environmental stressors. Possible benefits include:
Root growth stimulation, potentially improving the tree’s ability to access water and nutrients.
Improved resistance to drought and heat.
Higher chlorophyll levels, which can improve foliage color and help the tree tolerate stress.
More compact growth and potentially increased wood density.
However, responses vary by tree species, product, application dose, and site conditions. Excessive application, on the other hand, can have negative effects, so these products should be used carefully.
How Do Tree Growth Regulators Work?
A tree growth regulator inhibits shoot elongation (upwards and outwards), resulting in shorter, denser, wider growth. Plant growth regulators in general interrupt plant hormone pathways and aren’t toxic to grass, shrubs, or trees.
Tree growth regulators work mainly by modifying plant hormonal pathways that control growth and development, particularly the hormones responsible for shoot elongation. The tree continues photosynthesis and normal metabolism, but its rate and pattern of growth change.
Some of the most common tree growth regulators are:
Paclobutrazol: An active ingredient that inhibits gibberellin synthesis, a hormone that promotes cell elongation. Less gibberellin results in shorter shoots and reduced canopy growth, and it can also affect root growth and flowering. Effects can persist for multiple growing seasons, depending on dosage, tree species, application method, and site.
Uniconazole: A plant growth inhibitor that works similarly by inhibiting gibberellin production; it's generally quite potent: long-term tree studies have found effects lasting several years after a single trunk treatment.
Flurprimido: Another gibberellin inhibitor, reducing shoot elongation.
Mefluidide: This interferes with cell division/elongation and is used in some plant-growth-management applications.
Ethephon: This increases ethylene in plants, which can influence shoot growth, fruiting, flowering, and leaf/fruit drop depending on the tree and application. Its effects are much more context-dependent than the classic gibberellin inhibitors.
For mature trees, some growth regulator treatments are low-dose and infrequent because the active ingredients can remain effective for a long time. Application frequency can range from annually to once every several years, depending on the particular product and application method.
For example, some EPA-registered flurprimidol products specify approximately two-year treatment intervals for trees, using a water-diluted soil drench. Tree growth regulators used for long-term tree growth control are commonly applied to the root zone or by trunk injection, while mefluidide and ethephon are commonly used as foliar applications.
By altering the growth pattern, rather than denying the plant nutrients or damaging the tree, tree growth regulators aren’t considered harmful when used appropriately and properly. In fact, tree growth regulators are studied, promoted, and used by the International Society of Arboriculture (ISA) and their certified professionals.
Is There Controversy Around Tree Growth Regulators?
Some controversy surrounds the use of tree and other plant growth regulators, particularly repeated or unnecessary use, persistence in soil, and whether chemically suppressing tree growth is appropriate when pruning or choosing a better-suited tree could accomplish the same goal.
Soil persistence: Some can remain in soil for a relatively long time, so repeated applications can build up exposure.
Movement: Products can potentially move through soil or affect nearby plants if applied improperly.
Non-target effects: Because they alter plant growth hormones, unintended vegetation can potentially be affected.
Tree effects: Excessive dosing can suppress growth more than intended and potentially alter flowering, fruiting, roots, or the tree's response to stress.
Wildlife: Environmental toxicity varies considerably by chemical, and regulators evaluate effects on organisms such as aquatic species and soil organisms.
That said, not all use of plant growth regulators should be characterized as environmentally harmful. The specific chemical, dose, application method, frequency, and site conditions matter enormously. This is why it’s important to work with a certified arborist to manage tree growth treatments. In addition, these products are only available through specialty suppliers for professionals and are not available at regular home improvement stores.
Should You Have a Tree Growth Regulator Application in Your Yard?
Tree growth regulators can make it easier for certain trees to grow in certain spaces in urban and neighborhood environments, increasing the time between pruning needs and reducing potential damage to buildings or pavement, while supporting the health of the tree itself.
Your arborist may suggest using a tree growth regulator to help manage one or more of your trees for the reasons discussed above, but there are some situations where it isn’t necessary or appropriate to use the treatment to manage a tree specimen:
Severely stressed or declining trees.
During very wet weather or in very wet soil.
When a tree needs vigorous new growth and sprouts.
Young trees that are establishing themselves.
When a tree species hasn’t responded well to treatments, or doesn’t have documented responses for professionals to reference.
When there’s a site-environment problem (like major soil, irrigation, pest, or disease issues).
A tree growth regulator application doesn’t replace proper care, which is an ongoing need, whether a tree gets a treatment or not.
Learn more about working with a certified arborist in our article, “The Importance of Working With a Certified Arborist.”
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