What is Cambistat?

Cambistat is a soil applied tree growth regulator that reduces the growth of trees. The active ingredient in Cambistat is a molecule called paclobutrazol. This molecule is transported to the growing points of the tree where it interacts in the plant. Extensive research and field trials have shown that indirect effects of growth regulation provide numerous health benefits to the trees. By reducing the amount of energy allocated to shoot growth, more sugars and substrates are available for other uses throughout the plant. A single application can provide these benefits for multiple years.

The Science of Growth Control

Gibberellic Acid

Gibberellic acid is the plant hormone that is responsible for cell enlargement and elongation. It is produced through a series of reactions in the Isoprenoid Pathway. A lone electron pair on a nitrogen atom at the edge of the paclobutrazol molecule interacts with the central iron atom of the plant enzyme kaurene oxidase. This interaction blocks a key step in the formation of gibberellin. When gibberellin formation is inhibited, the plant responds with a reduction in vegetative growth.

Energy Re-Allocation

Carbohydrates produced by photosynthesis support all metabolic processes of life and allocation of this limited resource is very important. Energy not spent on vegetation growth is available for other uses throughout the plant such as:

• Increased fibrous root growth
• Increased defense chemical production
• Thicker more resilient leaves
• Storage for future needs
• Fruit and flower development
• Increased production of other plant hormones

Health Enhancement

Abscisic Acid

Abscisic Acid has a powerful role in helping plants withstand and respond to environmental stresses. Abscisic acid and gibberellic acid are produced from the same starting material, so when Cambistat inhibits gibberellic acid formation more abscisic acid can be produced. Abscisic acid is considered the plant stress hormone, and it induces responses that protect the tree. Some of the more notable responses include: • Increased leaf thickening
• Promotion of fibrous root elongation
• An increase in the defense response of the tree.

These electron microscope photographs show the leaves of treated and untreated trees. Notice how much thicker the treated tree leaf is. Research and Photographs courtesy of Qi, Knighten, and Chaney.

These electron microscope photographs show the undersides of leaves of treated and untreated trees. Notice how many more trichomes (hairs) have grown on treated trees. This response my help explain decreases in water loss and increased defenses of leaves on treated trees.Research and Photographs courtesy of Qi, Knighten, and Chaney.

Chlorophyll Production

Chlorophyll molecules have two distinct components: a ring structure with a magnesium atom at the center, and a long side chain called a phytol tail. The phytol tail is formed from a series of reactions that begin with the molecule geranylgeranyl pyrophosphate. When gibberellic acid formation is inhibited, more phytol can be formed and chlorophyll levels can be enhanced, resulting in a darker green leaf.

These two sets of leaves came from a pair of test trees. The leaves on the right were treated. Notice how much darker the treated leaves are. -treated in fall of 2000, picture taken fall of 2003

Stimulating Fibrous Root Growth

Re-directing Excess Energy

When Cambistat reduces the growth rate of the tree, it frees up energy, carbon and other substrates that would have been used for that growth. These get shunted back into the tree and are available for use for other functions. One of the responses by the tree is an increase in fibrous root growth.

Root to Shoot Ratio

A major issue with trees growing in lawns and other urban sites is a poor root to shoot ratio. This basically means that compared to trees growing in more natural settings, there is more top growth as compared to fibrous roots. This has obvious repercussions that lead to trees that are more prone to stress. By reducing the top growth and increasing the fibrous root growth, applying Cambistat will increase this important ratio.

Linden on the right was treated in 1992 to keep it out of the powerlines. The untreated tree was across the street. Photo courtesy of Dr. Bill Chaney – Purdue University.

Chlorosis and Root Growth

Chlorosis is a major problem with many urban trees. While soil pH may play a role in this problem, so too may a lack of healthy fibrous roots to utilize existing soil nutrients. While Cambistat alone is not the solution to chlorosis problems, it may stimulate root production and prevent trees from further yellowing. See our protocol for treating chlorotic trees.

Improves Water Relations

Stomatal Aperture

Trees treated with Cambistat may have elevated levels of the hormone abscisic acid that results from the interruption of the production of gibberellic acid in the isoprenoid pathway. Abscisic acid plays a key role in the aperture of the stomata. It is naturally produced in root tips in response to dry soils. From the roots it is transported through the xylem into the canopy and eventually the leaves. In the leaves, abscisic acid signals the guard cells of the stomata to close. By elevating levels of this hormone with Cambistat stomatal aperture is more conservative and water status is maintained. Research with growth regulators demonstrates this effect, as seen in the chart on the right.

Thicker Leaves with More Trichomes

When trees are treated with Cambistat there is an increase in Chlorophyll and Abscisic acid, which can cause the tree to produce thicker, darker green leaves. Leaves of some species also appear to produce a significantly higher number of trichomes (hairs), which may provide a physical barrier to water loss.

Reduces Stress, Increases Longevity & Durability

Tree Lifespan

Not Age But Size Related

Trees do not have an internal biological clock like humans. They do not replace cells – but add cells. The reason trees decline and die is related to the trees ability to make enough energy and gather resources to support its living tissue. Every growing site has limitations on resources it can provide. When the trees size surpasses these limitations, the tree will not be able to gather what it needs (both site resources and energy) and will start to slowly decline.

Increases the Lifespan and Durability of Yard Trees

Cambistat works by making the tree more conservative. Slower growth rate means energy is shunted into increased root mass, increased defenses, thicker leaves, and increased storage. A slower growing tree needs fewer resources - including water, minerals and energy. This reduced resource need raises the threshold of stress needed to injure the tree. For many mature yard trees, the deficiency level of the growing site is already past this stress threshold. By reducing the trees growth rate – health and longevity can be restored. As can be seen in the chart above, reducing a trees growth rate with Cambistat will:
• Reduce sensitivity to stress
• Decrease need for resources
• Increase energy reserves
• Increase tissue defense
• Reduce attractiveness to organisms
• Give longer lasting leaves

Frequently Asked Questions

When should I apply Cambistat relative to pruning?
Depending on the desired results:
* For optimum growth control, Cambistat should be applied 60 – 90 days prior to pruning while the tree is actively growing.
* For a more natural look, apply Cambistat at the time of pruning or up to 30 days after pruning.

What is the best time of year to apply Cambistat?
Cambistat can be applied anytime during the year as long as the ground is not frozen or saturated with water.

Will surrounding vegetation be regulated?  
Yes.   The growth of grass, shrubs, flowers, and other vegetation will be slowed if the roots of these plants come in contact with the applied Cambistat.

Will use of Cambistat replace pruning?
No.  Use of TGRs reduces shoot growth which reduces frequency of pruning.  If a strict cycle is maintained, Cambistat will reduce the amount of vegetative growth necessary to remove.

How long does Cambistat last?
Reapplication is recommended every 3 years.  However, on some tree species, effective regulation lasts longer, others may be retreated sooner.

What risk does Cambistat pose for people, pets & wildlife?
Cambistat has a CAUTION label.  Cambistat is diluted with 11 parts water and placed in the soil next to the tree by licensed and experienced applicators.  The tree roots absorb Cambistat, and it moves into the canopy which removes it from the soil.

What are the effects of Cambistat in groundwater?
Laboratory and field studies have been conducted as part of the standard battery of toxicological studies for acceptance by the EPA.  It has been concluded that paclobutrazol has low mobility in soil.  In a field study in sandy loam soil in North Carolina, with irrigation of 8.47 inches of water over 30 days, no paclobutrazol was detected below 6 inches of soil.

What is the background of this product – research, field performance?
Paclobutrazol (Cambistat) has been university researched and field tested for over 30 years and has been used in the utility industry in its present formula for more than eight years.  It has proven effective in all parts of the country.

Can I use treated leaves for mulch?
Yes.  Leaves from treated trees can be used as mulch, bagged for disposal or bundled.

Jim Neeser

Certified Arborist MN-4108 AU
Office: 952-252-0558
Cell: 612-280-3354
Email: jneeser@treecarescience.com