By Brandon Gallagher Watson
Understanding the uptake and translocation of chemicals within trees is important for arborists wanting to effectively treat trees for insects and diseases, but how much do we need to know to be effective?
Tree physiologists study how chemicals are absorbed into the vascular system and the roles various enzymes and metabolites play in transporting those chemicals throughout the tree. For you, the practical arborist, understanding uptake needn’t be as concerned with the molecular level as the physiologist, but we can take concepts from them and use them to better understand how are applications are getting into the tree and moving within them.
The term “uptake” refers to treatments being absorbed into tree through various parts of the tree — most commonly though the roots, bark, foliage or by injection. Translocation, then, is the movement and distribution of those chemicals within the tree. Many factors affect the speed at which uptake and translocation occur, including molecular size, solubility, time of year, water availability, tree anatomy and tree health. Similarly, these factors affect the translocation of the chemicals within trees, as does the volume of water with which they are applied and where on the tree they were applied. Probably the most important thing for arborists to know about uptake relates to what factors will affect the speed at which the treatments are effective.
Regardless if the treatment was applied by tree injection or soil application, uptake and distribution of chemicals is reliant upon the tree’s vascular system. Within the vascular system, water and nutrients are absorbed through the roots and transported up to the foliage by the process known as transpiration. This mechanism is the result of the cohesion properties of water. You know how you can float a paper clip on top of water? That’s because the hydrogen atoms in water molecules are attracted to each other like a magnet.
As water molecules exit the top of the plant through the leaves, the cohesion properties of water pull the next molecule into its place. From the tip of the leaves to the tip of the roots, all the water molecules in a tree are connected and keep moving up as the next one exits.
Arborists can take advantage of this upward pull to apply treatments to a small proportion of the tree and have that treatment moved internally to other parts of the tree. This what “systemic” treatments are doing, and they can applied to trees in many different ways, including through the roots, the bark, the foliage, and injected directly into the vascular system.
Soil-applied treatments enter the tree through the roots, and are the most common systemic treatments used by arborists. It is the fine fibrous roots that are primarily responsible for uptake, so it is important to target your applications to reach a high number of those fibrous roots.
Even as recently as five years ago, many soil-applied treatments, including imidacloprid, came with the recommendation of applying in a grid pattern within the dripline of the canopy. There were issues with timing and predictability with this method. First, tree roots come off the trunk in, more or less, a spoke pattern like the wheels on a bike. Applying in a grid manner means some of your application may be in between those spokes and not enter the tree at all. Second, a treatment that entered a root 20 feet from the trunk means it has to move 20 feet just to get to the base of the tree, slowing down the speed of the treatment.
Research has shown that a high concentration of fibrous roots can be found right at the base of the trunk where the tree enters the ground. As applications at the base of the tree are just as effective, work faster, and are considerably easier to apply, most soil-applied treatments today are applied in this manner. Besides application location, soil properties such as texture, pH, cation exchange capacity, and organic matter will all play a role in the speed of treatments.
Systemic bark spray
Taking the variability of the soil out of the equation entirely, systemic bark spray treatments have been increasingly popular over the past few years. Although not all molecules can be applied this way, highly soluble molecules, like dinotefuran (Transtect, Safari), can be mixed with water and sprayed directly onto the bark of the tree, from about head-height down to the ground. The treatment will absorb into the vascular system through the lenticels (the pores found on the bark of trees). This application method is gaining favor with arborists as the applications take just a minute or two per tree, the treatment quickly gets in to the vascular system, and, in many cases, utilizes less product than soil applications — saving dollars and reducing chemical use.
Injecting treatments directly into the vascular system changes the use of the term “uptake” slightly. Rather than referring to the days or weeks it takes for a treatment to move into and through a tree, “uptake” for tree injection usually means the minutes or hours it takes to get an entire dose into a tree. Factors that are going to affect uptake here will include the volume of solution, the health of the tree, the pressure of the application device used, the seasonal weather, and the properties of the chemical being injected.
Tree anatomy also plays a role in tree injection uptake. Many trees are able to conduct water and nutrients through several years of growth rings, so injection treatments can go fast as there is more active tissue for solution to enter. Other trees, such as elms and ash, only use this year’s growth ring for water conduction. In years of cool spring temperatures, this can result in a smaller growth ring and usually slower injection times for the remainder of that season.
No matter where on the tree the treatments are applied, their reliance on the vascular system and transpiration for movement means they have a serious limiting factor — water. Soil applications require water to move the treatment into the roots and water to move the treatment around the tree. Similarity, bark sprays and tree injections require water movement for distribution. No water means no movement; thus, treatments applied during acute or chronic drought will have a tough time moving effectively. So what can be done? Even in times of prolonged drought, watering trees a few days before and a few days after the application of any treatment can go a long way to speeding up and better distributing systemic treatments.
Understanding uptake and distribution of treatments doesn’t take a Ph.D. in plant physiology to be useful for an arborist. In most cases, the point of understanding uptake is about predictability and setting the right expectations with a client. Treatments are available that may work very quickly with a short residual or treatments that work slowly but last much longer. Combining the proper treatment with the right application method and setting the proper expectations is pretty much the basis of any successful tree health care program. And, when in doubt, water.
Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A). Photo provided by Rainbow Treecare Scientific Advancements.