By Shawn Bernick
Q: Can fall really be a good time to apply tree health care products if the trees are going dormant?
— Jim Johnson, Johnson’s Family Tree Service, Minot, N.D.
A: Thanks for the question. The short answer is “yes.” However, there are a few caveats for timing, depending on the application method that will be used to administer the treatment. Generally speaking, there are three tree health care application methods: tree injection, soil applications, and sprays. All of these services can be offered in the latter parts of the tree care season. A wide variety of issues can be treated later in the season, and the application timing window is often much wider than it is during the spring months. This allows the fall application season to be extremely operationally efficient, which can result in higher profit margins than the same treatment performed other times of the year.
Soil-applied tree health care products can be applied until the ground freezes for control the following year.Tree injection applications with fungicides for diseases such as Dutch elm disease or sycamore anthracnose, and for insect problems such as emerald ash borer, can be done in the fall up until trees begin to show fall color and lose their leaves. Trees treated with tree injection after this period may not get complete and even distribution of the treatment throughout the entire canopy. Although there is much more to learn about the process, the conventional wisdom is that as trees begin to lose their leaves they are no longer evenly pulling solutions throughout the canopy, which may result in uneven distribution of the products in the canopy of the tree. Furthermore, uptake time of tree injection products may be much slower once trees have dropped their leaves. For this reason it is recommended that insecticide and fungicide tree injection treatments be done while the tree has the majority of its leaves, and is still actively transpiring.
In contrast, tree injection treatments with micronutrients can be done later into the fall/early winter months. Micronutrient treatments for deficiencies of iron and manganese have been successfully injected into trees later in the season, and can result in significant reduction of chlorate symptoms the following season. Depending on location, iron chlorosis on oak can be treated with late-season tree injection applications in November and, in some years, even December. It is unknown where micronutrients injected into the vascular system reside in the tree until spring; we do not yet know whether the solution remains in the lower trunk until the spring sap rises, or if it continues to translocate to the canopy throughout the dormant season. More research is certainly needed to better understand the physiologic basis of many of our commonly prescribed techniques, but we do know from field experience that micronutrients can be applied later into the year than other tree injection treatments.
Applying tree health care products to the soil — either through soil injection or basal drench — can be performed up until the ground freezes for efficacy the following spring. Insecticides, tree growth regulators, fertilizers and other soil amendments can be applied during the fall months. Soil-applied insecticides, such as imidacloprid, can be applied in the fall to manage a broad spectrum of insect problems on a wide variety of tree species. Soil applications allow for flexible crew scheduling as they can be applied on cool days, warm days, windy days, and even during light snowfall. Most soil-applied tree health products are formulated to adsorb to the cation exchange capacity (CEC) of the soil. Negatively charged tree health products bind to the positively charged CEC and are released (or “exchanged”) over time into solution as the soil moisture allows. Once in solution, these products can enter the root system where transpiration pulls them up into the canopy. Depending on when in the fall season the soil treatment is applied, the product may partially translocate into the tree that year or may simply reside in the soil until the tree begins to pull solutions up from roots in spring. The advantage of fall treatments over spring treatments is that many soil-applied products can take several weeks to fully move into the upper canopy; especially on larger, mature trees. Applying in the fall ensures the treatment is available to the tree as soon as it begins the annual sap rise. Scheduling soil applications in the fall helps alleviate operational and scheduling challenges that companies may face during the spring season.
Spray applications are mostly thought of as spring treatments for foliar fungal diseases and growing-season applications of insecticides, but sprays during the later parts of the season can be an effective management option as well. Dormant sprays, as the name implies, are performed after the trees have lost their leaves or, for conifers, after temperatures have cooled. Horticultural oils are among the most common dormant-season spray treatments. Horticultural oils are highly refined petroleum products, similar to baby oil, which are sprayed on the trunk and branches of trees to control overwintering pests on the tree (such as spider mites and scale insects). Dormant-season sprays can also be used to control egg masses from destructive caterpillars such as gypsy moth. Oil applications are frequently used as part of an overall insect or mite management strategy that may include additional foliar sprays or systemic applications during the growing season to control severe infestations of insects.
Taking advantage of a slower season for tree care by performing many of your plant health care applications in the fall is a great way to extend your service season, ease the scheduling conflicts of springtime, and offer new and valuable services to your clients. It is important that the products and application methods you choose to employ during this time are backed by scientific research and sound arboricultural practices — the same as any other service you offer throughout the year. As you can see, even though the trees are beginning to bed down for winter, fall can be a productive season from both a biological perspective and an operational perspective as well.
Shawn Bernick is director of research and technical support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn.