The leaves are falling and many trees are bare, but, that doesn’t mean the trees are inactive. Most deciduous trees are devoting energy to root systems at this time. They are investing in roots for winter storage as well as in expanding the root network to exploit opportunities for nutrient and water uptake.
Since the trees are still growing, albeit underground and unseen, we can still make positive contributions to their health condition to make them more tolerant of freezing winter conditions and better prepared to begin above ground growth in the spring.
Watering is probably the most critical care option for trees overwintering in Minnesota. Soil moisture allows for continuing root growth and storage, and nutrient and water uptake. In addition, well watered root systems are better able to handle the many winter stresses including root desiccation.
For evergreen trees and shrubs (spruce, pine, arborvitae etc.) watering in the fall is very important (Figure 1). Since these trees retain their foliage throughout the winter, they are much more prone to issues with winter desiccation. Conifers going into winter with a dry root system will experience more issues with winter injury, including salt damage leading to needle drop and even death of terminal branches. Keeping irrigation on or watering up until the ground becomes frozen will help these plants hydrated through the winter (Figure 2). Heavy snowfall helps protect the roots of trees from desiccation. With snow on the ground, winter injury is more related to structural damage due to weight of snow.
Amending soils in the fall with prescriptive organic matter will also help root systems by creating more pore space for water and nutrient holding (Figure 3). The soil microorganisms that feed on the organic matter are literally soil builders. They will remain active until the ground is frozen and during that time they will naturally till the soil and reduce the bulk density of heavy clay soils. In sandy soils, the organic matter will help hold water and nutrients in the profile where the tree and shrub roots can access it this fall and into next spring.
Fall is also a great time to treat for insect scale problems on trees (Figure 4). Horticultural oils applied to branches and twigs in the fall will suffocate most overwintering scales. There is no risk of foliar damage on deciduous trees, because the foliage is gone. These treatments can however remove the bluish sheen from evergreen plants with blue characters such as Colorado blue spruce, concolor fir and some junipers.
In the winter, deer, rabbits and voles can cause significant damage to stem and trunk tissue on trees and shrubs. Putting metal window screening around the base of newly planted trees and shrubs or those with a history of damage will prevent rabbits and voles from getting to the delicate trunk tissues. The metal screens should be at least four feet tall and should be between 1-1/2 to two inches away from the bark of the tree. This prevents rabbits and voles from reaching over or sticking their noses through the wire to nibble. Metal screening should also be buried at least two inches into the soil to prevent burrowing beneath the screen. Chicken wire and other large gap screens are not effective at preventing access to the bark tissue.
Voles especially need cover from predators. They can become a problem if snow banks go up and over the wire mesh and the voles can tunnel through the snow to get to the bark. They also like the cover of mulch. Having mulch piled up against the trunk of trees is not a good idea in general, but with voles it provides a sanctuary from predators so they can feed on trees unabated. Pulling mulch away from the trunks of trees with about a foot of open clearance between the trunk and the mulch is enough to make voles feel uncomfortable.
Paper-based tree wraps, paints and sprays tend to not be as effective as the wire mesh physical barriers for rabbits and voles. Of course no one makes a screen large enough to prevent deer damage.
Deer do prefer saplings and young trees with big tasty buds. Often they will even reach their neck over fences to rip off the succulent tissues. The key with deer repellents is that by using them you make the plant less attractive than other plants in the vicinity. Thus, even plants treated with repellents can be fed upon if the deer are very hungry or if there aren’t any better feeding options for them in the area (Figure 5).
Most deer repellents contain putrescent egg solids as their primary ingredient. Think stinky sulfur egg smell. The best repellents are those that stay on the bark tissues even after a rain or snow event. Unfortunately, even those repellents will wear off over the winter. Repeated treatments are needed when and if the temperature rises above freezing throughout the winter to keep the trees stinky. An alternative that has shown some success is the use of capsaicin tablets that are formulated to be dissolved in the soil and taken up from the roots of trees into the tissue that is fed upon by deer. These tablets however require an active xylem stream within the tree to get the capsaicin where it will be effective. That’s probably not going to happen this late in the season, but planning early for to apply them in September of next year is certainly an option.
Dr. John Lloyd, Ph.D. of Plant Health Doctors, LLC contributes to the Ostvig Tree Care website blog. He is also available through Ostvig Tree Care to answer questions on tree care and plant health issues. Feel free to send any questions you may have about this blog or any other tree issues to firstname.lastname@example.org.