As we start to leave fall and head into winter you may think your tree and shrub pest problems are over – well, think again! Yes, it’s true that most insect and disease problems are long gone by the time November and December roll around, but not all. And some problems are just getting started and other for other conditions winter is the best time to treat them.
What about Scale Insects? Many species of scale insects overwinter under a waxy coating for self-protection. They do not move during this stage in their lifestyle, but many will continue to feed if temperatures remain warm enough. Horticultural Oil at this time is very effective in controlling many species of scale. As with all applications it is important to follow all label directions for the product you are using – and Oil can cause phytotoxicity on some plants during cold temperatures. Be sure to consult a professional arborist for specific recommendations.
Spider Mites can also be a problem in the fall and winter. Some species are actually more active in colder weather – and these are usually referred to as cool season mites. Eriophyid mites and the Spruce Spider Mite both fall into this category. You should check all needle bearing plants, especially spruce and pine, for the presence of spider mites. Warm days – over 45 degrees following a period of cold weather is enough to “wake up” these pests and cause them to start feeding. This can often happen on plants near reflected heat and sun such as along roadways, at the corners of buildings and near windows. The best approach is to treat plants in late fall to reduce populations. Make an attempt to check your trees and shrubs (especially in areas with radiant heat such as against buildings and windows) during and warm periods during the winter.
The Hemlock Wooly Adelgid is another insect pest that will do damage during the winter. As with the above pests, late fall treatments and warm day inspections are the best approach.
The many different types of fungi that cause disease for our plants are not active during the winter months, but there are some things you can do during the late fall and winter to help suppress these problems. A critical part of any disease management strategy is pruning – pruning out diseased areas will help to reduce the spread of infection during the following spring and summer. And since it is often not advisable to prune diseased trees during the growing season as this would do more to spread the infection than to slow it down, pruning in the dormant season is recommended. During the winter the twigs and the diseased areas are easily noticeable and can be removed and discarded. General pruning should also be done in the winter months where appropriate and practical. Removing dead and broken limbs can be done at any time, but general cleaning and thinning is often best during the winter.
What about fertilization? Fertilization has been in the news a lot lately. Many people frown at fall and early winter fertilization, but if properly done it can be an effective time to help your trees and shrubs. First, never use a high salt, quick release fertilizer in the late summer, fall or early winter. Second, as you have heard me say many times “ get a soil analysis”. This will help determine the nutrient needs of your plants. More specifically, I want to address the micronutrient Iron. It is a great example of how a soil analysis and science are essential to proper tree care. Iron is essential for the growth and development of trees. When a tree lacks iron, less food is produced and the tree will start to decline. If a tree starts declining, it may be more susceptible to damage from insects or disease. The most common cause of iron deficiency is high soil pH. Something that can be easily determined from a soil analysis! When soil pH exceeds 6.5 to 7.0 the availability (not the presence) of iron in the soil is greatly reduced. Your soil can have tons of iron but your plants will not be able to absorb it and utilize it if the pH is too high. Continually adding iron as part of the fertilizer will not have any positive effect. In this case, adjusting the soil pH is the treatment needed, not addition of iron.
Spend some time this fall with an Arborist and go look at your trees and make a plan for their health.