Winter Scouting for Ornamental Plant Problems

Woody plants can be checked for a number of potential problems during the winter. For deciduous plants, winter allows for a full inspection of the branching structure of trees and shrubs. Broken and hazardous branches can be removed. Also, pruning can be done to improve form. If there is no snow cover, the base of trees can be checked to determine if there are any girdling roots that could be cut away. The mulch layer can be checked to ensure it is not excessive over the root zone and is not against the base of the plant. If it is, it can be raked to a depth of three inches and/or pulled away from the trunk/stem.

Pennsylvania has a true dormant season; insects (with a few exceptions) and diseases are not active on outdoor plants. This is good news! However, you can still use the time to scout for some.

Viburnums can be checked for egg-laying sites of the viburnum leaf beetle. Female beetles insert eggs into small twigs; these are in the overwintering life stage. Twigs with egg-laying scars can be pruned out to remove the eggs, which will prevent larval feeding damage in late spring.

Eastern tent caterpillars, which often feed on ornamental crabapples and cherries, overwinter as eggs in a mass that looks like grey Styrofoam. They can be pruned out if located on small twigs or scraped off if they are on larger branches.

Broadleaf evergreens, such as Japanese andromeda, azalea, and rhododendron, can be checked for lace bug damage. The top surface of the leaf will have small discoloration spots, typically whitish, that were caused by lace bugs feeding damage from the previous years. This damage is called stippling. The bottom of the leaf can be checked for dark fecal spots and shed exoskeletons to confirm that lace bugs caused the foliar symptoms on the upper surface. If lace bugs are confirmed, then a plan can be developed to treat the lace bug nymphs in the spring.

Even though winter is a time of dormancy, there are a couple of ornamental plant pests that may be active. Hemlock woolly adelgid and spruce spider mites can be active during the winter. Spruce spider mites overwinter in the egg stage, but if winter is mild with days in the 40s, they can become active. This is especially the case for needled evergreens that are in protected locations near buildings that might have a warmer microclimate. Tapping branches over a white sheet of paper will dislodge the mites so their activity can be confirmed. A hand lens (jeweler's loop) makes this easier. If the plant has noticeable stippling damage, and the temperature has been above freezing and is projected to stay mild, a horticultural oil application could be made to the symptomatic plants. Hemlock woolly adelgid goes into winter as a nymph (immature) and develops into an adult from late February into March. Again, an oil application could be made to manage this pest if the temperature is mild.