Q. My boxwoods have suffered from leaf miners. Can I trim all the diseased or damaged foliage at this point in the winter?
A. Your objective, to remove all the damaged foliage, is a very good idea. Any boxwood foliage that carries an infestation of the boxwood leaf miner will show severe damage or death in the late spring. As the larvae eat the interior of the leaf, then hatch and fly to puncture new foliage, the damaging cycle is repeated and the plant suffers. You will see yellow or dried foliage, and the overall look of the plant is thin and weak.
However, our climate is very capricious, warm weather follows cold and then newly trimmed plants may put out tender growth that is damaged by the next cold snap. In general, the best advice is to prune in late winter as you wish, but wait until mid- to late February to do it. Pruning often stimulates plants to produce new foliage quickly that may then be damaged by late freezes. If you wait until February, there is little time between your pruning and the warmer weather of spring.
Be sure to remove all the clippings from the ground and do not compost them. Bag them or burn them (be sure to get a burn permit if it is available in your area) but do not leave any clippings on the ground. You can remove old mulch as well.
Fertilize the soil so that new growth will be strong and put out a new, clean insulating mulch. Boxwoods are surface-rooted plants, so rake gently.
By removing an entire cycle of boxwood leaf miners your plants can produce healthy new foliage. If you see any signs of leaf miners hatching around mid-April, their usual time to hatch and fly, spray with a nursery-recommended insecticide. You can cure leaf miners with carefully scheduled pruning and good plant hygiene.
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