Gardening Tips: Controlling Boxwood Leafminer and other boxwood problems

Boxwoods are one of the most popular landscape plants in North Carolina. Well-maintained boxwoods can provide a landscape with a formal, elegant look. However, boxwoods are not immune to insect and disease problems. In fact, one might argue that they have more than their fair share.

Above: Create a formal, elegant look by shaping boxwoods into a circular profile. Below: Boxwoods sculpted into topiary forms.

Those of you who grow boxwoods, particularly the American type (although the English and Japanese are susceptible as well), may have seen damage caused by an insect called boxwood leafminer. The symptoms of boxwood leafminer injury include smaller than normal leaves with yellow blotches covering the top part of the leaf. The bottom of the leaf will be bumpy and blistery, as the leafminer has laid its eggs inside the leaf. Gradually the insects will hatch from the eggs and chew their way out of the leaf. The adult insects then puncture the underside of the leaf and lay eggs back into it.

In addition to making a healthy boxwood quite unattractive , these insects are difficult to kill. Adult leafminers are active in mid-April, searching for new leaves to lay eggs in. If needed, this is the best time to treat with a contact insecticide, though they can also be treated with preventative systemic insecticides throughout the year. As always, follow the label directions if using an insecticide.

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Above left: Yellow blotches, caused by the boxwood leafminer insect, form on the top part of the leaf. Above right: The bottom of a leaf infected by boxwood leafminers is bumpy and blistery.

Right photo: Boxwood, left to grow naturally in a pot

There is no simple cure for phytophthora root rot, a devastating disease affecting boxwoods. As the name suggests, this disease causes the roots of the plant to rot, preventing them from taking up water and nutrients normally, and eventually results in the  plant's  leaves  turning  a

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light brown color. Once this happens, there is nothing you can do to save the plant, and you should not replant boxwoods in the same area, as they will be likely to develop the same disease, as it is permanently in the soil. Phytophthora is worse in poorly drained soils, so taking steps to improve drainage, such as adding compost or sand to clay soil, may be helpful as a preventative measure.

One of the most common problems of boxwoods is neither an insect nor disease but rather a physiological problem. Many types of boxwoods, particularly the Japanese types, tend to turn a bronze, orange, or reddish color during the winter. Not all leaves will turn this color, perhaps just a few on the tips of branches. This is normal and is caused by winter winds and may be worse when the soil is dry, the pH is too acidic, or if the temperatures vary drastically during the winter. This will generally not kill the plant, but will definitely make it less attractive. In the spring the new foliage often returns to a normal green color. Keeping the plants moist and adjusting the pH to about 7 by adding lime will lessen this type of winter damage.

Boxwoods of all types tend to perform best in soils that stay on the dryer side, and in areas of partial to even full shade. As is often the case with landscape plants, planting the right plant in the right place is perhaps the most effective way to limit future problems.

Matthew Stevens is the County Extension Director and Horticulture Agent for North Carolina Cooperative Extension’s Pitt County Center. If you have questions about this article or gardening in general, please contact the Pitt County Extension Master Gardeners at [email protected] or 252-902-1705.