The preferred time to  prune boxwood  is late winter or early spring in the New York area, once the coldest plunges of temperature have passed, around March 15. Boxwood takes to light cutting and shaping quite well but little is required other than to correct the shape by reducing a branch here and there. If you respect the natural shape of the plant you have chosen and prune simply to maintain tidiness, you will limit the potential for plant stress and die-back. Do not prune when the temperature is projected to be below 40°F for several weeks.

If you are regularly cutting most of the branch tips of your shrub and altering its natural shape significantly, that is considered shearing. June is best; do not risk die-back by shearing after mid-August. This type of reduction is the most stressful to the shrub and, for dense plant types, should be accompanied by thinning. Boxwoods do not reliably regenerate leaves from bare wood, so you need to restrict shearing to no more than ¾ of the way into the current exterior layer of leaves. The top of the plant should remain less wide than the lower branches so that sunlight can reach all parts.

Thinning to remove some of the branches at the outer edge of the plant is different from pruning or shearing, and particularly important for the dense common (American) and English boxwood types. Thick growth reduces sunlight into the center of the shrub, which causes foliage drop, weakening the plant. The resulting leaf debris accumulates in the lower branches, causing abundant and vigorous aerial roots to grow in this moist and dark environment. Eventually, the exposed aerial roots will die, shocking the weakened plant. Lack of air circulation will also promote disease if thinning is ignored.

Thin dense boxwoods from early spring to late fall if the foliage completely hides the view of the interior branches. Do not thin when extreme temperatures are predicted. To thin, using sharp bypass pruners, reach inside about 6 inches, and remove a twig. Continue to prune until the small interior twigs become intermittently visible, along with the older, light green interior leaves. When pruning is completed, the shape and size of the boxwood should look unchanged. Use clippings for propagation or holiday decoration if desired.

Learn more about boxwood care, selection and troubleshooting in our Guide to Boxwood.

Courtesy of the NYBG Plant Information Service