While boxwoods don’t require pruning, many gardeners choose this plant precisely because it can be formed into elegant and interesting shapes. Pruning improperly, however, can drastically affect boxwood’s health. Here are some things to keep in mind before you pick up the shears.
Minor maintenance and shaping can be done at any time throughout the growing season. Be sure your pruning tools are sharp and will make nice, clean cuts; ragged cuts heal more slowly and are more likely to invite pests and disease.
Shearing boxwood plants into neatly clipped hedges is a common practice. A good time for shearing is in June, after the major flush of growth. To keep lines even, place wooden stakes at either end of your hedge and tie a string between them. Using a line level, raise or lower string to the height at which shrubs are to be pruned. This can be done either with electric or hand pruners; but keep in mind that while hand pruning may be a bit more work, it also offers you more control. A shaky hand or tiny waver with electric pruners and you’ve got a noticeable variation in your uniform lines. Be sure to wear protective eyewear if using electric trimmers, and take care not to tangle them in the string.
One of boxwood’s most desirable features, its thick, lush foliage, is the very thing that threatens the shrub’s health. Excessive thickening of leaves at the tips of branches will block light and air from reaching the plant’s center, leading to disease-inviting conditions. These conditions also inhibit the growth of leaves at a branch’s base so that, should deeper pruning be necessary, there will be nothing but bare twigs left and no way for the plant to make food. All types of boxwood tend towards excessive thickening, and regular shearing exacerbates it. Thinning helps to prevent this problem. Cut back branches at regular intervals to just beyond the green growth. This will leave spaces, but if thinning is done uniformly and at regular intervals, the effect will not be unpleasing. This is not a job to be rushed; take the time to stand back often to assess your work. The ideal time to thin your boxwoods is between late November and early December, which serendipitously makes the branches available just in time for holiday decorations.
Don’t toss your summer boxwood scraps either! Cuttings from one-year-old wood taken between July and November can be rooted to make new plants. Fill a container that has good drainage with sterile rooting medium. Rooting mediums can be purchased at most gardening centers, or you can make your own. While there are several acceptable recipes for rooting medium, the American Boxwood Society recommends using equal parts pine bark, coarse builders’ sand, and perlite. Remove the bottom one inch of leaves from the cutting and insert into the medium. You can treat the end of the cutting with rooting hormone if you choose, but boxwoods will root without it. Keep soil moist, spraying plants regularly to maintain high humidity, which increases rooting success. Plants should set roots within two to three months.