Prune Boxwoods in Winter’s Cold
Healthy plants grow, and unless pruned, they often outgrow their function in the landscape. This problem is most often resolved by pruning during the spring and summer. But pruning boxwoods during these months often spreads canker-causing microorganisms between the cut surfaces, infecting the branches. Cankers damage boxwoods and are difficult to control with fungicides.
At the ground line, prune away canes that produced fruit last year. They are easy to identify because the dried clusters of calyx are still attached. Next, remove all weak canes — they tend to lie on the ground — and all upright canes smaller in diameter than a pencil. To assure an abundance of large, plump raspberries, thin the remaining stems, leaving four to six inches of space between them. Thinning allows adequate room for new, strong shoots to emerge from the roots and rhizomes.
Many diseases associated with boxwood decline, including cankers, are spread from plant to plant with pruning tools, especially during the growing season. In warm weather, diseases produce microscopic spores and mycelia that can be carried by your steel pruners and deposited on moist, fresh-cut surfaces.
Prune your boxwoods on a winter day. When the wood is cold and brittle, the branches snap easily when bent sharply. As no pruning tools are being used, there is no chance of spreading diseases. Branches up to one inch diameter can be easily snapped. If you need to prune larger branches, use clean pruning tools and swab them with rubbing alcohol between cuts.
Boxwoods tend to grow a dense outer layer of branches that excludes light from the center of the plant. By reaching down 12 to 18 inches into that heavy clump and snapping out handfuls of branches, you will thin the outer layer, thus allowing light to penetrate to the inner stems. A good healthy boxwood plant should have an abundance of new shoots emerging from the stems to eventually replace those in the outer layer.
Thinning the outer layer of branches also helps alleviate the trash problem. The dense growth tends to collect dead leaves as well as man-made trash. Leaf-spot diseases like wet foliage are one of the consequences of poor air drainage. Keep the canopy airy.