The single most important maintenance activity for keeping English boxwood healthy is thinning. Ibis improves the air and sun circulation through the interior of the plant, reducing the chance for infection by diseases such as Macrophoma and Volutella. First, thin the plant to reduce the dense foliage and prune out dead twigs. A vigorous shaking of the branches will then force the debris to fall to the ground. Finally, a leaf rake can be used to collect the debris, which should then be removed from the site.
The soil pH should be between 6.5 and 7.2 for English boxwood. Boxwood should therefore not be planted near acid-loving plants such as azaleas or hollies. If the pH is below the recommended range, add dolomitic lime.
Early fall is the best time to plant or transplant. This allows the boxwood to produce new roots before the new foliage appears. The root ball ought to be at least as wide as the drip line. The depth of the ball is usually determined by the height of the plant. A 3 to 1 ratio provides a general guideline. For example, a 6′ tall boxwood should have a root ball 1 ½’ to 2′ deep. When planting consider how large it will be at maturity. Future problems of overcrowding can be avoided by properly spacing the plant to account for its ultimate size.
English boxwood does best with partial sun during the growing season. The site should offer protection from sunshine and wind during the winter. Plants exposed to continual, direct sun in winter will have reddish-brown or yellow leaves due to rapid temperature changes. Boxwoods planted with exposure to the south or west sun in the winter often experience winter bronzing.
Apply and maintain mulch to a depth of one inch. Avoid mounding mulch under branches, which encourages adventitious rooting, or next to the trunk, which may attract moles.
Tying string around English boxwood will protect it from snow and ice damage. First, tie the string securely to the main trunk at the base of the shrub. Then wrap the string in an upward spiral pressing the branches upwards and inwards. Work up to the top of the plant then back down and tie the string onto the trunk again. The rows of string should be about 8″-10″ apart to provide the best support.
Propagation by stem cuttings can be successfully accomplished from July to December. Cuttings are taken from one or two-year-old branchlets. Remove the leaves from the bottom 1″ of the cutting. Treat this bottom portion with a rooting hormone, then place the cutting in a container with a media mix. Successful mixes have an equal portion by volume of pine bark and perlite, or coarse, sharp builder’s sand and perlite. Rooting usually occurs in two to three months. During this time, the cuttings benefit from high humidity. The plants can be planted outside in a protected area the following spring.