Boxwoods

Source(s)

  • Lynn Batdorf, Curator – National Boxwood Collection, National Arboretum in Washington, DC
  • Dr. Gary Wade, Extension Horticulturist, University of Georgia College of Environmental & Environmental Sciences

Lynn Batdorf, Curator of the national boxwood collection at the National Arboretum in Washington, DC spoke at the Landscape Planning Short Course in Athens on January 29, 2009. He offered some interesting insight into the genus Buxus as he described in detail the history, culture and many cultivars of boxwood. I summarized my notes from his lecture in bullet form below.

  • There are 97 species of boxwood worldwide, but only 7 are temperate plants. The rest are tropicals. There are 182 cultivars of the temperate species in the national boxwood collection at the National Arboretum.
  • Boxwood roots grow shallow, within the top 15 inches of soil, and the roots extend out several times the canopy spread. A mature boxwood is difficult to transplant due to the extensive root mass and percent of root loss during transplant.
  • Boxwood prefers an alkaline pH, in the range of 6.8 to 7.5 for optimum growth. It often suffers nutritional deficiencies at low pH. Dolomite lime is recommended to increase pH because it contains magnesium which boxwood likes. Do not plant boxwood adjacent to azaleas, camellias, gardenias or other acid-loving plants.
  • Boxwood responds to fall fertilization because it promotes root growth, and roots grow all winter. Fall fertilization also minimizes winter leaf bronzing, since this is often linked to nutritional deficiencies. Tip bronzing, for instance, indicates magnesium deficiency.
  • Hand thinning is much better for boxwood than shearing. Shearing results in a thick, dense outer canopy, poor air flow within the foliage, and encourages leaf and twig diseases. Branch die-back can often be attributed to shearing and poor cultural conditions.
  • Boxwood leaves remain on plants for 3 years before they shed. It’s important to keep them on the plant as long as possible by preventing inner leaves from becoming shaded. It’s also important to maintain leaves as far down within the canopy as possible.
  • Use hand pruners to make selective thinning cuts inside the canopy on selected branches. Thinning gradually controls plant size, but more importantly it opens the canopy and improves air flow and light penetration which are important for maintaining leaves.
  • Boxwood does not respond well to severe pruning. Exposing the old wood often results to frost and winter injury and sunscald on the trunk and branches.
  • There is no such thing as boxwood decline. Boxwood problems are caused by numerous insects and diseases, including leaf miner, mites and scales, but most of these are encouraged by poor cultural/management conditions or improper soil pH.
  • English boxwood does not get leaf miners because the leaves contain an alkaloid that kills the insect.

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