Crossville Chronicle, Crossville, TN

March 17, 2014

Plateau Gardening: Prune boxwoods to keep them healthy, strong

By C. Rae Hozer
Chronicle contributor

CROSSVILLE — Many thanks to readers who emailed topic suggestions or yard and garden questions to MGardenerRae@frontiernet.net. Keep them coming! Watch throughout the 2014 season for "Plateau Gardening" articles based upon reader submissions.

However, since answers to email inquiries may not appear in print right away, residents with a plant problem that needs immediate attention should contact their county University of Tennessee Extension office for quick, reliable diagnoses and prescriptions for treatment. If there is no urgency, homeowners may look through images and questions posted on the UT Soil, Plant and Pest (SPP) Center’s Facebook page at http://www.facebook.com/SoilPlantPestCenter by others with yards and gardens in the state. Browse pictures at the top of that page to see images of plant diseases such as peach leaf curl (photo 40), cedar apple rust (photo 50) and black knot disease on plum (photo 54).

Learn what good bugs and bad ones look like. Photo 18 shows Japanese beetles and photo 61 a flat headed borer, both are insect pests. The damsel bug in photo 36 is a beneficial insect that preys on bad bugs. View pictures of flowering plants like Cornus angustata ‘Elsbry’ also called the Empress of China dogwood (photo 37) and fungi like Coprinus comatus (shaggy manes) in photo 34.

Individual posts on the SPP Center’s 2014 Timeline show images of recently reported problems like winter damage to broad-leaved evergreen foliage or yellow bellied sapsucker damage to a sugar maple. In addition, find announcements of events like the UT Gardens Gala to be held on Friday, April 25, from 5:30 to 9:30 p.m. at the UT Gardens Friendship Plaza in Knoxville and news of robotic equipment at the lab used to determine the pH of soil samples. I recommend bookmarking the site. It’s on my favorites list. 

Last week, a resident asked when boxwoods should be pruned. He normally cuts his back in November and wished to affirm that was correct. The worst time to prune trees and shrubs of any species is between the end of July and Nov. 1. Lee Reich, author of The Pruning Book, explains why in an article on pruning boxwood at the Gardens Alive website. Pruning stimulates plants. That may be manifested by new grow, but even lacking such evidence, cells near the cuts are stimulated. The process of entering dormancy is delayed due to such pruning making affected plants susceptible to winter injury from freezing temperatures and cold dry winds. Boxwoods are especially vulnerable.    

Boxwoods (Buxus species) are slow-growing, broad-leaved evergreen shrubs with no flowers of significant ornamental value. Buds for new shoots form mostly at branch ends. Little pruning is necessary other than thinning. Thinning is normally done when temperatures are above freezing in late autumn (November or December). Boxwood branches are brittle. Pruning when the weather is too cold can break or injure branches. Thin by removing six to eight inches at the tip of small branches around the plant's exterior to reduce foliage density.

When the outer foliage is too thick, it blocks sunlight and air. Given enough sunlight, interior branches stay strong, but branches at the plant’s center become thin and weak without sufficient light. Poor air circulation creates an overly-moist interior which favors fungal diseases. The size and/or shape of boxwoods in the landscape are often managed by the technique of shearing. That type of pruning is usually done in early June with a follow up trim in July. The risk of disease is greater with regular shearing because it promotes overly-dense exterior foliage. Incidence of fungal disease is less when the shrubs are thinned rather than sheared.

Next question: When should tall ornamental grasses be cut back?

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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) for answers to horticulture questions, free publications and to learn about the Master Gardener program. Send email comments or yard and garden inquiries to Master Gardener Rae (MGardenerRae@frontiernet.net).