My 3 year old Korean boxwood has tips that have leafed out nicely, but there are also bare branches that appear thin. This year, we’ve fertilized it and have a new watering system, so it should be doing well, but it seems spindly. Is it sick? Should we cut the thin branches out?
This sounds like something that might be due solely to the cold winter we had last year, which could have caused buds and whole branches to die off. As well, when winter temperatures fluctuate significantly, a plant may start to emerge from dormancy (which will include conductive tissue starting to fill with water) only to be hit by freezing temperatures that will freeze the water and kill surrounding tissue. Cold dry winds will remove moisture from branches, in essence causing “freezer burn”. These conditions can affect the entire plant, certain sections of it or only branch tips.
However, there is also a disorder known as “boxwood decline”, which is not well understood, but can be caused by fungi, injury from cold, stress from drought and roundworms called nematodes. As well, improper soil pH (it should be 6.5-7), poor nutrition, irrigation and drainage problems can also contribute. Symptoms include randomly occurring dead/dying branches, premature leaf drop, yellow foliage. Cankers can develop along the branches. Experts advise nematode and soil analysis, and evaluation of drainage.
Prune the dead stems – disinfect pruning shears often in bleach (diluted 1:9 with water) or rubbing alcohol (for 10 seconds). As it’s November, don’t prune any live branches (e.g., the ones that have live buds). Next spring, if you prune too early, you may cut off live buds that are a bit delayed in developing. Thinning the boxwood will permit better air circulation. Avoid over-watering or too much fertilizer, but make sure to water until just before the ground freezes, to make sure the roots have moisture. If the area is highly exposed, the plant may need wind protection. If you confirm boxwood decline disease, do not plant another boxwood in the same site (unless you find a replacement that resists decline).
Note that it’s best to water the boxwood deeply, at least once a week, as opposed to frequent shallow watering (which may not reach the root area of the plant). Adding a 2-3 inch (5-7.5 cm) layer of mulch (but don’t cover the trunks) is recommended. Clean up the dead leaves/stems from around the shrub and dispose of these.
Finally, be aware that there is also a fungus that causes boxwood blight, which would include leaf spots that affect the whole surface of the leaves, infected stems that form cankers (which are dark brown/black) – this kills the plant. This could be identified by a leaf sample (wrap this in plastic wrap if bringing it to a nursery). Fungicides would be needed to kill the blight.
Clemson Cooperative Extension. Boxwood Diseases & Insect Pests https://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2052.html
TMG Ask a MG. Boxwood hedge https://www.torontomastergardeners.ca/askagardener/boxwood-hedge/
Virginia Cooperative Extension. Best Management Practices for Boxwood Blight in the Virginia Home Landscape. https://pubs.ext.vt.edu/PPWS/PPWS-29/PPWS-29-pdf.pdf
Gardening Know How. Information on boxwood care. https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/boxwood-care.htm
TMG Ask a MG. Yellow boxwoods. https://www.torontomastergardeners.ca/askagardener/yellow-boxwoods/
Iowa State University. Prepare your plants for winter. https://www.extension.iastate.edu/news/2009/oct/061201.htm
Cooperative extension. Ask an expert. Boxwood turning brown. https://ask.extension.org/questions/185876#.VjZLXcshxUM