Many landscapers and home gardeners reach for boxwood (Buxus spp.) when they want a hedge plant or a hardy, attractive specimen shrub. Boxwoods have dense, evergreen foliage that responds well to shaping. They are the classic hedge and topiary plant, and many species and cultivars even thrive in the shade. If all that evergreen goodness goes yellow, however, the shrub's attractiveness diminishes greatly. In most cases, this can be avoided with proper care.
Old Man Winter
Boxwoods enjoy cool climates. In general, they grow best in U.S. Department of Agriculture plant hardiness zones 4 through 8, although this varies by species -- some can tolerate winters in USDA zone 4, but others can't tolerate climates lower than USDA zone 6. In fact, it is winter weather that is one cause of yellowing leaves on boxwood plants. Damage occurs when an unseasonable warm winter day gets the water flowing in the plant -- water that quickly freezes when evening arrives. This damages the plant, and as a result, the leaves turn light yellow -- sometimes so pale they look almost white, according to the Missouri Botanical Garden. To help the boxwood recover, wait until all danger of frost is past, then prune off any branches that have split wood, making the cut below the split. Sterilize your pruning tools in a solution of 1 part bleach to 3 parts water for about five minutes first, then allow to air-dry. This will prevent the spread of diseases. Then, remove all the damaged branches. If the entire shrub is damaged, cut it all the way back to between 12 and 18 inches above the ground. In some cases -- especially if the boxwood is young -- you might have to replace the plant.
Like most plants, the boxwood will struggle to survive if its roots are damaged. Damaged or dying roots are not able to absorb nutrients, and as a result, the leaves start to die, turning yellow and brown. Several things can damage boxwood roots, including soil nematodes and fungal diseases. The latter are the most common. Once the roots are damaged, the plant cannot usually be saved, but these problems are preventable with proper care. Overly wet soil is a haven for fungi, so plant your boxwood in a well-drained area and in light, fertile soil -- not heavy clay. Avoid depressions where water tends to collect. Strong, healthy plants rarely get sick, so water deeply during periods of drought -- water stress can also cause yellowing leaves -- but water at the soil level and not from overhead.
The Aging Process
Sometimes, yellow leaves are not a symptom of a problem at all. Rather, they are a normal sign of the aging process. Boxwoods grow slowly, so you might not notice it for a few years, but the plants tend to drop the oldest leaves as new ones come in. If this is the case with your shrub, those yellow leaves will be found on the inner part of the plant, but new growth on the outer part will still be green.
The boxwood leafminer is the most serious pest of boxwoods, according to Clemson Cooperative Extension. The tiny orange flies that swarm the plant aren't so much the problem as their larvae are -- when the eggs hatch, the tiny larvae feast on the leaves. Leaves infested with leafminer larvae usually present with misshapen, yellow or brown swollen spots and often drop from the plant. A severe infestation can defoliate and kill a boxwood. Unfortunately, insecticides are not recommended in most cases unless the entire plant is severely affected. If you use an insecticide, use one that contains malathion and carbaryl. Apply the insecticide in mid-April or early May when you see the tiny adult flies. Instructions will vary depending on the product, but in general, dilute 1.5 oz in 1 gallon of water and fill a sprayer with that solution. Apply on a calm, warm day, thoroughly coating the tops and bottoms of the leaves. Repeat applications may be necessary to kill all the larvae, but do not apply more than once a week.