Dense, compact boxwood (Buxus spp.) shrubs make excellent hedges or borders in well-drained soil and full sun or heavy shade. For example, American, or common, boxwood (Buxus sempervirens) thrives in USDA zones 5 through 8 and grows up to 20 feet tall and 15 feet wide. Hardy in USDA zones 5 through 9, littleleaf boxwood (Buxus microphylla) grows up to 4 feet tall and 4 feet wide. Diseases and pests, however, can turn a healthy boxwood shrub brown. Some solutions call for chemicals. Wear gloves, goggles, long pants and long sleeves when you use pesticides, and have someone else apply the chemicals if you are pregnant. Keep pesticides under lock and key, and keep children and pets away from plants you are treating, as well as away from application tools.
If boxwood leaves turn light brown or tan, and spring growth gets off to a slow start, the plant may have canker disease. On infected plants, the bark easily peels off the base of affected branches, and the leaves lie against the stem and may have pinkish fruiting bodies. The fungus Votuxella buxi causes this disease. To control canker, remove dead branches and leaves before new spring growth appears. Thoroughly spray the shrub with a mixture of 2 teaspoons of copper fungicide in 1 gallon of water.
You can’t see nematodes with the naked eye, but you will have no problem seeing the damage these microscopic worms do to your boxwood. As the nematodes feed on boxwood roots, they cause stunting, bronzing of foliage and general decline. A 3-inch layer of organic mulch around the boxwood and regular irrigation during drought will help infested plants live longer. The American boxwood species tolerates stunt nematode and resists root-knot nematode, according to Clemson University Cooperative Extension.
3. Boxwood Leafminer
Boxwood leafminer larvae measure 1/8 inch long and cause blisters and brown or yellow leaf blotches as they feed on the leaves. The leaves may be smaller than normal and drop before the usual three-year period. You should not use insecticides on leafminers unless your boxwoods are heavily damaged, cautions the Clemson University Cooperative Extension. To kill adult leafminers in the spring, spray boxwood leaves, stems and branches with a mixture of 4 teaspoons of carbaryl in one gallon of water.
4. Other Causes
Straw-colored leaves and gradual weakening are symptoms of English boxwood decline, which occurs in drought-stressed boxwoods of 20 years or older. Possible causes of this disease include fungi, parasitic nematodes and poor soil drainage. American and littleleaf boxwoods show resistance to English boxwood decline. To control decline, remove dead branches and leaves and change the mulch every year. Cold winter winds can cause boxwood leaves to turn reddish brown or bronze. Korean boxwood (Buxus microphylla var. koreana), which grows in USDA zones 4 through 8, has evergreen leaves that naturally turn light brown to purple in the winter.