American and English boxwood varieties are susceptible to root rot. Leaves turn a light green, and then can proceed to turn a straw-color, bronze or yellow. The leaves tend to curl upward. The bark will separate from the base of the plant as it dies. By the time you notice the symptoms, most of the roots could be dead and brown in color instead of white. Root rot in boxwoods is the result of wet or soggy soil.
English Boxwood Decline
English boxwood decline is a disease that usually affects plants that are more than 20 years old. The symptoms will resemble those of root rot, but the disease is the result of drought stress. Boxwood plants affected by this disease will have discoloration of the veins in the main stem. By the time you notice the drought stress that rots the roots, the plant will be severely affected.
After transplanting boxwoods, you need to give the plant at least 1 inch of water each week during the summer until the plant becomes established. In some areas, it may take two years for the plant to become established. Keep a 2- to 3-inch layer of mulch around the plant to help retain the moisture in the soil. If the plant doesn’t receive 1 inch of water per week during fall, leading up to winter, water the plant until the first freeze to help keep the plant from bronzing during the winter.
Plants need 10-10-10 (nitrogen-phosphorous-potassium) fertilizer in late winter or early spring before any new growth is noticed. Pour the fertilizer around the base of the plant, but 6 inches away from the stem of the boxwood. If you fertilize the plant in late summer, the plant will continue to grow and become susceptible to cold and frost damage, which will cause the plant to die. If you haven’t fertilized the boxwood and it has yellow leaves on the bottom of the plant or the leaves are falling off of the plant, it could mean a lack of nitrogen.