Boxwood has shallow roots and fertilizer applied directly to the roots can cause burn damage. Apply fertilizer out 6 inches from the central stem, avoiding getting it on the plant, and provide a good deal of moisture after fertilizing. It is possible to kill boxwood by over fertilizing, according to North Carolina State University Extension. Signs of over fertilization include yellow or brown foliage and dried branches. Never guess at soil composition when it comes to boxwood. A detailed soil analysis at least once a year is necessary to supply the correct amount of nutrients to boxwood.
Prior to Planting
Take a soil test in the area where boxwood will be planted in spring during the fall. If fertilizer is needed, add it per package or soil analysis instruction before winter sets in. This gives the fertilizer time to disperse nutrients to the soil before boxwood planting.
It is a good idea to do another soil test analysis before planting new plants in the spring. Apply fertilizer after planting boxwood if absolutely necessary, but most likely no extra fertilizer will be needed until next season.
Fertilize established plants in early spring after another soil test is taken. If the analysis indicates no need for fertilizer, do not apply it. Check pH level of soil prior to fertilizing. Boxwood requires a pH of 6.5 to 7.0. Low pH inhibits the effects of fertilization by not allowing the plant to take up and use the fertilizer as easy since it needs other nutrients for optimum growth and vigor.
Boxwood in cold climates needs to be fertilized once a year, but those in warm climates may benefit from two. The first is in early spring, prior to any new shoots and the second is in late June or early July. Look for signs of nitrogen deficiency before providing a second fertilization. Signs include a uniform yellowing of older leaves on the plant, thinning and diminishing size of leaves and turning bronze during winter instead of staying green. Leaves stay on branches for three years, even in cold climates. Leaf drop is another sign of nutrient deficiency. Do not fertilize plants during late summer in climates with cold winters, as it encourages growth that is easily damaged by frost.
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