Fertilizing
How do I know if I should fertilize my boxwood?
Fertilization is unnecessary unless the site has a proven deficiency. Thus, there is no regular schedule to guide fertilization of boxwood. The most reliable guide to applying fertilizer is by testing the soil. Soil samples analyzed by the Cooperative Extension Service will provide appropriate fertilizer recommendations for a specific site. Applying excessive amounts of fertilizer may result in the death of the foliage, a branch, or even the entire shrub.
Perhaps the most common nutritional deficiency occurs with nitrogen. The earliest symptom of nitrogen deficiency is yellowing of the leaves on the lower branches. The leaves will have a rather uniform yellowing, which is more pronounced on the older leaves, inside the plant. As the deficiency becomes more acute, the younger leaves become stunted in size and may become bronze in winter. In general, boxwood leaves will stay on the plant for three years. If the leaves fall off prematurely, this is typically another symptom of nitrogen deficiency.
What kind of fertilizer is best and when should I use it?
Granular fertilizers provide the best results. Most tend to have high concentrations of nitrogen, indicated by the first of three numbers. For example, a common fertilizer, having a 29-3-4 analysis contains 29% nitrogen (N), 3% phosphorus (P), and 4% potassium (K). Results from a soil test provide the best guidance for fertilizer, including the exact formulation and rate if it is necessary. The application of liquid fertilizers, with their fast-acting and concentrated properties, will over-stimulate boxwood and should not be considered for those planted in the ground.
Fall applications of nitrogen, which promote healthy root growth, provide the best overall results. As winter approaches, the soil with a lower average temperature and higher moisture content makes nitrogen less available to the plant. Boxwood should not be fertilized in the spring, as this encourages mostly foliage growth. Spring fertilization causes the shrub to produce new large dark green leaves, which appears beneficial. However, such a shrub would have more leaves without a corresponding increase in roots, creating a stressful condition. Boxwood should not be fertilized in summer, as this promotes late season leaf growth. These leaves are not able to harden off before the damaging winter freezing occurs. While soil test results provide the best guidance, generally annual application of fertilizer is seldom necessary.
What is the best method to apply fertilizers?
Fertilizer should be applied by broadcasting it around the base of the plant at and just beyond the drip line. Most of the active roots are located in this area. Surface application is the easiest and most effective technique. Fertilizer particles that come into direct contact with the roots of unmulched boxwood can cause root burn. This can be avoided by broadcasting fertilizers only on mulched ground when the soil has adequate moisture. Fertilizer particles that come into direct contact with the leaves can cause foliage burn. This can be avoided by forcefully rinsing the leaves with water to dislodge particles and remove dust.
Deep root application of fertilizer is another method. This involves drilling holes in the soil, then filling them partially with fertilizer. While this approach does eliminate the volatilization of urea and ammonium (sources of nitrogen in fertilizer), it is not worth the effort, nor does it benefit the boxwood. Boxwood roots grow best when near the surface. Encouraging them to grow deeper by using this method is not recommended.
Site Selection
Location is very important when planting boxwood. All too often, design elements are considered before site conditions. This has resulted in many healthy boxwood, planted in a poor site, growing weak or even dying. Each individual site will have its own set of conditions. The following is a list of site conditions to consider.
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