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Feeding Fall's Hungry Plants

Understanding plants' nutritional needs is the key to good gardening

Yellowing leaves on a holly are a sure sign of nitrogen deficiency.
     This year’s fruit on my American hollies is very heavy. That gives me a job to do. Unless I give them additional nitrogen by mid-September, their foliage will be yellow-green instead of a rich dark green that will better show off the bright red berries.
     Plants that produce a heavy set of fruit tend to have lighter-colored foliage. This happens because the nitrogen in the plant is mobile. If the roots cannot obtain sufficient nitrogen from the soil to satisfy the needs of the developing fruit and seeds, the nitrogen will drain from the leaves. Nitrogen concentration in the foliage has a major effect on chlorophyll content. Holly leaves with a rich dark green color contain approximately four percent nitrogen. When less nitrogen is available, older leaves turn yellow-green first.
     When you consider that the purpose of a plant is to flower and produce fruit with viable seeds, you appreciate the importance of proper nutrition. The same is true for a pregnant animal or human.
     If you fertilize your hollies every year, it is unlikely that you will see any change in the color of the foliage. However, if you are only applying mulch around your hollies and they are laden with fruit, now is the time to feed them with a nitrogen-rich ­fertilizer or compost.
     If you are using a high-nitrogen lawn fertilizer, apply approximately one-half cup for every three feet in height. The fertilizer should be spread evenly around the drip line, where most of the feeder roots of holly are concentrated.
     For LeafGro, Orgro or Maine Lobster compost, apply a layer about two inches thick in a two-inch-wide band around the drip line of the holly. If I use Bloom, my two-inch wide band needs be only about an inch thick. 
     Consider using the same treatment on your azaleas and rhododendrons. Azaleas and rhododendrons are not producing seeds, but they are growing flower buds that will bloom in the spring. This growth requires additional nitrogen. If the roots cannot supply it, the nitrogen will migrate from the lower leaves to the developing flower buds. This is why many azaleas only have a small cluster of leaves in mid- to late winter and in spring when they flower. When azaleas have an ample supply of nitrogen, they are evergreen.
     If your azaleas lost most of their leaves last winter, it means that the plants are starving for nitrogen. However, azaleas and related species can only absorb ammonium nitrogen. Both compost and Bloom are rich in ammonium. If you use fertilizer on acid-loving plants always use one that contains ammonium sulfate or blood meal.
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