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Gardening: Beware of too much late-summer fertilizer in gardens

gardening by Mary Robson, WSU Extension Agent
There's a common popular message that "plants are like people," which isn't really true. The biology of plants differs almost completely from that of people, and the words we use like "plant food" tend to encourage beliefs that plants need regular meals every day, like us.
   In this belief, gardeners often over-use fertilizers, which are various combinations of plant nutrients and minerals that most plants use, along with water and sunlight, to make its own food. (Parasitic plants, such as dodder, attach themselves to another organism and let it make their food.)
   In late July and throughout August, most trees and shrubs are slowing down their growth rates. Twigs and leaves produced in spring are "hardening off," growing stiffer and ceasing to add a lot of new growth. The combination of less water available and light levels dropping back from their June 21 peak will cause plants to reduce new growth. This natural mechanism, which can be seen in rhododendrons that have fully expanded new shoots and are now forming flower buds, protects the plant. If woody plants continued to grow without stopping, right into fall and up to frost, the newest growth would risk frost damage and the plant as a whole might suffer winter-kill.
   In spite of these facts, gardeners often feel that by adding more fertilizer to shrubs, they help the plant. New, succulent growth that might emerge now will give plants a temporary look of health, but it's not desirable in the long run. Roses, for instance, require a regular monthly feeding up to the middle or end of July, but no later. The growth that's now forming will stay on the rose until frost, blooming again and then setting rose hips (attractive red seed pods) to carry the plant through winter. Roses are susceptible to winter kill, if temperatures fall abruptly to Arctic levels, as seems to happen every few years. Too much fertilizer now will add to the possibility of winter kill by leaving a lot of vulnerable new foliage on the plants.
   Disease and insect difficulties can be worse on plants that have received too much fertilizer, especially those that have too much nitrogen applied. Tender, new overfertilized growth attracts more aphid attacks, aphids being sucking insects who enjoy the plant juices loaded with carbohydrates. And powdery mildew, a common late-summer shrub problem, will also be more serious on over-fertilized plants, since it generally attacks newest foliage first.
   In general, the best time to fertilize trees and shrubs is from March through May, in the most active growth phase. Plant growth then coincides with available rainfall to take fertilizers down to the roots. In late summer, natural water supplies are far more restricted. Fertilizer must be accompanied by water; plant roots can't absorb the nutrients unless the material is washed into the soil and, in the case of organic compounds, broken down by soil microorganisms. Do not fertilize shrubby plants if not prepared to water them. It's better to allow the natural, slower rhythms of late summer to move into the garden rather than attempting to force growth with fertilizer, and with too much water. Also, excess nitrogen and phosphorus, applied in quantities the plants cannot absorb, may be washed into surface water, lakes and Puget Sound, where the nutrients cause over growth and algae and other undesirable effects.
   As much as we dislike thinking of winter, from the gentle mid-days of July, it's time to allow our trees and shrubs to adapt to the coming season. Keep that liquid fertilizer squirter in the garage or basement until spring. Should you spread compost? Yes. Applying composts and mulches, right now and right through winter, is fine for all landscape plants, because they do not contain high percentages of plant nutrients. All the benefits of mulch, such as water retention and weed suppression, are excellent for plants in late summer and into fall.
   Is there anything that still needs fertilizing? Unlike roses, trees, and shrubs, some plants do require fertilizer now. Any flowering annuals or perennials that are being kept in pots should be fertilized at least once a month until frost to keep the new leaves in good health. These plants, such as petunias, marigolds, and impatiens, are being kept in artificial states of youth in order to keep them blooming. Use a fertilizer with a fairly low nitrogen content--fish fertilizers supply trace elements and are excellent for container plants. Fertilizer boxes are marked with numbers indicating the percentages, by weight, of the nitrogen, phosphorus and potassium in the mix. A 5-10-10 is a good local all-purpose fertilizer (5% nitrogen), as is a 10-20-20 (10% nitrogen).
   "Bloom More" type fertilizers contain no nitrogen, just phosphorus and potassium, since nitrogen tends to stimulate green leaves rather than flowers. However, for container plants containing annual flowers like petunias, some nitrogen helps, since the emergence of new green leaves helps keep the plant looking fresh until frost.
   If your vegetable garden is being re-planted with fall and winter crops in late July, dig in about 5 pounds of 5-10-10 per 100 square feet of vegetable garden to add nutrients that were removed by the summer crops. If a vegetable or flower garden will be empty over winter, plant a "cover crop" using seeds like vetch, crimson clover, or buckwheat to grow a quick crop that will suppress weeds and be dug into the ground in early spring to add fertility to the soil.
   Fruiting strawberries that produced early summer crops (not ever-bearing strawberries) should be side-dressed in August with 5-10-10.
   Houseplants can still use fertilizer during August, but like the plants outdoors, they will slow growth in fall and winter, and shouldn't receive much fertilizer once days get dark and short.
   So, remembering that plants aren't like people, stop adding fertilizer to areas of the garden that don't need it, can't use it, and may be positively harmed by it at this season of summer. It's possible to be too kind to the garden.

Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.