FEBRUARY 10, 1997
Gardening: Feeding, techniques for spring fertilization
by Mary Robson, WSU Area Extension Agent
Even with the wild weather this winter, landscape plants have begun to revive slowly and leaf buds are swelling with new life. Spring, as plants move into active growth, is the ideal time to add plant nutrients in the form of various fertilizers. How can fertilizers be used effectively, economically, and safely on trees and shrubs?
"Plant Food" is a common term, and it often appears even on bottles and boxes in garden sales departments. It's an inaccurate phrase. Green plants make their own food, synthesizing carbohydrates from sun, water, and nutrients in the marvelous process named photosynthesis. We depend on the ability of plants to make food as plant materials are the source of most human and animal food. What we loosely call "plant food" is actually mineral nutrients that help the plant manufacture its own food.
For health, most green plants need the carbon, hydrogen, and oxygen they get from air and water. Most of the plant nutrients needed are normally present in soils. However, gardeners often want to add nutrients to enhance or improve plant growth, particularly if plants show signs of poor growth. Plants that are stressed for lack of mineral nutrients may have very small leaves, little growth in a year's time, yellowish leaves, and poor fruiting or flowering. Many dead twigs and branch tips can also mean the need for additional nutrients.
Be careful about concluding that these symptoms mean only a lack of nutrients. They could be caused by an insect or disease difficulty. They could also mean root rot, root damage, poor soil drainage, or a plant in the wrong place, such as a sun loving plant in a shady area. Check the roots of any plant you suspect of looking peaked for lack of fertilizer. Fertilizer won't help a plant with root problems; it will make the condition worse because the plant has no ability to take up the nutrients and the damaged roots may be burned or further injured.
If you determine that the shrub or tree has healthy roots, and just needs a growth boost, consider a spring fertilization. If a landscape is newly installed, fertilize trees and shrubs during their second and third year of growth, and perhaps up to the fifth year. Don't fertilize plants when first installing them; use only mulch for the first full season of growth. If a plant has suffered serious limb damage or breakage in winter storms, proper fertilization can help the plant reestablish new growth. Vegetable and fruit gardens and plants in containers need regular fertilizer. Add nutrients to the vegetable garden site yearly. Feed container plants in active growth about every three weeks, because the regular watering needed to keep them growing leaches out nutrients.
For established garden trees and shrubs five years old or older, no additional fertilizer may be needed, even in spring. It's important to keep in mind that over-fertilization, especially with processed fast-acting products, can be harmful to surface and ground water by running off. Many people use fertilizers too often and in too much quantity. The main plant nutrient needed by trees, shrubs, flowers, and vegetables is nitrogen, which contributes to green leaf tissue growth and general plant health. Gardeners reach for boxes of fertilizer that add nitrogen, phosphorous and potassium. Phosphorous and potassium contribute to root and stem strength and general plant health, but they persist longer in soils and are not as likely to be deficient as is nitrogen. Follow label directions and do not add more fertilizer than suggested by the product. Apply fertilizer across the root zone of plants, and be sure it's watered in.
Plant roots can take in nutrients only in solution. Putting dry fertilizer on dry soil doesn't work. Gardeners often wonder whether organic fertilizers are preferable to processed fertilizers. The processed type, chemically produced from petrochemicals, dissolves rapidly in water and is readily available to plant roots. These types are easy to misuse, because they can burn plant roots if over-applied. If used property, processed fertilizers work effectively to promote plant growth.
Organic type fertilizers are derived from previously living substance and rock powder, and can be as diverse as biosolids, animal manure, cottonseed meal, bone meal, and granite powder. Plant nutrients from these sources aren't necessarily superior to other forms, but they do have some distinct advantages. They are slowly broken down by soil micro-organisms into a form that's available to plant roots. They supply nutrients gradually and contribute to the health of the soil fauna (soil micro- and macro-organisms), especially when used in concurrence with mulches. What form of fertilizer the gardener chooses depends on philosophy and garden need.
Compost, the final break-down product of "green" wastes such as tree and shrub trimmings, leaves, and weeds, isn't a fertilizer. It does contain measurable quantities of essential plant nutrients, but not in sufficient amounts to classify it as a fertilizer. Research on compost use is revealing that compost offers benefits to soil health and is an invaluable soil addition.
Choosing and using fertilizers should be done in the context of total garden management. Add organic mulches, 2-3 inches a year, for weed control and soil protection. Renew mulches as they break down. Work compost into the soil when preparing new garden sites (not just in the planting soil, but across the entire area.) Soil amendments contribute to the long-range health of the garden. Fertilizers, applied in the spring (February-April), give plants a "boost".
If land is sandy, slopes, has a very shallow water table, or is near a well head, be extremely careful with fertilizer applications in order to protect water quality. With fertilizer, it's all too easy to use too much of a good thing. For more information, check with your local Cooperative Extension office for relevant publications: EB1722 "How Fertilizers and Plant Nutrients Affect Groundwater Quality," and EB1034 "Fertilizing Landscape Trees and Shrubs." Both publications carry a small charge, plus handling and mailing. In Pierce County, call 206-591-7170 9 a.m. to 3 p.m. weekdays; in King County, call 206-296-3900.
Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.