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Standing in a garden center looking at fertilizer packages can be confusing. What to apply? When to apply it? The best time to provide fertilizer to all types of landscape and house plants is from now forward through spring, continuing during the period while plants are in active growth. And in order to do this, it's helpful to understand some specifics about the way fertilizers work.
There's a common popular message that "plants are like people," which isn't really true. The biology and metabolism of plants differs from that of people. Words we use like "plant food" tend to encourage beliefs that plants need regular meals every day like us, or should be fed their "vitamins" often.
Assuming that "plants are like people", gardeners often misunderstand fertilizers. Fertilizers are various combinations of plant nutrients and minerals that most plants use, along with water and sunlight, to make their own food. Green plants, the most common in our gardens, manufacture carbohydrates through photosynthesis, using nutrients and light to create their food. (Parasitic plants, such as dodder, attach themselves to another organism and let it make their food.)
Humans and other animals are able to gain nourishment by eating plants precisely because the plants have made, and stored, food supplies. We depend for our own lives on the abilities of plants to make and store food.
Perennial plants, such as trees, shrubs, and herbs, store food supplies during dormancy (over winter) and use these stored supplies to begin spring growth. The purpose of adding fertilizer is to support the plant's needs as it uses the stored food up in leafing out and in early spring growth.
What needs to be fertilized? Plants that are leafing out, plants that were installed in the garden last summer, and plants like ground covers and hardy flowers can use moderate amounts of fertilizer now and during active growth season. Twigs and leaves produced in spring will gradually grow, harden off and cease to add new growth as the season progresses. Looking carefully at a twig, it's possible to see the spot where last year's growth starts and stops.
In general, the best time to fertilize trees and shrubs is from February through May, during 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 applications must be put on moist soils and accompanied by watering or rains. Plant roots can't absorb the nutrients unless the material is washed into the soil. Soil microorganisms break it down into compounds that plants can use.
Choices of which fertilizers to use depend on the nature of the landscape. Evergreen conifers, rhododendrons, and azaleas benefit from acid-based fertilizers. Look for packages specifically labeled for these plants.
Fertilizer packages are marked with numbers indicating the percentages of plant nutrients, 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.)
Other landscape plants will do fine with a standard 5-10-10 analysis, or a 10-20-20. The nutrient most needed by plants during the spring is nitrogen, the first listed component on fertilizer packages. Follow the instructions on the label as to how much to apply, and don't get in the habit of over-fertilizing plants.
For landscape plants, one spring fertilization combined with a good mulch of compost or other organic matter should suffice throughout the year.
Some plants and garden areas do need fertilizer more than once a year. Consider what's growing, and the period of growth. Container plants, annuals or perennials that are being kept in pots should be fertilized at least once a month until frost to keep them growing well. These plants, such as petunias, marigolds, and impatiens, continue to put on active growth throughout the entire summer season. Use a fertilizer with a fairly low nitrogen content if the intention is to keep plants blooming.
"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.
For the vegetable garden, dig in about 5 pounds of 5-10-10 per 100 square feet of vegetable garden to add nutrients that were removed by last summer's crops. Side dress when you harvest and replant crops, since vegetables are in continuous active growth and utilizing soil nutrients throughout the season.
Roses require a regular monthly feeding up to the middle or end of July, but no later. The growth that's now forming will support the blooms, and each rose requires about 40 healthy leaves to produce one flower. A good program of rose fertilization is essential until mid-summer. Over-fertilized roses are susceptible to winter kill, if temperatures fall abruptly to arctic levels, as seems to happen every few years. Too much fertilizer in mid-summer will add to the possibility of winter kill by leaving a lot of vulnerable new foliage on the plants.
Consequences of over-fertilization: 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 over-fertilized 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, most succulent foliage first.
Also, excess nitrogen and phosphorus, misapplied in quantities plants cannot absorb, may be washed into surface water, lakes and Puget Sound, where the nutrients cause over growth of algae and other water polluting effects. Do not apply fertilizer where it will wash down slopes during heavy rains.
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