Many gardeners have the impression that the more fertilizer they apply the more the plant will grow. Fertilizer is not plant food. Plants use water, carbon dioxide, elements from fertilizer, and energy from the sun to produce their own food. Synthetic and natural fertilizers provide the nutrients for plant growth, but not the actual food for growth. In other words, fertilizer doesn't feed a tree, it makes it easier for a tree to feed itself.
Correct fertilizer amounts promote healthy flower production and foliage growth, while excessive fertilizer decreases plant health and even leads to decline and death. Over application or incorrect fertilizer applications contribute to polluting our waterways. Excess fertilizer can even increase the incidence of some plant diseases.
A moderate rate of growth and good, green color is best for most woody plants. Excessive vigor, which is evident by lush, green leaves and long shoot growth is often undesirable. Such trees are more susceptible to freezing injury, are more likely to break during wind/ice storms, and require more pruning than trees exhibiting moderate growth.
Don't fertilize to make a young tree bigger, faster. Think of it as feeding your children more food so they'll reach a mature size faster. It just doesn't happen that way. Overfeeding our children creates obese kids that have troubles now and in later life. The same goes for raising trees.
Wait until spring to fertilize fall planted trees and shrubs. Wait 6 — 8 weeks to fertilize trees planted in the spring. Use slow release fertilizers in a light band along the perimeter of the planting hole.
Newly planted trees are under stress and should receive only a light application of fertilizer. For 1 gallon container plants, apply 1 teaspoon of a nitrogen fertilizer or 1 tablespoon of 10-10-10, for larger plants apply 2 — 3 tablespoons.
Ideally you should base fertilization rates on soil test results, plant age, current and desired growth rate, plant type, or by using a few general guidelines. The rate should also be influenced by rainfall and soil type.
A wet growing season will normally increase the need to fertilize, especially in sandy soils. During periods of dry weather, reduce the amount of fertilizer. Fertilizer encourages water-demanding new growth and can injure roots of trees and ornamentals experiencing drought stress.
A nitrogen application will have its greatest effect 3 — 4 weeks later. Like lawns, woody plants can absorb nutrients as long as the soil temperature is above 40°F. Root growth occurs during cool weather even when the foliage appears dormant. Root growth of woody ornamentals is most active in fall and late winter/early spring but slows during hot, summer weather.
Fertilize trees and shrubs in the spring or fall. Spring fertilizer application should be made before new growth starts. Fall fertilization should be made approximately 1 month after the first killing frost. Many gardeners are reluctant to fertilize in late fall for fear it will stimulate new growth if a period of unseasonably warm weather occurs. However, this is unlikely if applied late in the season. Late fall fertilizer is more effective in promoting plant growth than a spring fertilization.
Late summer fertilization (mid August) should be avoided since it actually may stimulate late growth that will not harden off before frost. Applying slow-release fertilizers around trees or shrubs should not be applied late in the season (after July 15) because they may keep the plant growing rapidly late in the summer. This late season growth may not "harden off" completely, and winter damage may occur.
Fertilizer should be spread evenly over the entire root zone which can extend 2 — 3 times the width of the branches. Remember that some of the root zone may have already been fertilized when fertilizer was applied to the lawn or flower bed. Sprinkle the fertilizer on top of the soil or mulch, and water lightly. The fertilizer will move quickly through the mulch and there is no need to remove the mulch or trying to place the fertilizer below the mulch.
Spread the fertilizer evenly under the branches. Don't dump fertilizer in one spot. This can cause roots below the fertilizer to burn and die. Avoid getting fertilizer on the leaves. If fertilizer lodges in the whorls of plant foliage, use a broom to brush the fertilizer off.
Placing fertilizer in holes around mature trees has been tested and research indicates that surface application of fertilizer is sufficient and the hole method is not necessary since most of the tree's feeder roots are in the top 12" of the soil.
However, if you have compacted soil, bore holes 4" — 6" deep, 2' — 3' apart using a punch bar or drill with a 2" auger. A good tool is a bulb planting auger sold in garden centers. Start 2' from the trunk and continue out 2' beyond the branches (drip line). Divide the fertilizer into as many equal parts as there are holes and place in the holes. Boring holes in soil can increase soil aeration and water penetration into the root zone.
Fertilizer spikes and stakes driven into the ground contain satisfactory amounts of nutrients. Unfortunately, the spacing is such that very little fertilizer comes into contact with most of the root system. Lateral fertilizer (side-to-side) movement in the soil is very limited.
Foliar sprays of a liquid or water soluble fertilizer on the foliage can be used for correcting deficiencies of minor elements such as iron or manganese. This method should not be used to provide all of a plant's fertilizer needs.
Benefits from foliar sprays are short lived. Since nutrient deficiency is often caused by a disease or improper soil condition (pH, drainage, soil compaction) foliar sprays give only temporary relief and do not correct the main problem.
Tree injections of micronutrients is another method of fertilization that should be used only as a last resort. Trees can be permanently injured by drilling holes. Decay could develop and may out weigh any benefit the fertilizer might provide. Any benefit from the fertilizer will be temporary at best and it does not change poor soil conditions.
See: Mauget Injections
Do not use weed-and-feed fertilizers under trees or shrubs unless the label says it is safe. Some plants, such as dogwoods, are very sensitive to dicamba herbicide that is contained in many weed-and-feed lawn fertilizers.