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- 7. It's Still to Early to Fertilize Landscape Plants - Top
Don't be too hasty to get plants off and growing with early fertilization. There is still a good chance that we will have freezing temperatures. Fertilization now will promote premature growth that can be damage by cold weather.
Some plants like bougainvillea, verbena and lantana began breaking winter dormancy in February with new growth. Subsequent cool weather has staved off full emergence of the spring growth flush. Hopefully cool weather will continue, holding plants in a protective dormancy until the danger of freezing temperatures has past in mid-March.
We cannot control unseasonable weather and it's altering effects on normal plant growth, however, we can wait to fertilize these plants until it is appropriate to do so.
Fertilization of landscape plants should be delayed a few more weeks. Most trees, shrubs, ground covers and vines respond well to an application of fertilizer in mid-March, just prior to the normal spring flush of new growth. This is the time when the bulk of growth for the year will occur. If you wish to increase a plant's size, the application of nitrogen will accomplish that. If you want to maintain a plant at it's current size, then nitrogen should not be applied unless plants are clearly deficient.
Nitrogen deficiency is manifest by stunted growth and small, off-colored leaves. If leaves are lighter green than is typical for a specific species of plant, then it is likely that nitrogen is lacking. It is possible to correct nitrogen deficiencies with light applications of nitrogen without stimulating excessive growth. A light application can be made by applying half of the normally recommended amount of fertilizer.
In the early spring, even with warm temperatures, the soil has not heated up sufficiently to allow the use of organic sources of nitrogen. These materials; manure, blood meal, fish emulsion, and others have to be broken down by soil microbes that only become active when soil finally warms in April.
The best sources of nitrogen for early spring fertilization are chemical forms such as urea, ammonium, and nitrate. These are listed under the ingredient statement on all fertilizer labels. Probably the least expensive of all is ammonium sulfate (21-0-0). It is available most places where garden supplies are sold.
The secret to success with spring fertilization of landscape plants is in the placement of the fertilizer itself. Spreading the fertilizer evenly over the roots is the only way to get efficient uptake of the nutrients. Keep in mind that roots, regardless of the type of plant, extend out well beyond the spread of the branches. At a minimum, spread fertilizer evenly under the branches and out several feet beyond as well.
For trees, apply one pound of fertilizer for each inch of trunk diameter, measured one foot up from the ground. For example, a tree with a 3 inch thick trunk would require 3 pounds of fertilizer.
For shrubs, apply one-half "cup" of fertilizer for small shrubs up to 3 feet high, and one cup of fertilizer for larger shrubs. In the case of low, spreading shrubs used as ground covers, apply 1 pound of fertilizer for every 100 square feet of planting area. In the process of applying the fertilizer to low-growing plants, be sure to wash any granules off of the leaves using a strong spray from the garden hose. Granules left on plant leaves can cause fertilizer burn.
Regardless of the amount of fertilizer or the plant it is being applied to, water the fertilizer into the ground soon after applying it. The best method of "watering-in" fertilizers is by using a soaker hose or portable lawn sprinkler.
Cacti, succulents and palms should not be fertilized in early spring. These heat-loving plants will not respond to fertilization until day temperatures have risen and are consistently maintained at 80 degrees and higher. Fertilizing in cooler weather will at best waste fertilizer, and at worst cause plant injury.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
- Updated: February 23, 2003
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