University of Vermont
Department of Plant and Soil Science

Spring News Article


Dr. Leonard Perry, Horticulture Professor
University of Vermont

Have you fed your trees recently?  If not, they may be quite hungry, if not starving.  The best time to fertilize is in spring, after the frost leaves the ground but before trees begin active growth.  On sandy soils, where nutrients rapidly are “leached” through them with rains, you may need to apply fertilizer twice—in early and mid-spring. 
If you don’t get around to fertilizing in spring, the next best time for trees is October.  This is late enough to not stimulate new growth, but early enough that some nutrients will be taken up by roots for use the coming season.  Since we don't see the roots, we often don't realize that they continue growing and absorbing nutrients long after the leaves fall, often into December, and begin work again in the spring before the leaves return.  After all, they must be absorbing nutrients and water to enable the leaves to resume growth.
A tree may be getting adequate nutrients from the soil already, but it may benefit from additional fertilizer to keep it growing at its best.  A healthy, vigorous tree is much less susceptible to attacks from disease, insects, and other stresses.
The 1998 ice storm showed the difference between healthy and stressed trees.  One of the hardest cities hit by this storm, Montreal, lost many street trees to ice damage-- trees stressed by many factors including inadequate fertility.  Trees in the botanic garden, however, were little affected-- trees which had received proper care.
Trees benefit from all of the elements, but usually respond more to applications of nitrogen.  Often there is adequate phosphorus in soils from previous fertility.  In fact, unless a soil test calls for phosphorus, you should not apply it as this is illegal in many areas (since it washes into and pollutes watersheds from stimulating algal growth).  A soil test (kits are available from Extension service offices and some complete garden stores) is your best bet to know how much fertilizer to add, or if any specific nutrients are lacking or are needed and in what amounts, so you don’t add too much or too little.
Trees planted in a lawn will benefit from the same fertilizer as put on the lawn, so if you have fertilized the lawn last spring or early fall, there is probably no need to fertilize trees planted in it.  If not, a complete fertilizer (one such as 10-0-10 containing nitrogen-zero phosphorus-potassium, respectively), put on with a fertilizer spreader at 2 to 4 pounds for each 100 square feet, should be adequate.  Trees will need differing rates of fertility, depending on their stage of life, and growth as dictated by other cultural and environmental factors.
If a tree is newly planted, it should only need one to 2 pounds (per 100 square feet) of a 10 percent nitrogen fertilizer. In subsequent years, use the 2 to 4 pound rate.  When mature, back off again to the lower rate, or perhaps none.  If a tree has put on over 6 inches of new shoot growth the previous season, no fertilizer likely is needed.  If shoot growth was between 2 and 6 inches, fertilize with 2 to 3 pounds of a 10 percent nitrogen fertilizer.  If shoot growth was less than 2 inches, fertilize at the 3 to 4 pound rate.
Other signs that a tree may need fertilizer are:
1) it makes very little growth, even though it is established and there is adequate rainfall;
2) its leaves in midsummer do not have a good green color, but are yellowish;
3) its leaves gradually become smaller, year after year;
4) its leaves turn to their autumn color and drop in August or early September.
Yellowed leaves also may indicate that the soil acidity, or pH, needs correcting.  This controls a plant’s ability to absorb nutrients.  A soil test will let you know this level and, if too low (often the case in our region), how much lime to add to raise the soil pH. With 7.0 being neutral, most trees grow best in slightly acidic soils (pH 6.0 to 7.0), while evergreens such as spruces and pines often grow best in more acidic soils (pH 5.0 to 6.0).  A soil test also will let you know if a specific nutrient is lacking, causing leaves to yellow.
For trees not planted in lawns, you also may broadcast fertilizer on the surface.  Research has shown this to be effective, reaching tree roots, even in lawns.  Just make sure NOT to use a fertilizer containing weed killer herbicides, or this may be taken up by tree roots and harm or kill your trees.  Instead of broadcasting fertilizer, you may choose to “root feed.”
A common method of root feeding for home gardeners is to buy spikes of tree fertilizer and drive these down into the soil.  Another method is to make holes in the soil with a crowbar, or similar tool, and pour fertilizer into these holes.  Holes should be about 18 inches deep and 1-1/2 to 2 feet apart.  They should start about 6 feet out from the trunk of older and larger trees, and extend out about 6 feet beyond the spread of the branches—the same area as when you broadcast fertilizer.  For younger trees, make holes about every two to three square feet.
Another method of root feeding is to use a tube you attach to the hose.  On the hose end is a container to add fertilizer tablets.  Simply push into the ground, turn on the water, and the fertilizer solution is injected into the root zone.  Use similar spacing as above.  This is the method usually used by tree care professionals.  Such wands can be found in complete garden centers and specialty garden supply catalogs.

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