University of Vermont
Department of Plant and Soil Science
TREES NEED FEEDING?
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
2) its leaves in midsummer do not have a good green color, but are
3) its leaves gradually become smaller, year after year;
4) its leaves turn to their autumn color and drop in August or early
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
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
Return to Perry's Perennial