Fertilizing trees refers to the practice of supplementing soil nutrients with some of the 17 chemical elements required for growth and development.
Trees are autotrophs: they use nutrients to feed themselves by making sugar in the leaves that can be used right away, or stored as starch for future needs.
Do I need to fertilize my trees?
Often, you don't. Most soils have enough nutrients for most trees.
Trees adjust their growth and development rates to the level of nutrients, and will usually make out all right as long as the roots can continue to grow.
In nature,
trees get nutrients from air, organic matter, and soil minerals. Organic matter can be restored for urban trees by keeping composted leaves and twigs under the crown.
Clayey soils
(common on urban sites) present special problems in the availability of micronutrients like iron for some trees. A future factsheet will discuss this topic.
Lack of water and compacted soil have been shown to be much more limiting to the growth of urban trees than low nutrient levels in most cases.
What conclusions are supported by research?
Young deciduous trees
benefit from some additional nitrogen (e.g., 10-6-4). Since plant growth is irregular and nitrogen is mobile in the soil, use a slow-release fertilizer.
Conifers rarely need fertilization at all,
since most are adapted to low-nutrient soils.
Serious pest and structural problems result on trees that are overfertilized.
Highly fertilized trees often attract more diseases and insects. Their shoots are weaker as well.
Surface application
is the easiest, cheapest, and most effective method of fertilizing ornamental trees on most soils. Other methods also work, but are rarely necessary.
Aeration and watering
of urban trees often produce the same growth response as fertilization. They facilitate root growth and nutrient mobility through the soil.
Trees surrounded by grass
benefit from the application of a complete fertilizer every few years. If you apply winterizing fertilizer to the grass, this is unnecessary.
When should I fertilize the trees?
Slow-release low-nitrogen fertilizers
should be applied in early fall or early spring. They will be used by the roots in the fall, and by roots and then shoots in the spring.
Standard fertilizers
should be applied in late May or early June.
Where can I get more information?
"Fertilizing Trees and Shrubs" (Cornell University, 1989, rev. 1997), available from your local office of Cooperative Extension or the DEC, or call us at 716-461-1000.