Trees need nutrition, just like we do. When one or more nutrients are deficient, they will not reach their full landscape potential. Poor nutrition can make your trees more susceptible to diseases and insects, and may lead to a shorter life span and inferior growth than a similar well-fertilized tree.
The nutrients that trees (and all plants) require can be broken into two categories:
- Macronutrients: Nutrients that are taken up in large amounts. These include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). When you buy a bag of fertilizer you may notice the formula 10-3-4 or some similar arrangement of numbers. This indicates the pounds of N, P, and K per 100 pounds of fertilizer. For example a bag with a formula of 10-8-6 contains: 10 pounds of nitrogen, 8 pounds of phosphorus (as P2O5), and 6 pounds of potassium as potash per 100 pounds of fertilizer. A fertilizer with all three elements (i.e. N, P, and K) is called a complete fertilizer.
- Micronutrients: Micronutrients are required on a much smaller scale but are also important to plant health. They include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), chlorine (Cl), and molybdenum (Mo).
Urban trees are often growing in an environment with many stresses that include low moisture, compaction, and competition from turf grass and other landscape plants. Providing adequate nutrition for your tree can help mitigate these stresses.
The best way to tell if you need to fertilize is to contact a tree health specialist, who may perform a soil test. Using soil sampled from the area around the tree, a soil test can tell you the levels of macro and micronutrients available to the tree.
The color of the leaves may also be an indication of nutrient deficiency. If the leaves on your tree are yellow or “off-color,” you may need to fertilize. Keep in mind the color will naturally change from bud break to full leaf-out
A good time to fertilize is in the spring right before, or as new growth starts or wait until mid-summer when most new growth has stopped. Fall fertilization is also very beneficial because it allows the trees to store more energy over the winter and prepare for an extra special spring.
For established trees apply the fertilizer at the drip line of the tree, not at the base of tree. The drip-line is the area right below the edge of the canopy.
Do not fertilize a thirsty tree. It is difficult for trees experiencing drought stress to absorb the fertilizer and it could actually damage the roots.
I hope this helps you in your gardening endeavors. I recently read that urban trees make people happier, not to mention the shade, beauty, and improved air quality they provide. Therefore your efforts to beautify your landscape are a benefit for others as well as yourself.