Large trees are often overlooked by homeowners when they undertake a fertilization program on their property. Well established shade and ornamental trees seem to be self-sufficient.
However, they too, benefit from periodic fertilization, which envigorates them and makes them less vulnerable to insects and diseases. To determine whether a particular tree needs fertilizer, check its twig growth. If new growth over the past year (the length of the twig from its tip to the slightly raised ring indicating where growth started last spring) is less than two inches, consider fertilizing. If it is more than four inches the tree probably does not need feeding. As a rule of thumb, fertilize any tree that has not been fed for several years, or that is not growing at a normal rate.
To fertilize trees effectively, it is important to understand how their roots are structured. A tree's root zone spreads over an area about a third again larger than the breadth of its branches. Like the branches, the larger roots are near the trunk and the small fibrous hair roots that actually pull in the nutrients are farthest away. So, the roots that actually feed the tree are in a circle beginning about a foot or two from the trunk and continuing to several feet beyond the spread of the branches. It is important to spread fertilizer in this zone. If it is put any closer to the trunk, it is likely to be wasted.
Once ornamental and shade trees reach maturity it is difficult to feed them effectively. It is recommended that professional arborists be hired to evaluate and feed valued specimens and those trees that have health problems.
The Main Meal
We recommend fertilizing young trees to give them a good start, whether you use organic mulch or not. After the tree is about 20 feet tall, we believe that the organic mulch will provide enough food for the tree. If later in its life a tree needs fertilizer, the job is for a professional who uses special tools and fertilizers, not available to homeowners.
Feed newly planted or transplanted trees 6 months after they are planted, then once every fall for the first 3 to 4 years. Then feed them once every 2 or 3 years until they reach 15 to 20 feet, in late fall as leaves are dropping. Apply an all-purpose slow acting granular fertilizer using a (drop)seeder spreader. Yardeners should not apply fertilizer too close to the trunk. The tree’s feeder roots begin about 1 to 2 feet further out and radiate in a circular zone to several feet beyond the farthest spread of the branches. As a rule of thumb, use ½ pound of slow-acting fertilizer for each ½ inch of trunk diameter as measured 4 feet from the base of the tree. Spread the fertilizer within an area at least 1-1/2 times the size of the area under the tree’s drip line.
For more information see file on Choosing Fertilizer.
Calculate Fertilizer Area
Tree spikes, sold to fertilize trees, are not effective in providing a large tree with all the nutrients it needs. If doing the job themselves, homeowners should use a standard slow release all-purpose granulated fertilizer for more satisfactory results. Sprinkle this on the soil over the root zone, the size of which determines how much fertilizer is needed.
1. Estimate the number of feet from the base of the trunk to the farthest edge of the root zone. This may be a foot or two beyond the reach of the longest branches.
2. Multiply this distance by itself to arrive at a number of square feet.
3. Since most general purpose fertilizer packages indicate amounts needed for every 1000 square feet, it is easy to calculate from this how much fertilizer is necessary for a specific tree.
Foliar sprays can be used for speedy correction of micronutrient deficiencies in trees, but they do not serve as the primary source of nutrients for the tree.
The best way to avoid a deficiency problem is to see to it that your trees get a yearly feeding of nitrogen. Starting when the trees are three years old, add 0.05 pounds of nitrogen per tree for every year of age. Stop increasing the dose when the trees turn ten.
The tree's growth rate will also be sluggish. Here are some general guidelines that can help you gauge whether your tree is growing as it should. Established peach trees should have shoots that grow 8 to 18 inches a year, while the shoots of young trees should grow at least 12 inches. Long shoots (24 inches) that have grown too fast generally don't produce many flower buds, nor do shoots on the short side (3 to 4 inches). The shorter shoots also tend to produce smaller fruit. A very good way to avoid a nitrogen deficiency is to establish a regular feeding schedule. Every spring apply 0.05 pounds of nitrogen for each year of the tree's age. Start when the tree is three years old; once it reaches ten years, stop increasing the dose. Be careful not to overdo it with the fertilizer, though. Too much nitrogen will result in soft fruit with a washed-out color and insipid flavor.
For information about the various kinds of fertilizers go to Fertilizers in the Tool Shed section.
Snacks and Vitamins
Besides a routine annual “main meal,” you can give your young trees optional “snacks” and “vitamins” to boost growth. These compounds are applied as foliar sprays that are absorbed directly through leaf tissue instead of through the roots. While not essential to survival or basic health, they give trees an extra kick that will help them better withstand drought periods and insect attacks, and may even help them produce brighter flowers and larger fruits. Although not really practical for large trees, snacks are most appropriate for small trees during their first 5 years.
The best time to do this is late June or early July as the plant is moving into the high heat of summer. Use a compression sprayer and any water-soluble garden fertilizer. Fish emulsion is best, but there are many others that are satisfactory. Mix the spray and apply it to upper and lower leaf surfaces in the morning before the sun gets too hot. You can use this same technique to give your trees some extra “vitamins” in the form of a seaweed or kelp-based plant tonic that contains trace micronutrients such as boron, magnesium, and zinc, which can make a difference in the tree’s overall health.
What About Tree Spikes?
The most common fertilizer sold in nurseries and garden centers specifically for trees and shrubs is available in the form of “spikes.” They look like tent pegs and are designed to be pounded into the soil around the tree, where they are supposed to slowly dissolve to release nutrients. Spikes are not really satisfactory in most situations. They tend to concentrate the nutrition in a few small areas, out of reach of the many widespread feeder roots. Also, many more spikes than the number indicated on the label are needed to properly feed a single large tree. Although 4 or 5 spikes might work for a 5-foot-tall sapling, a mature oak tree would require 80 to 100 spikes to supply adequate nutrition, even though the package says 10 are enough. At this rate the cost of spikes becomes prohibitive. Granulated fertilizer can be spread more evenly and is less expensive than spikes.