WEST FEATURES
Should you Fertilize Trees?

by Marcia Passos Duffy
How to determine if your trees are getting enough nutrients

Forest trees grow and thrive without the addition of fertilizer, which can give the impression that trees in a turf landscape will also do just fine without additional nutrients.

However, shade and ornamental trees in residential, commercial or street landscapes live in a different, and unnatural, environment compared to forest trees. Trees living in a turf landscape, while they may look fine, live only a fraction of their 100-plus year life expectancy. According to research by the USDA Forest Service, the average downtown urban tree lives only about 10 years; life expectancy for the same tree in a rural area is 150 years. The reasons: pollution, salt, soil compaction, drainage problems, insect, diseases and, most importantly, nutritional deficiencies.

Arbor Green Pro can be applied by injection.

While trees in the forest benefit from soils naturally rich in humus—replenished yearly from the decay of leaves and other organic litter on the forest floor—trees in a tidy landscape are often lacking sufficient nutrients for optimal health.

“Research has shown that forest trees get half of their annual requirement of nutrients from leaf litter,” said Dr. Roger Funk, plant physiologist with Davey Arbor Green Pro (www.arborgreenpro.com), a tree preservation company that also provides shrub and lawn care throughout North America. If you remove the leaf litter, you remove half the nutrients, which include nitrogen, phosphorous and potassium. Studies estimate that leaf litter that is left in place to decompose recycle at least 1 pound of nitrogen per 1,000 square feet of surface area.

Turf landscapes pose a unique problem to trees because grasses are in direct competition with trees for nutrients. “That’s why you don’t see a whole lot of trees in grass plains, because they are both trying to extract the exact same nutrients from the soil,” said Funk.

Apply fertilizer, but in the right place

The solution is not to apply more fertilizer to the turf so that the trees can have their share, but to apply fertilizer deeper into the root system so that the turf can’t get at the fertilizer that is intended for trees, said Funk, whose company uses an injection launch to place the fertilizer laterally into the ground near the root system of the tree (4 to 12 inches deep into the soil).

While the fertilizer needed for trees is similar to turfgrass (nitrogen, phosphorous, potassium), the ratio is slightly different; trees and shrubs require 3:1:1, as opposed to turf’s 4:1: 2. Trees also prefer slow-release nitrogen, similar to the way leaves decay. “If you fertilize with nitrogen that is not slow releasing, it can harm the tree,” said Funk. The key, he says, is to follow nature and avoid putting down quick-release nitrogen.

Funk recommends using fertilizer two times a year: during active tree growth during leaf drop and bud growth in the spring to imitate nature’s own release of naturally occurring nutrients. “Remember, there is usually no leaf litter decay in the heat of the summer or dead of winter,” said Funk.

Mimicking nature’s own release of nutrients is the optimal way to fertilize a tree, agrees Mary Tebo, community forestry educator at the University of New Hampshire Cooperative Extension. “It is important not to overdo it,” said Tebo, the author of the recent book, “Following Nature’s Lead: Integrated Landscaping for New Hampshire” (published by UNH Cooperative Extension). Overfertilizing can push an already-stressed tree over the edge, said Tebo. She suggests a more conservative approach by liming trees around the root system in the fall and fertilizing in the spring.

Treated with Arbor Green Pro (on left) compared with untreated.

Mulching

You can avoid the fertilizing question completely if you mulch trees over the entire root system and keep grass growth away. Tebo suggests mulching one and a half to two and a half times the width of the canopy of the tree if the mulching is to be effective in bringing nutrients to the roots. “If you can mulch, it is really the best thing for the tree,” said Tebo, adding that even a little mulch is better than nothing.

If you do mulch, make sure the mulch does not go deeper than 3 inches and stays clear of the bark of the tree. Mulching will also help keep down weeds, retain water and keep the soil temperature stable, and, as the mulch decomposes, it will add nutrients to the soil. “Even if you mulch, try not to clean up too much around the tree in the fall, because this takes away nutrients,” said Tebo.

If mulching is not an option, choose shrubs or ground cover before planting turf underneath the canopy. If the tree is in an urban environment, however, it is best to consider a regime of fertilization.

Test the soil

Just because a tree is exhibiting decline or stress does not mean you should automatically fertilize the tree. “Always take a soil test first,” said Tebo. Adding fertilizer is not like giving vitamins to a tree, as some may believe. “Fertilizer is a stressor, an overwhelming situation for the tree to handle. If the tree is already stressed, adding fertilizer can put the tree over the edge,” said Tebo.

Treated (on left) and untreated ash trees.

Tebo also suggests avoiding fertilizing the tree when it is first planted; opt instead for plenty of water, which is what a newly transplanted tree really needs. Also avoid fertilizing during drought, she says. “If there’s enough soil volume, the tree can adjust to dry spells.” Funk, however, noted that using a low-salt index fertilizer could eliminate the added stress of fertilizing the tree during drought or transplanting.

Do all trees in the landscape need to be fertilized? If the trees are in a highly naturalized environment where leaf litter is left on the ground, probably not. However, most commercial, residential and street landscapes do not fit this profile.

Some trees are more tolerant to the turf conditions that others, such as London plane and sycamore trees, which have been adapted to flood plains and can better tolerate the similar flood/draught conditions of the turf landscape; evergreens also adapt well to the turf environment.

However, maple trees and most ornamentals are not easily adaptable to the turf environment and require a high degree of care and attention if they are to thrive. Fertilization will improve the appearance of these trees and enable them to withstand pests and diseases.

Marcia Passos Duffy is a freelance writer based in Keene, NH.

SIDEBAR 1

Steps to Take Before Fertilizing Trees

1. Get a soil analysis. This will let you know for sure what nutrients the soil is missing. Apply lime first if the test recommends it—this will balance the pH of the soil and make the soil more amenable to the tree absorbing nutrients.

2. Watch for telltale signs of nutrient deficiency: pale or yellow leaves, yellowed leaf veins, reduced leaf size and retention, premature fall colors and leaf drop. Also: reduced twig and branch growth and over all ill health of a tree. These visual signs, along with a soil analysis, will help you determine if the tree needs to be fertilized. Leaves that appear yellowish could mean there is a phosphorous deficiency in the soil; areas of dead tissue on the leaf edges could mean the tree needs potassium. Minor nutrient deficiencies (such as iron, magnesium or manganese) do also happen occasionally and can be analyzed by symptoms or a leaf tissue test. Your local cooperative extension can help make these determinations.

3. Establish a fertilization schedule. Use only a slow-release fertilizer annually or biannually. Do not overfertilize.