Most Trees and Shrubs in a Fed Lawn Need No Fertilizer

Most people asking when to feed do not need to feed.

The lawn covers the largest group. Colorado State is direct about it, saying that feeding a tree growing in fertilized turfgrass will likely not be beneficial, and Clemson explains why, since shrub and tree roots intercept nutrients from the fertilizer applied to the grass above them.

New plantings are the second group, and the answer there is the same for a different reason. Clemson says water rather than fertilizer is the most important element for successful establishment.

It also helps to know what the bag actually does.

Fertilizer corrects a shortage of a nutrient. It does nothing whatever for a plant short of water, short of root room, planted too deep, or sitting in soil that will not drain.

In practice the nutrient in question is nitrogen. Shortages of phosphorus and potassium are uncommon in landscape trees, so a general purpose feed is usually solving one problem and adding two things the plant did not ask for.

The readers left are those with a woody plant in an unfed bed, showing a real shortage, in soil low in organic matter. That is a much smaller group than the people currently standing in a garden center.

Three things point at a shortage, and only one of them is a color.

Leaves lighter green than normal is the first, and normal is doing a lot of work in that sentence. It means lighter than the same plant used to be, or lighter than the same species growing nearby, rather than lighter than a photograph.

Reduced annual twig growth is the second, and it is the one worth measuring. Less flowering than the plant used to give is the third.

Twig growth is measured between two narrow bands of fine rings on the bark. Each band marks where a season started, so the length of smooth bark between them is one year of growth, and a short run on a plant that is otherwise healthy is the evidence a feed is answering.

A soil test finishes the job properly. A soil test or a plant tissue analysis comes back with the kind and the rate, which is a different thing from a guess with a bag.

Two windows are recommended, and they sit at opposite ends of the dormant season.

Early spring is the first, four to six weeks before bud break, so the nutrient is in place when the roots start working.

Late autumn is the second, after the leaves have dropped, provided the soil is still above 40 degrees Fahrenheit. Roots keep taking up nutrients after the canopy has finished for the year, which is what makes that window work at all.

The sources disagree about late summer, and it is worth knowing why rather than picking one.

Colorado State advises avoiding late summer and early autumn feeding because it can interfere with winter hardiness.

Clemson lists late summer through early autumn among its windows.

Climate settles it.

Where winter arrives hard and early, a late feed pushes growth that meets the cold soft, and the Colorado answer applies.

Where the season runs long and the winter is mild, the Clemson window is available.

Summer has one absolute. Do not feed a drought-stressed plant, because nutrient uptake depends on soil moisture, and fertilizer going onto dry soil is salt rather than food.

A rate on its own is not a quantity, and this is where most applications go wrong.

Slow release nitrogen goes on at 2 to 4 pounds of total nitrogen per 1,000 square feet a year, and never above 6 pounds in a year.

Fast release nitrogen goes on at 1 to 2 pounds per 1,000 square feet where there is no turf in the treated area, and never above 4 pounds a year. The turf caveat matters, because a lawn under the tree is already receiving its own rate.

The area runs from the trunk out to the outermost branches. For a mature tree Colorado State goes further and treats a circular area twice the diameter of the canopy, since the roots run well past the branch tips.

Establishment is deliberately light. A tree in its establishment phase takes 0 to 1 pound of nitrogen per 1,000 square feet, and a mature tree on soil low in organic matter takes 2 to 4.

Measure the circle before you open anything. The rate is per thousand square feet of that area rather than per plant, which is the step almost everybody skips on the way to a handful thrown at the trunk.

Every one of these mistakes looks like diligence at the time.

Feeding late in the season pushes growth that has no time to harden before the cold, and the damage shows the following spring as dieback on the newest wood rather than on the day it was applied.

Feeding a thirsty plant does nothing useful. Uptake depends on soil moisture, so the nutrient sits in dry soil as salt around roots that are already struggling.

Feeding instead of diagnosing is the expensive one. A plant declining from root damage, compaction or bad drainage gets a season of fertilizer and another season of decline, and the real problem goes another year unexamined.

Over-application has its own signature. Excess salts damage roots and can show as a crust on the soil surface, and heavy nitrogen produces abundant but lanky growth, which on a woody plant means weak wood rather than a fuller shape.

The principle holds across all of them and the details shift.

Evergreens keep their foliage for several years, so a shortage builds slowly and a correction takes just as long to show. Judging the result of a feed on an evergreen by next month is judging too early.

Flowering shrubs answer heavy nitrogen with leaves. A shrub fed generously and flowering poorly is often a shrub that was fed generously, and the pruning timing usually deserves a look before the fertilizer does.

Fruit trees in a regularly fertilized lawn need nothing separate, for the same reason every other tree in that lawn does not.

Acid loving shrubs are a different question wearing the same clothes. A pale rhododendron or blueberry in alkaline soil is usually unable to take up what is already there, which is a pH problem rather than a shortage.