Using Compost Around Trees and Shrubs

A compost mulch can benefit trees and shrubs just as it does other plants. A circle of mulch is particularly useful around trees and shrubs in a lawn; it protects them from lawn mowers and weed trimmers, as well as reduces the competition from surrounding grass plants for food and water.

Spread a 1/2- to 1-inch layer of compost on the bare soil under the tree as far as the drip line (the soil beneath the outer edge of the foliage). The compost will protect any roots that are protruding because of compacted soil. As the compost breaks down and moves into the soil, it becomes available to the vast network of feeder roots near the soil surface. If a ground cover surrounds the tree, sprinkle compost among the plants to condition the soil for them and the tree.

When you spread compost on bare soil, cover it with a 2- to 3-inch layer of some other kind of organic mulch, such as chopped leaves, pine needles, or wood chips. The mulch will hold the compost in place and keep it from drying out. If such a mulch is already in place and is too difficult to remove, just lay the compost on top. The rain and soil organisms will work it into the soil.

Mulching a large tree takes a considerable amount of compost. Home composters often reserve some of their compost supply for trees that have sentimental, historical, or horticultural value or that need extra care because of injury or illness.

To get maximum impact from the compost, use it with fertilizer. The only truly effective way to fertilize trees is to inject nutrients into the soil the way professional arborists do. In the soil under the drip line and, if possible, 1 or 2 feet beyond, drill 1- to 2-inch-diameter holes. The holes should be 18 inches apart and 12 inches deep. Fill the bottom third of each hole with a slow-release all-purpose granular fertilizer, and top off the holes with compost. This treatment will provide steady sustenance to a tree for two to three years. Under a shrub, drill the holes only 8 to 10 inches deep.

Trees suffer from problems other than a lack of nutrients, of course. One such problem is surface rooting. Sometimes, homeowners try to hide surface roots, which look unsightly and may present a hazard, by shoveling soil on top of them. Unfortunately, this harms the tree and may kill it. To cover the surface roots of trees safely, use a very light soil mix and spread it thinly over the area. Mix equal parts of soil and compost, then spread a 2-inch layer over the roots. A thicker covering will smother the exposed roots, which have become accustomed to getting oxygen. Replenish the covering as needed. Aerating the soil beneath the tree is also helpful.