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Research done at the Morton Arboretum in Lisle has shown that when grass roots and tree roots compete for water and nutrients, the grass always wins! If you remove the grass from around your trees and replace it with an organic mulch, the tree quickly starts producing more roots. But, be careful because over mulching is rapidly becoming the number one cause of death to shrubs and trees.

Why? Because too thick a mulch (over two inches) suffocates plant roots. Roots must respire and take in oxygen. When oxygen levels in the soil drop below 10%, root growth declines, and soon the plant dies also. Problems from over mulching accumulate slowly and it may take three to five years for the damage to show. When mulch is placed against the stems and trunks of trees more damage is done. Roots can survive continual moisture; but, tree trunks can not. This contact of mulch and trunk causes the trunk tissue to decay and fungal and bacterial disease to develop. Many times bark beetles and borers will assist in the decline of the tree. The continuous use of the same type of mulch can change the pH of the soil. Monitoring the soil and rotating the type of mulch used can be a way to correct any pH problems. Placing piles of mulch against a tree trunk will provide cover for small rodents. They will live in the mulch and chew on the tender nutritious inner bark to get at the sugars. Many fresh and non-aged mulches may cause nitrogen deficiencies. Bacteria and fungi which break down the mulch must have an ample supply of nitrogen to do their job. They use up the nitrogen in the soil.. Some experts suggest when using wood chips that you add a high nitrogen fertilizer to the mulch.

Grass Clippings: Dry or compost before using and mix with other materials to reduce matting.

Hardwood Chips: Apply a nitrogen fertilizer. (see above)

Composted Leaf Litter: This is an excellent source of natural nutrients.

Manure: Compost before using and mix with coarse textured materials to avoid burns.

Mushroom Compost: This is readily available and a good source of nutrients.

Peat Moss: It is not recommended because it packs down and dries out.

Pine Boughs: This is a very good covering for perennials in winter.

Pine Needles: This is good for providing acid, but should be mixed with other materials.

The use of organic mulches mimics the natural processes that occur in a deciduous forest where a blanket of leaves is deposited on the forest floor every fall. Leaves insulate the soil during winter and are the mechanism by which carbon and mineral nutrients are recycled through the ecosystem.

The Illinois Department of Natural Resources, and the U.S. Forest Service funded a project to donate a copy of the Tree and Shrub Handbook published by the Morton Arboretum to each Tree City in Illinois. Our copy is in the Palos Park Library. It is a part of the Tree Body Resource File and can be used by any library patron. This is a loose leaf notebook with material on a wide variety of topics about the selection, and care of trees and shrubs, including pests, diseases, planting, pruning, and other maintenance topics.