Building Soil Under Trees

 All the products referred to in this soil building section for trees can be found in our Yardener's Tool Shed; click here

Mulch Feeds Soil Food Web

A major problem in America is that most people use no mulch around small trees. Or if they do use mulch it is not the best kind for the health of the tree.

Yes, most forms of mulch keep down weeds, slow the evaporation of water from the soil, and protect the small vulnerable tree trunk from damage from lawn mowers and string trimmers. But as important, mulch around a small tree needs to serve as food for the soil food web inhabiting the soil in which the roots of that tree must live.

Soil with sufficient organic content will have over 5 earthworms per cubic feet and a tablespoon of that healthy soil will hold over a billion beneficial soil microbes (bacteria, fungi, and protozoa). The collective community of all the soil critters from big (earthworms) to small (bacteria) make up the soil food web. They break up compaction, help soil store water, and create food for plants. A well fed soil food web leads to healthy plants. If there is no layer of organic matter on the surface to feed the soil food web, the soil is essentially dead. Dead soil gives plants little support.

So here is the complication. There are very few types of organic matter available in stores or from landscapers appropriate as mulching material that also serves as food for the soil critters. Bark chips and roughly shredded bark perform all the other mulching functions, but they are not appropriate as food for the soil food web. Earthworms can’t take bark chunks down into the soil.

The best organic materials for feeding the soil food web are chopped leaves (more available in the fall), finely shredded bark (shredded with a tub grinder), or a mixture of commercial compost and Canadian sphagnum peat moss. You do not need four inches of this type of mulch around your small tree or shrub. A half an inch is plenty for the season. On top of the thin layer of critter edible mulch you can layer the bark chips or roughly shredded bark up to but no more than four inches. Now you have the best of all worlds. You are feeding the soil food web and you are keeping down weeds and protecting the tree from those clunky lawn mowers. Remember; doughnuts not mountains.

Mulch Is Critical To Health Of A Tree

The most important job in caring for any tree on your property, large or small, is making sure it has either groundcover or mulch around its trunk as far out towards the drip line as you can allow.

Trees living in turf lose out to the grass plants for water and for food. Groundcovers act as “living” mulch and do not compete with the trees as much as turf. Groundcovers hide decaying leaves that provide the same benefit to a tree as does a thick layer of organic mulch. Those leaves provide organic matter to the soil that is critical for having a healthy tree; this same phenomena is what happens in a healthy forest. The soil with sufficient organic material (3 to 5%) has earthworms and billions of nitrogen-fixing bacteria producing food taken up by the tree. That organic material holds water to help when the dry periods of summer come around.

Organic Mulches

When a tree is young, under five years, mulching the tree with some kind of organic mulch is easy and takes little time. This means putting a circle of some kind of organic stuff around the base of the tree, avoiding having that mulch pile up against the base of the trunk. Chopped leaves, pine needles, wood chips, bark mulch or similar organic materials make suitable mulch for trees. We specify “organic materials” because we want them to break down and be pulled down into the soil to feed the earthworms and other critical microorganisms inhabiting a living soil.

Stone mulches or commercial landscape fabrics keep down weeds certainly, but they offer no benefits to the soil and thereby become a detriment to the health of the tree instead of a help.

Organic mulches should always be 2 to 4 inches deep and kept in place 365 days a year. Any higher slows access of oxygen to the soil and any thinner allows weeds to germinate. Be sure to maintain it over the winter to buffer the soil from the effects of freezing and thawing, which disturbs shallow tree roots.

While we don’t recommend using landscape fabrics around trees, we do make an exception for what are called “mulching pads” for trees in their first two years in your yard. Made from recycling rubber, these pads keep in soil moisture and prevent weeds. While they do not add organic matter to the soil, they serve well to keep lawn mowers and string trimmers away from that tender trunk; a major cause of tree problems as the tree matures. After two years, these pads should be replaced with organic mulch.

For more information see file on Using Mulch In The Yard.

Avoid Mulch Mountains

The mountains or volcanoes or whatever you want to call them are back. I’m referring to those mounds of mulch that many landscapers and homeowners load up around the base of small trees. Those mounds sometimes cover as much as ten inches of the trunk of the tree. They drive me nuts.

Guess what? Those mulch mountains are slowly killing those trees. It is tree murder. That piled mulch keeps moisture up against the bark of the tree and leads pest insects and disease spores into the tree like an open doorway. Why do people and landscapers continue year after year to slowly kill small trees? It is a mystery to me.

The rule is simple. Never let mulch around the base of a small tree touch the bark of the tree. The circle of mulch can be three to four inches deep, but in the middle of the circle the trunk is kept bare. The mulch layer should start about 6 inches from the trunk. We want doughnuts not mountains.

This is not new information. It has been general knowledge among reputable tree care professionals for 25 years. The tree care companies that make mulch mountains are just plain ignorant and apparently don’t spend much effort to learn the right methods for mulching small trees. Homeowners see these mulch mountains and figure if the professionals make mulch mountains, maybe I should do the same.

Compost Tea For Soil Under Trees


Compost tea brewing systems can be found on our Yardener's Tool Shed: click here


Add Beneficial Microbes To Soil Under Trees


Beneficial Microbes cab be found on our Yardener's Tool Shed; click here

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