The Dirt on Healthy Trees

Construction season is in full swing. Bulldozers and graders crisscross former farm fields to force the land into submission. Water will run away quickly into detention basins and won’t pond in people’s yards. Building sites will be level without any surprises for the builders. After the homes are built, a few trees will be planted, giving the impression that one day the neighborhood will have a healthy canopy and ample shade.

If you plant it, will it grow?

Fast-forward twenty years, and these new subdivisions will look much the same as they do today. Most of the trees will have been replaced at least once. Why? The soil.

Soil isn’t just dirt. Soil is a living, breathing, complex organism that plants of all shapes and sizes need to survive. Healthy soil is about 45% sand, silt and clay, 5% organic matter and 50% air space with half the air space filled with water.

The many insects, worms, fungi and other small organisms that live there need air and water to survive – just as we do.

Plant roots work their way through the soil particles forming dense networks that support the plant and absorb nutrients and water for the plant. If the soil is dead – if it cannot provide what the plant needs – the plant will be stunted or even die.

After a development site is mass-graded, the earth left behind is more akin to concrete than soil. This crushed, compacted dead dirt is the un-weathered parent material that was formed when the glaciers pressed through the area 12,000 years ago. Try to dig into it and you might break your shovel. A pick-ax or jackhammer may be necessary due to the soil’s highly compacted state.

What does this have to do with trees? It’s the soil. Just because a tree is larger than a flower doesn’t mean it is better able to manage living in bad dirt. If a tree is placed in a hole surrounded by concrete, that tree is going to have trouble. The roots cannot penetrate into the concrete, so they never develop a strong network to support the tree. Thus, the tree will never grow very large. Trees know their limits.

These suburban trees do not live as long as their rural cousins – life is too stressful and the dirt is bad. There is not enough air or nutrients in the soil and no room for their roots to spread out.

Relatives of mine live in the southwest suburbs in subdivisions that were built 15 year ago. They all have had to replace the original trees that were planted when they moved in. And, driving through their neighborhood today, there still are no tall trees. People build gazebos in their backyards to have shady places to go in summer.

The way development trends are going, our suburbs will have no tall trees in our lifetimes. It may take 100 or more years for the compacted dirt left behind to develop enough natural characteristics that it can be considered soil. Only then will trees be able to grow up and out the way only they know how.

Tips for suburban trees:

- Preserve existing trees. If there are healthy, native trees existing on a development site, don’t cut them down. Work them into the site plan. There are few good reasons to cut down a healthy native tree. They are too hard to replace.

- Minimize stress for trees. This includes avoiding construction under or near trees, not mowing under trees, pruning trees correctly and keeping the root zone free of pavement and other impervious materials.

- Don’t mound the mulch around the tree trunk. (An arborist I know calls these “Tree Volcanoes.”

- Minimize mass grading during development. Work with the land and the soil the way they exist today. Healthy soil will mean healthy trees.

Contact the McHenry County Master Gardeners for advice on tree care, 815-338-3737.