Know Your Tree's Roots

Learn more about complex tree root systems.

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Mature Tree Roots

When it comes to trees, virtually all of our attention is focused on what grows above ground. After all, that's the only part of the tree that we actually see. But it's only half the story, because what grows below ground is just as important. And when you consider the root systems of trees, you gain a whole new understanding of how trees grow and what it takes to keep them healthy.

The majority of trees have root systems that extend horizontally from the base of the tree, in most cases, well beyond the outermost branches. And up to 80 percent of those roots lie within the top 18 inches of soil. Moreover, the essential feeder roots (those that take up water and nutrients) begin several feet away from the base of the tree and extend outward in every direction.

Why is it important to know the nature of a tree's root system?

  • You'll be less likely to disturb or damage them when planning various projects, from an irrigation system to a new deck or patio. Or when trying to save established trees while building a new home; it's something more and more contractors and homeowners are committed to doing.
  • To effectively water your trees, you'll want to make sure your sprinkler throws water well beyond the outermost branches to where the feeder roots are.
  • You'll also be able to come to terms with why it's often so difficult to get grass to grow beneath mature trees, whose root systems tend to suck up all the available moisture and nutrients.
  • You'll recognize the importance of mulched tree rings, especially for young trees, which allow young roots to develop and grow without competition from turf or other plants.

Speaking of young trees, the first year of growth in the ground is by far the most critical, so take time to prepare the planting hole properly. Be sure to dig a saucer-shaped hole at least two or three times the diameter of the rootball, but only about 75 percent as deep. Loosen the sides of the planting hole with a garden fork so the roots can penetrate easily. After positioning the tree, backfill the planting hole with the excavated soil, water well and apply a 3-inch layer of mulch.

Do all that, and your tree will be well on its way to surviving that first year, assuming you keep it well watered (at least 10 gallons of water a week) more if your soil is sandy, less if it has a lot of clay. You place it around the base of a young tree, fill it with water and over time the water slowly seeps into the root zone of the tree.

One more tip: If you live in an area where summer comes on strong and stays a while, wait until fall or the following spring before planting a tree. The summer heat can be brutal on young trees.

But once a tree's roots have had a chance to grow into the subsoil where water is more abundant (say, by the second or third year in the ground) drought is less of a problem.

If you're planning on moving a tree, it's best to wait until fall or early spring, but understanding a tree's root system will help guarantee a successful move, especially if you prune the roots in advance of the move.

Root pruning, which involves severing the roots by digging a trench around the tree a season or up to a year before you move it, will encourage feeder roots to develop closer to the base of the tree.

And more feeder roots means you can dig up a much smaller, much lighter and much more manageable rootball. It also means the transplanted tree will settle into its new home with little or no transplant shock.

Tree roots can also be the source of numerous tree problems, the most common of which is girdling. Girdling occurs when a tree root grows in a circle rather than out into the surrounding soil. And when that happens, the tree may be susceptible to falling over in high winds, because the girdling roots can't anchor the tree properly.

The condition is seen most often in container-grown trees that have spent too much time in their containers.

So if you pull a tree from its container and notice roots growing in a circle, cut them out with pruners.

Tree roots can also be attacked by various fungi that cause rot, the most obvious of which manifest themselves in the form of mushrooms. But unfortunately, you might not even recognize the problem until it's too late, and in many cases there's no cure for the condition anyway. Often it's just a matter of time before a strong wind comes along and literally uproots the tree.

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