Green Horizons

Preventing Stem Girdling Roots: How deep is too deep? If there is more than 1" of soil over the flare/branch root area, you are asking for trouble. How can I check for proper depth? An easy way is to take a knitting needle and probe downward through the soil around the circumference of the stem. You are looking for the large roots that are running for the large roots that are running away from the stem. Once you hit such a root, mark the spot on the needle with your finger and remove it from the soil. If the depth is greater than 1", the tree is too deep. What can I do? A large tree that has been in the ground for several years with a well-developed SGR is a lost cause and should be removed. For smaller and more recently planted trees, remove the excess soil from around the trunk to expose the root flare. If you find an SGR, cut the root with a hand pruner. If you just brought a tree home from the nursery and are waiting to plant it, remove the top of the root ball until you expose the root flare. You might be surprised how much excess soil you have bought!

They are out there, everywhere. Landscape trees planted years ago that just don't seem to be as tall as they should be. They leaf out later and drop their leaves sooner than their natural cousins. Even with frequent summer rains, their leaves look like someone took a blow torch to their edges. Then it finally happens. One spring they don't leaf out at all. Or during a summer thunderstorm, they suddenly topple in a gust of wind.

These are not the notoriously fast-growing yet shortlived trees most people love to plant, like Bradford pear, Siberian elm and silver maple. We are talking about oak, maple and ash.

Stem girdling root (A) encircles the tree when the soil line (B) is more than 1-2” over the tree roots. Tell-tale above ground signs of possible SGRs include slow-healing wounds (C) and stem sprouts (D).

What is the reason? A new exotic insect or disease? A new pollutant to our air or waterways? No! More often than not, we are the problem. We are simply planting our trees too deep.

The result? Stem girdling roots. Stem girdling roots, or SGRs, are the result of roots encircling the tree stem and eventually strangling the stem tissue. Water and nutrients cannot travel up the trunk, depriving the leaves of the building blocks required for photosynthesis. What little food the leaves do produce cannot travel down the stem to the roots, preventing vital water and nutrient absorption. You get the picture.

You may ask how a root can grow up around the stem of a tree. It's simple. By planting a tree too deep, or heaping mounds of mulch around a young tree, we create an environment for roots to grow abnormally and develop into SGRs.

While SGRs are not necessarily new, (the first cases were reported back in 1937) the phenomenon seems to have become more prevalent during the past 15 to 20 years. Some arborists point to the widespread use of the hydraulic tree spade and the push to get trees out of the nursery and into the landscape. Economic concerns have resulted in the lost art of hand-digging trees in production nurseries.

As a tree makes its way from the nursery pot, to the liner nursery, and finally to the landscape, there is an opportunity for planting a little deeper in the ground. The result is the trunk of an improperly planted tree resembles a telephone pole instead of the trunk's normal flaring observed in trees growing in the wild. Lest you think this is not a common problem, or one that does not happen in your community, consider this: At a recent urban forestry conference held in Missouri, we airexcavated (used air pressure to blow away the surrounding soil) and found SGRs on seven out of eight trees examined. All eight trees were within 100 feet of each other — right in front of the local park department's maintenance facility!