How to prevent stem girdling roots
Many factors can place damaging stress on trees, including insect pests such as aphids and gypsy moths, and diseases such as oak wilt and fireblight. Drought, excessive shade, deer browsing and freeze drying also make life tough for trees. Although possibly less known, stem girdling roots are one stress factor that is easily preventable.
Stem girdling roots, or SGRs, encircle a tree’s stem, adversely compressing the stem’s woody and nonwoody tissues over time. Evidence suggests that SGRs can lead to catastrophic stem failure and depletion of anchoring roots.
Jeff Kleinboehl, an instructor in the DCTC Landscape Horticulture program, reported that improper planting methods were routinely taught in the nursery industry and at horticulture schools for decades. Kleinboehl pointed out that matters began to change after a study conducted by Gary Johnson, an associate professor of urban and community forestry at the University of Minnesota, and Richard Hauer, a tree health specialist with the Minnesota Department of Agriculture. Johnson and Hauer reported that prevention is the best way to treat SGRs.
“Growing trees in containers often forces roots to encircle the stem,” Kleinboehl said. “If you let girdling roots grow unchecked, they will ultimately damage the tree, causing die back and structural weakening due to stem compression. When you plant your new tree, make sure you don’t plant it too deep, which could cause the encircling roots to become SGRs.”
Example of encircling roots on container-grown tree
Example of SGRs on mature tree
Example of SGR damage
Planting too deep means burying the root collar, which is the swollen or tapered zone on a tree where stem and roots differentiate. The root collar must be planted at soil level. Do NOT rely on the soil line in the container or ball as your guide. Dig down to examine the root flare⎯the enlarged area where lateral root tissues begin appearing on the stem.
Remove excess soil as needed from the root ball and keep the flare slightly above or at the surface of the soil when planting. The deeper you bury the root system, the fewer roots the tree will have to establish itself in the ground. Ninety-five percent of tree roots are found in the first 12 inches of soil.
“Also, consider purchasing younger trees,” Kleinboehl said. “Not only will they exhibit fewer encircling roots, but they will also grow considerably faster than trees that have been in containers too long.”
For more information about SGRs or other landscape topics, or to learn more about the Landscape Horticulture program, contact:
- Jeff Kleinboehl
Landscape Horticulture Instructor
651-423-8478 - Matt Brooks
Landscape Horticulture Instructor
651-423-8392
Beware Mulch Volcanoes
Take it easy with the mulch around the base of trees. Don’t make mulch volcanoes. Keep your mulch at two inches, four at the most, which allows the tree’s root collar to remain closer to the soil surface. —Jeff Kleinboehl
Photo courtesy of Fredric Miller Miller, Joliet Junior College, Bugwood.orgImproper mulching technique results in "mulch volcano"
