Gardening is uplifting, whether we’re working on our lawn and landscape or planting vegetables, flowers and fruits.
Some of the things we do are downright funny, such as making sure our mowing pattern is straight so the neighbors don’t snicker at our crooked lines or making everyone wait while picking the last of the string beans before heading down the road on vacation.
There’s little humor, however, in a dead tree. There’s a silent killer working among our trees — and it’s not an insect, disease, or tough winter. The tree’s own root system is strangling affected trees in a disorder called “stem girdling roots,” and we humans are primarily to blame by the way we’re planting our trees.
The problem isn’t new, but it's becoming increasingly evident, and just this past week I encountered it twice. I noticed an ornamental crab tree that just didn’t look healthy, and on inspection I noticed heavy roots tightly choking the trunk slightly below ground level.
Then I received the following email from a Moorhead resident: “My beautiful quaking aspen died of root girdling. It's about 20 feet high and has been in my yard eight years. When it didn't leaf out this spring, I contacted the county agent and he informed me that the tree had been improperly planted by the nursery, and it died from root girdling. It saddens me to lose such a tree and also to deal with its removal and replacement. I’ve attached photos, and you can see the girdling roots.”
I contacted Joe Zeleznik, North Dakota State University Extension Forester, who described the disorder as serious and frequent. The University of Minnesota estimates that nearly half of all trees that topple in storms likely are affected by stem girdling roots.
Not all losses are that sudden. The disorder can shorten a tree’s normal lifespan by as much as 80 percent, meaning a tree with an average expected lifespan of 100 years could die after 20 years when affected by stem girdling roots.
What exactly are stem girdling roots (often abbreviated SGRs)? Normal roots grow outward from the tree trunk like spokes on a bicycle wheel. Dysfunctional roots grow in a circular pattern around the tree instead of outward. As both trunk and roots expand in size with normal growth, the encircling roots contact the trunk and squeeze and compress the trunk like a boa constrictor.
The root in too-close contact may be on one side of the trunk, or circling entirely. The compression can severely restrict or stop the flow of water and nutrients between the tree and its roots. Trees become less healthy with the reduced flow of materials. Weakened trees are often attacked by insects, disease, and winter injury, which are often blamed, but aren’t the “root” cause.
At the point of strangulation, tree trunks are weak, and can snap off. SGRs are not always visible at the soil surface, but can be found by digging down. Symptoms in affected trees include unusual leaf color, small leaves, scorched leaf edges, early leaf drop, early autumn color, dieback of 1 to 2 feet of branch ends, a thinning canopy, death of major branches, leaning trunks and cracking trunks or bark.
Note that other factors can also cause these symptoms. The serious problem of SGRs is caused primarily by improper planting. When you remove a tree’s pot, the roots are commonly growing in circles around the soil ball. If the rootball is planted as-is, the roots often continue in circles instead of outward. Expanding roots can soon contact the trunk, becoming SGRs
The problem needs to be prevented at planting time.
Preventing the problem
Extension Forester Joe Zeleznik offers the following advice.
After removing a tree from its container, look carefully at the roots on the surface and sides of the soil ball. On the top of the rootball, locate the topmost flare roots, which are the large, main roots radiating from the trunk. It may be necessary to scrape away surface soil as trees may have been planted too deeply in the pot.
After you’ve located the top flare root, examine the sides of the soil ball. Gently pull circling roots outward like the spokes of a bicycle. If necessary, cut away the mass of soil and tangled roots on the outer perimeter of the rootball.
When planting the tree in its final location, the topmost flare root should be just barely below soil surface, or even partially visible. SGRs become worse when trees are planted too deeply.
Disrupting a tree’s root system by pulling the roots apart when it’s actively growing in full leaf causes stress to a new tree. To mitigate the stress, plant the tree quickly after correcting the circling roots, and water immediately and deeply.
For a complete discussion of SGRs, search online for the downloadable bulletin “Stem Girdling Roots: The Underground Epidemic Killing Our Trees” by the University of Minnesota and USDA Forest Service, which was an information source for this column.
Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler’s Greenhouse in Fargo. Readers can reach him at firstname.lastname@example.org.