The St. Paul Park and Recreation Department expects to lose several hundred mature boulevard trees this year—including Norway and sugar maples and greenspire lindens—to a problem know as stem girdling roots, or SGR, said arborist Chris Boche. Trees already are being marked for removal.
Leaf scorch, a possible warning sign of stem girdling roots.
SGR is a problem in many urban forests. Trees get planted too deep or soil gets piled next to the trunk. The tree roots head for the surface, seeking oxygen and water. In some cases, the roots circle back around the tree instead of heading away from the trunk. As both the tree and the roots grow, the roots act as a tourniquet, squeezing the tree and preventing the life-giving sap from flowing.
The problem is preventable. Boche said planting practices have improved in the past 10 years. However, many older trees were not planted correctly and are starting to show signs of SGR. The drought hasn’t helped. ”Trees 20 to 25 years old should be in great health,” he said. “They are suddenly dying.”
Boche gave a short driving tour of boulevard trees, including a stop near Summit Avenue and River Road, to point out the warning signs of SGR. One giveaway is the tree goes into the ground straight-sided, like a light pole, instead of having a root flare at the base. Other warning signs include small leaves or die off at the top of the tree; premature fall color, early leaf drop; and large seed crops (as the tree responds to the stress).
Sometimes the stem-girdling root will pop to the surface and it is easy to see it wrapping around the trunk.
Gary Johnson, professor of Urban and Community Forestry at the University of Minnesota, has studied stem girdling roots for more than a decade. He said it’s a nationwide problem.
His involvement started in the mid-1990s when Northfield city officials asked him and a fellow researcher to look at why their sugar maples were dying. The trees were 20-25 years old, the replacements for elms lost to Dutch elm disease. Johnson said they looked at the maples and ruled out disease. Then they noticed the trees didn’t have the typical root flare at the base.
Normal root flare
“We started excavating,” he said. “It was tree after tree after tree—stem girdling roots.”
When several straight-line windstorms hit the metro area in the late 1990s, it provided further evidence of the problem. Examining the tree damage, Johnson noticed that a number of trees had simply snapped at the base instead of being uprooted. The most damage occurred among trees six to ten inches in diameter.
“Almost half of those snapped off four or more inches below ground at a point where stem girdling roots had squeezed the stem, weakening it,” he said.
Ash and maple trees have shown signs of stem girdling roots in storm damage, but given their relative number in the treescape, they are underrepresented with the root problem, Johnson said. Little leaf lindens, the third most commonly planted boulevard tree in the state, are more prone.
“Over 74 percent of the little leaf linden that went down in the storms snapped off at stem girdling roots four or more inches below the ground,” Johnson said. “They were grossly overrepresented.”
Planting trees at the right depth is one way to solve the problem, he said. He also recommends that people not pile mulch or wood chips next to tree trunks. Mulch is nothing more than pre-soil, he said. Twelve inches of woodchip mulch piled against the stem today will be eight to ten inches of soil in three to five years.
Johnson estimates that 65-75 percent of urban trees are buried too deep. Not all those trees will develop SGR.
Life for a street tree is tough. The average life for a boulevard tree is 25-35 years, a number Johnson calls terrible. “Box elders, in their native sites, live to 100-125 years,” he said. “Silver maples live 125 years or more. Burr oaks live 250-450 years and we have trees dying in 25-30 years. That is such a waste of time and money.”
Minneapolis goes bare root
Ralph Sievert, director of the Minneapolis Park and Recreation Board’s forestry division, said Minneapolis probably is losing trees to SGR, but doesn’t have any guess as to how many.
Research has improved planting practices, Sievert said. Also, the Park Board has switched to planting bare-root trees instead of the balled-in-burlap trees. In part, cost savings drove the decision to switch. But the change to bare-root trees also makes it easier to get the tree planted at the right depth, with the first lateral root just below the ground.
Planting a burlap-balled tree makes it more difficult to get the depth right. The soil over the top of the root ball gives an artificial level. “If you aren’t probing to find the root flare, you could inadvertently plant it deeper than you thought you were,” Sievert said.
Johnson said metro area forestry departments have improved planting practices. But he still does a lot of sampling at private residences, homeowners associations and commercial landscapes. Some people are still chronically planting trees too deep, he said.
Scott Russell is a journalist. He wrote for the Southwest Journal and Skyway News (now the Downtown Journal) in Minneapolis from 1999-2005. He also wrote for The Capital Times, a Madison Wisconsin daily, from 1993-1999. Email: firstname.lastname@example.org