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Extension > Garden > Yard and Garden > Trees and shrubs > A practioner's guide to stem girdling roots of trees > Formations of SGRs

Formations of SGRs

Gary R. Johnson
University of Minnesota

Richard J. Hauer
Minnesota Department of Agriculture

Copyright © 2002 Regents of the University of Minnesota. All rights reserved.

Relatively little research has been conducted on the formation of SGRs and the frequency of their occurrence. In a survey by Hauer and Johnson (1997), practicing professionals suggested the formation of SGRs was related to confined rooting areas (boulevards, sidewalk tree pits), but Tate (1981) found no statistical relationship between boulevard planting sites and SGR incidence in Norway maple. Johnson and Borst (1999) also did not find a relationship between SGRs and boulevard width.

Roots are often forced into an encircling growth pattern when trees are grown in containers (Figure 15). If these trees are subsequently planted in the landscape several inches lower, encircling roots can eventually enlarge and become SGRs. Even wire baskets can later induce girdling (Figure 16). However, the frequency of this is unknown.

Fig. 15 - This container-grown sugar maple formed encircling, woody roots as a result of the length of time it had been growing in this container. The first main order roots were 6 inches below the surface.

When tree roots hit obstructions such as curbs and compacted soils and gravel, growth is redirected. If roots are redirected toward the stem, they might eventually contact and compress stem tissue. More commonly, however, roots will follow the obstruction (Figure 17). Watson et al. (1990) concluded in their study of maples, honeylocust, green ash, and littleleaf linden that root pruning during transplanting encouraged the production of new roots, often in directions that eventually conflicted with stem tissues. In many cases, SGRs were already present (though not stem compressing) in the soil balls of transplanted trees, or formed within two years of transplanting. They observed no relationship between depth of soil over the roots and increased incidence of SGRs. However, two recent studies (Johnson and Johnson 1997; Johnson and Borst 1999) observed a relationship between SGRs in green ash, maple, and lindens and planting depths. The mechanism that leads to SGRs in deeply planted trees, however, is unknown.

Fig. 16 - The secondary roots that formed after transplanting were directed in an encircling pattern by the wire basket that was left intact at planting time - 14 years earlier.

Fig. 17 - When roots hit an obstruction such as a road or curb, they typically redirect their growth to follow that obstruction.

Although there is anecdotal evidence that excess soil depths over primary roots might stimulate secondary and tertiary roots that eventually become SGRs (Johnson 1999), no published research supports that speculation. However, if in fact roots are produced within two years of planting and are positioned to eventually conflict with and compress stem tissues, excess soil depths over root collars would certainly disguise the developing problem. If stem tissues were not buried, SGRs would be obvious and correctable at an early stage.

Wilson’s (1967) research on how roots grow around barriers might shed light on root growth of deeply planted trees. Roots growing in their normal horizontal plane were forced to grow around barriers. Those deflected at angles of 30 degrees or less returned to their original plane of growth upon growing past the barrier. As the barrier angle increased beyond 30 degrees, the angle of correction (adjustment back to the original plane) was less than the deflection angle. For example, a root deflected by 30 degrees that grew beyond the barrier and returned to the original plane had a 30-degree angle of correction. Roots deflected between 60 and 90 degrees would have an angle of correction of approximately 40 degrees.

Tree roots grow in areas favorable to growth. Deeply planted tree roots might grow toward the surface to areas conducive to growth. If the roots that grow to the surface respond as did those studied by Wilson (1967), it would be expected that some roots will grow toward the stem. Roots that grow toward or tangential to the stem could become SGRs. Redirection of tree roots toward the stem from deeply planted trees has been observed in Minnesota by Johnson (1999) and in Wisconsin by Miller (1999).

Fig. 19 - The first main order roots were buried by 9 inches of soil in this balled-in-burlap tree.

Fig. 20 - Note the random direction of the secondary and tertiary root growth. The white band on the tree stem was the soil line, 13.5 inches above the main order roots.

Root collars and developed branch roots can be excessively buried by a variety of situations and practices. Trees might be transplanted with excessive soil over the root collar and contained within the burlapped soil ball (Figure 19). Six to nine inches of soil over the root collar flare of transplanted or containerized trees is not uncommon. Trees might be planted in the landscape at a depth greater than they were grown in the field or container (Figure 20). A 1997 survey by Johnson and Johnson, and a 1999 survey by Johnson and Borst observed a range of less than 1 inch to more than 10 inches of soil over the root collars of 3- to 9-inch d.b.h. sugar maples, green ash, and lindens. Construction and surface regrading can add extra soil to the rooting surface of newly or recently planted trees. Applications of mulch against the stems of young trees also can hide developing stem girdling problems.

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