It is not easy to know where roots are located under the soil surface. Preventing deep roots at planting is easier than correcting them later. The roots must be located prior to planting in order to be sure they won’t be too deep for life. This discussion focuses on the most common size of tree planted, 5-8 cm (2-3 in) caliper, and should only need slight modification, if any, for larger or smaller trees.
Roots can be too deep in the root ball when a tree leaves the nursery. This can be corrected at planting, if not too severe. The functional root ball volume of the B&B tree shown in the photo (red line) is only 25% of normal because the roots were too deep in the root ball. This tree did not survive and should have been rejected before planting.
How do you Determine Root Depth?
Recent research results and recommendations of arborists have been considered in revising the recommended procedures for determining the depth of roots. Seeing is believing, so wherever possible, removing soil and container substrate is preferred to probing.
When trees are being purchased in quantity, checking every tree may not be possible. Then it is even more important to have confidence that your grower produces quality trees. Checking for root depth on a few trees of each species is advisable. This can be done in the growing fields, or in the root ball.
The root flare is easily recognizable, but is a zone, rather than a specific point that can be used to measure root depth. The point of reference used to measure root depth should be where the main structural (large woody) roots become distinct from the basal trunk swelling. On a young tree, this point is typically covered by soil.
Horizontal root system View larger
Examples: Maple, elm, birch honeylocust, poplar
Oblique root system View larger
Examples: Ash, hawthorn, Malus, linden, Pyrus, oaks, ginkgo, Kentucky coffeetree, bald cypress
The angle of the structural roots on young trees can vary from horizontal to steeply angled, depending on species and growing techniques. On a large number of species commonly used in the landscape, the angle may be over 30 degrees (oblique root system), making it difficult to locate individual roots reliably by probing alone.
Trees produced in nurseries sometimes have one or two roots more than a few inches above the main grouping of roots. Don’t be misled when you find just one or two isolated roots.
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Where should the roots be?
Sometimes it is easy to tell that the roots are too deep. When the roots of a young tree are too deep, trunk movement of a young tree will cause a gap to form at the soil line. This can be seen in containers, in the field nursery, and after planting, but not in the root ball because the burlap and twine usually cover the base of the trunk. (Illistration by: S. Wegener)
Many tree cultivars in the northern half of the US are bud grafted; this is less common in the southern half of the US. Evidence of the graft union and cutback wound should be visible above ground at the base of the trunk. Cultivars on their own roots (rooted cuttings) and seedling stock are also cut back in field liner production. Ask how the tree was produced and insist on seeing the change in bark texture, or crook in the trunk above ground, if they are expected as a result of the production method. The roots could still be too deep, so always investigate further.
An inch of soil was removed in order to see the roots. View larger
The best way to judge root depth is to find where the roots join the base of the tree. If at least three distinct individual roots are visible at the surface, there no need to do anything more. Typically, on a young tree, the point where the main structural (large woody) roots become distinct from the basal trunk swelling will be just below the soil surface, and some soil will have to be removed to see the roots. A swelling at the base of the tree should not be interpreted as roots; it could be something else, such as a swollen graft union.
The roots must be covered with soil. If the roots are too shallow on a small tree, especially on one with horizontal roots, they may not be covered with enough soil to prevent exposure by erosion or frost heaving. As roots increase in size over time, the amount of soil over them will be reduced.
The upper surface of the uppermost root should be no more than 5 cm (2 in) deep where it joins the base of the trunk on field grown trees. Surface soil conditions in nursery fields can discourage root growth in the top inch or two. In containers, the uppermost roots should be just under the substrate surface.
Common sense dictates that there must be at least 3-4 roots distributed around the base of the tree to support it. A stool cannot stand without at least three legs, preferably four. Neither can a tree. If 3-4 roots are exposed at the same depth, there is no need to dig further.
You may have to go a little deeper to find 3-4 roots, because the main structural roots can vary in depth by a few inches, and their angle may differ as well. There should be 3-4 roots no more than 10 cm (4 in) deep. If not, then the tree may not meet even the most generous interpretation of the American Standard for Nursery Stock requirement that “Soil above the root flare shall not be included in the root ball depth measurement.” (2004, www.anla.org) A tree with all the roots found together at 10 cm (4 in) depth may have difficulty on poorly drained sites.
It may be difficult to excavate enough soil to clearly see individual roots, especially on species with deeply angled roots. After partial excavation, probing the soil with a surveyor’s chaining pin or stiff wire to verify locations of roots can make the task easier and faster.
When burlap over the root ball cannot be removed to locate the roots before placing the root ball in the planting hole, use a chaining pin used to probe through the burlap and estimate root depth. With a little practice you can tell when you encounter roots a few inches below the surface. After the tree is in the planting hole, the burlap can be removed and root depth confirmed visually.
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