Tree root systems show a great deal of individual and species
variation. As a general rule for young nursery-grown trees,
there should be two or more structural roots within 1-3 inches
of the soil surface, when measured 3-4 inches away from the
trunk.
Planting is the best time to make sure to ‘get the roots right’. When
inspecting B&B root balls, remember that that the American Standard for Nursery
Stock (2004, www.anla.org/publications/index.cfm)
states that “Soil above the root flare shall not be included in the root
ball depth measurement”. If the resulting depth measurement of the root
ball does not meet the minimum, the root system will be undersized and the tree
can be rejected.
Sometimes there are easy-to-see visual indicators of root depth. Trunk
movement of a young tree will cause a gap to form at the soil
line if the roots are too deep. This can be seen in the 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 are bud grafted. The graft union and
cutback wound should be seen 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.
Sometimes the developing root flare will be visible even on young trees. You
may need to remove a little soil, but if you can see the root flare, the roots
are at the correct depth.
There is a simple method of probing for roots when visible evidence
is lacking. An inexpensive surveyor’s chaining pin is a
convenient and effective tool. A stiff piece of wire can also
be used. Simply insert it into the soil around the trunk. It
is easy to tell when you encounter a root a few inches below
the soil surface. This method can be used in the nursery, in
root balls before planting, and in the landscape after planting.
Probing for roots a few inches away from the trunk, reduces
the risk of mistaking the graft union or some other swelling
at the base for the root flare.
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