Finding Your Roots
Having colorful flowers beneath your trees means knowing where the roots lie.
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The gardener had just moved to a '20s craftsman-style home in the city and was on her knees in a flower bed, trying in vain to clear a spot for some new plants. What were these bundles of fibrous reddish roots that infiltrated every square foot of the bed? The answer took only seconds. There was a large Norway maple at the corner of the yard. Although it was some distance away and the flower bed was clear of its canopy by about 20 feet, its feeder roots were still out and about, nibbling away at the rich organic matter in the flower bed. She rethought her garden and found another bed for the flowers and put others in large containers. She knew that in a competition for the same nutrients, the Norway maple would win, hands down.
The roots of a tree usually extend well beyond its canopy, and most occur within 12 inches of the soil surface. The feeder roots, of course, are usually very near the surface. Norway maples are notorious for vigorous feeder roots that successfully out-compete anything you put in their path, but many other trees are not so aggressive.
If you're looking for a shade tree that's better behaved underground, consider the sugar maple, pin oak or red oak. In one study, where the Norway maple showed the greatest density of roots in the top six inches of soil when compared to several other species, the sugar maple and pin oak had about one-quarter the density of roots. Linden had slightly more than sugar maple, and the red oak was the lowest of all.
But no tree is perfect. Given the choice of bare soil, turf grass or mulch, feeder roots will choose the mulch every time, no matter what kind of tree they belong to.