Answer: The ultimate size of any root mass will depend a great deal on soil type and growing conditions. But, a good rule of thumb is that the roots of most shrubs remain in the top 12-18" of soil and extend out about twice as far as the width of the plant. Since a typical Amelanchier spreads by rhizomes, the root mass will continually expand, which will extend the root system farther out than most other shrubs. As an example, if you were growing Daphne odora, with a mature height of 4' and width of 6', you can expect the roots to extend out 10'-12' from the center of the plant when it is fully mature. An American arborvitae (Thuja), with a columnar growth habit can reach 20' high and yet remain only 2-3' wide. The roots of these plants will grow down about 18" (to anchor the plant), but will extend out only 4-6' from the crown of the plant.
Hope this sheds some light on the root growth of most shrubs.
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