Answer: Roots appear on the surface of the soil as they grow in diameter, forcing what was a thin, shallow root above the surface. They also surface as erosion washes away soil, exposing the roots. There's nothing you can do to stop the roots from surfacing but you may have to redesign your landscape so you can plant groundcovers or shrubs in the areas where the roots are surfacing. They won't be quite so noticable with groundcovers planted over them.
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