begin quote (from www.ces.ncsu.edu/fletcher/programs/xmas/environment/pesticides.html):
Many growers use a technique called "weed suppression." Originally developed at North Carolina State University by Dr. Walt Skroch and others, weed suppression uses lower than labeled rates of herbicides to stunt rather than kill weeds.

For instance, Roundup is labeled at 32 to 160 ounces formulation per acre. Growers use rates of 4 to 8 ounces per acre to stunt the weeds so that they don't grow too tall. This treatment lasts 4 to 6 weeks before another application is required. Growers may treat two or three times through the growing season with Roundup at low rates, then come back in the fall and treat problem weeds such as briars and poison ivy at the full rate to eliminate these weeds.

This type of ground cover management results in a shift of common ground covers away from grasses which are competitive to the tree to small woodland perennials that aren't killed by low rates of Roundup. Clover is a common groundcover that fills in the places between trees with this type of management practice, as are dandelions, nimblewill, wood sorrel, and violets. Clover and bird's foot trefoil are also being sowed by growers in cleared land or other sites where appropriate ground covers are not present; these are then managed further through weed suppression with Roundup.