In a well-maintained orchard, summer weeds are naturally suppressed by the trees' shade. Occasionally, a postharvest application of Roundup at half rate is made in the fall. Research by Swiss scientists suggests the best treatment for fruit quality is a weed-free program only in early summer. Weeds are sprayed in mid-April with Roundup. By mid- to late summer, weeds begin to return.
Weeds present in fall and winter help to reduce erosion and to minimize nitrogen leaching.
When weeds are killed the following spring, the nitrogen is released back into the soil for the tree's use. Swiss scientists also have looked at a number of different mulches to determine the potential for weed control. At their research farm in Guttengin, where annual precipitation is 32 inches, quality and quantity of fruit was best when a landscape tarp w as used around the bases of trees. The edges of the tarp were buried 2 inches in the ground to hold it in place. Scientists found plastic provided higher temperatures, more moisture, and better nitrification than other products.