Members of OSU's Horticulture and Landscape Architecture department are studying combinations of mulch and irrigation to promote growth on newly-established pecan trees. Using a wood-chip mulch in combination with irrigation has shown an additive effect on growth of young trees. Alone, either practice will increase tree growth. Used conjointly, the effect is additive, producing more growth than either practice conducted singly.
The mulch helps maintain cooler soil temperatures, which creates
a water-conserving environment. Even one or two weeds near small
trees substantially reduced growth. Trees in this study ranged
from 18 inches to six feet tall when established. Trees kept mulched,
irrigated, and weed free were considered to produce 100 percent
growth.
Measured against that, trees with one primrose weed nearby showed
32 percent reduction in winter and early spring growth. One pigweed
resulted in a 17 percent reduction in the summer. One pigweed
and one primrose grown in succession within a foot of the tree
produced an additive effect of a 39 percent overall reduction
in growth.
In the second year, one primrose produced a 17 percent reduction
in growth, and one pigweed a 31 percent reduction, with a 48 percent
reduction shown when grown near young trees in succession. The
plants rob the young trees of a lot of nutrients and water, but
the researchers note there are also some allelopathic chemicals
put out by certain weeds which inhibit the growth of competing
plants such as trees.