EFFECTS OF WOOD CHIP MULCHES ON WEED SUPPRESSION AND WOODY PLANT ESTABLISHMENT
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Suppressing weeds and enhancing desirable plant establishment are important when installing or maintaining landscapes. Two woodchip mulches of different particle sizes and origin were applied at different depths to assess these two criteria in transplanted Douglas fir (Pseudotsuga menziesii) and snowberry (Symphoricarpos albus) in a historically weedy field. Arborist chips and chips made from recycled wood were used, in both coarse (1.09-7.62 cm) and fine (≤1.09 cm) particle sizes, at depths of 10, 20, or 30 cm. Weed biomass was significantly affected by mulch depth; the average annual weed biomass collected was 59.4 g/m2 from non-mulched plots, and 10.9, 2.6, and 0.5 g/m2 from plots with 10, 20, and 30 cm of mulch, respectively. For two growing seasons, plant health was visually rated on a 1-5 scale (with 5 being the most healthy), and data were collected on plant height and trunk caliper. Douglas fir trees were significantly affected by particle size in both arborist and recycled wood chips, with increased health ratings, caliper, and height in the coarser chip mulch. Mean Douglas fir health ranged between 3.4−3.5 in coarse chips compared to 2.4−2.8 in fine chips. Mean Douglas fir caliper ranged between 8.0−8.2 mm in coarse chips and 4.2−6.5 mm in fine chips. Mean Douglas fir height increased in coarse chips by 19.8 cm from year 1 to year 2, but only 4.3 cm in fine chips. Snowberry was not significantly affected by either mulch treatment; given its aggressive spreading nature, this is not surprising. Ideally, wood chip mulches should be coarse (at least 1 cm) and deep (at least 20 cm) to both suppress weeds and enhance establishment of woody landscape plants.