Properly applied woody mulches are effective soil conditioners and weed suppressors, attributes which enhance the establishment of newly installed trees and shrubs. However, mulch layers that are too thin will stimulate weed growth, and finely textured mulches can inhibit water and gas exchange between the soil and atmosphere. This experiment was conducted to establish a mulch depth and particle size that optimize both weed suppression and plant establishment.
Wood chip mulches from two sources were chipped to two particle sizes (coarse and fine) and applied at three depths (10, 20 and 30 cm) around newly installed Douglas fir (Pseudotsuga menziesii). Regardless of source, coarse mulches performed better than fine mulches in both controlling weeds and enhancing tree growth. Increased mulch depth likewise increased weed suppression and tree growth. In none of these treatments were soil parameters including pH and C:N ratio significantly changed from the control, refuting the popular claim that wood chips acidify the soil and create nitrogen imbalances.