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Educational Sessions

Arboriculture Research and Education Academy (AREA)

Weed Control and Growth of Douglas fir (Pseudotsuga menziesii) are Increased with Deeper, Coarser Woodchip Mulches
Wednesday, 12 August 2015
2:15 PM — 2:45 PM
Osceola 1, 2, 3
A, T, M, L, Bp

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.

Presenter Information

    • Linda Chalker-Scott
      Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture. She is the author of four books, most recently publishing "How Plants Work: The Science Behind the Amazing Things Plants Do." She and three other academic colleagues host “The Garden Professors” blog and Facebook pages, through which they educate and entertain an international audience. 

Presentation file information