05-28-1991 Council Packet - Laserfiche WebLink

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r|; I'/. 1 >1 I. Hardcover Status of Weed Control Fabrics May 22, 1991 Page 4 of 5 Plastic vs Fabric - The manufacturers of various weed control fabrics have made a variety of claims as to the effectiveness of fabrics. While some manufacturers allude to the elimination of sun light as the weed controlling mechanism, others suggest that the impenetrability of the fabric by common weed species is the significant inhibitor. Polyethylene sheeting and geotextiles are both petroleum products, and deteriorate under the ultraviolet component of sunlight. Most weed control fabrics on the market have been "treated" with UV inhibitors to retard deterioration. The bigger issue for the City is permeability and the absorption of water into the soil below the plastic or fabric. New plastic is highly impermeable, but does deteriorate after a period of years to a point where it allows weeds to thrive, suggesting that it becomes permeable to a degree. Fabric Peraeability - Fabrics are highly variable in their permeability. The most significant determining factor in permeability is whether the fabric is woven or non-woven. Non-woven fabrics, consisting of bonded random-oriented strands, typically allow 10 to 100 times as much vertical water flow as woven fabrics, which have strands of fibers oriented in geometric patterns. And, there are dozens if not hundreds of vari'^us fabrics manufactured for various purposes, which may result in extremely variable degrees of permeability. Fabric permeability is typically measured as the rate of vertical water flow, or the ability of a fabric to permit the passage of water through it. Vertical permeability is typically measured in gallons per minute per square foot. Typical values i Xifor non-woven landscape fabrics available at local landscaping outlets and garden centers, are 200-300 gallons per minute. The woven fabrics such as Dewits Weed Barrier are in the 25 gallons per minute range. There are also some "shade cloths" on the market which are a very open weave, which would have a high permeability. Although intended for above ground placement as a sunlight screen, these shade cloths could conceivably be used by scmm people as a ( probably ineffective ) weed barrier. Rsfvlew of Bxhibits - Staff would direct Council's attention to the Exhibits attached to this memo, especially the article entitled "Goodbye, Polyethylene". For a more technical review, please see the excerpt from "Designing with Geosynthetics".