Review of Tarping and a Case Study on Small-scale Organic Farms

Covering the soil surface with opaque plastic sheets to kill vegetation is referred to as tarping and is used by small-scale and organic growers to control weeds before planting crops. There are few published studies on tarping, and here we present a review of the literature in combination with observations from two on-farm case studies, one carried out in northern California and the other in northwestern Washington. An advantage of tarping is that it enables growers to control weeds without herbicides or tillage equipment, which can be cost-prohibitive for small-scale growers. Tarping is also suitable for no- or reduced-tillage systems, which is a primary goal for many small-scale and organic growers. Silage tarps that are 5 to 6 mils thick and black on one side and white on the other are most commonly used for tarping, are readily available new or used from some local agricultural suppliers or online, and can be reused for six or more seasons. Tarps are placed with the black side up to warm the soil, which encourages weed seed germination. When the soil is tilled and then tarped, a 3-week period with sufficient soil temperature and moisture is sufficient to kill emergent weeds in the top ≈1 inch of soil and provides a 95% to 100% weed-free surface at tarp removal. When a tarp is applied from autumn until spring to a plot that has established weeds, winter annual weeds can be controlled for several weeks after tarp removal, and then soil disturbance results in germination of additional weed seeds. For established perennial weeds, it may be necessary to extend the tarp application time to several months during critical weed growth phases or a full year to break the vegetative life cycle. Tarping does not reduce the weed seed bank, thus minimal soil disturbance after tarp removal is needed to maintain a reduced weed population during the cropping period.