Clear plastic. A non-chemical herbicide

Jim Boone
School of Forest Resources and Institute of Ecology, Cooperative Park Studies Unit, University of Georgia, Athens, GA 30607

Elaine Furbish
Institute of Ecology, Cooperative Park Studies Unit, University of Georgia, Athens, GA 30607

Kent Turner
Cape Hatteras National Seashore, Manteo, NC 27954

Susan Bratton
Institute of Ecology, Cooperative Park Studies Unit, University of Georgia, Athens, GA 30607

Phragmites communis is a serious pest plant in many eastern marshes. Because of its many undesirable characteristics, attempts have been made using various techniques to control the spread of this plant, but with little success; only herbicides have been effective in controlling it. Phragmites, however, typically grows in wetlands where the use of herbicides is difficult, if not impossible, due to regulations and environmental concerns. The inability to use chemical herbicides in many situations has lead managers to look for alternatives. Heating the soil to temperatures lethal for plants by covering with sheets of clear plastic was used to remove Phragmites in a North Carolina salt marsh (Boone, et. al. 1987).

Sheets of plastic were spread over stands of Phragmites after cutting or burning to remove stems. Each roll of plastic was six-mil, 12x27 m, and 51.8 kg. The plastic was too heavy to carry into the marsh and was airlifted. The plastic was difficult to anchor in place due to strong winds. The sheets were initially taped together with construction grade duct tape, but the tape deteriorated after 2-4 days of exposure to sunlight and high temperatures. Wind then carried the plastic off the plots and pulled up the edges that were buried under 2-3 dm of soil. To hold the plastic in place, sheet edges were rolled together and weighed down with sandbags weighing 5 kg spaced 1 dm apart. In addition, sandbags were placed in a regular grid on the plastic about 2 m apart. Sandbags were made from wastebasket-sized plastic bags. These measures created many small holes in the plastic, but did not seem to interfere with heating.

As long as the plastic was in place, it prevented sprouting. A complete die off of all species was noted in 3-4 days. Underplastic temperatures were recorded as high as 76°C (mean = 59°C) when ambient temperatures were 38°C. Underplastic temperatures averaged 33°C 5 cm beneath soil surface, and 28°C at 20 cm. This compares with mean temperature of 25°C and 23°C under standing Phragmites, and a mean of 26°C and 24°C under various types of marsh vegetation.

The main problems with plastic involved moving it into the marsh, holding it in place, and deterioration due to sunlight. There was no advantage to using large sheets of plastic. Sheets 12 m square would be much more manageable and would only weigh 23 kg. There was no advantage to burying the edges of the plastic, as there was a complete kill of vegetation within 1 m of the edge of unburied plastic. There is no reason to roll the edges of the plastic together where the sheets meet if they are overlapped. The plastic was held in place, even in strong winds, using a grid of sandbags spaced about 2 m apart. The life span of 4-mil and 6-mil plastic was about the same, roughly 2 months. This suggests that less expensive and more available 4-mil plastic would work just as well. After 8-10 weeks in the sun, the plastic deteriorated. It was imperative to remove the plastic from the field before that time because it suddenly became brittle and fragmented into thousands of tiny pieces.

The short lifespan of the plastic can be overcome. We took no measurements of underplastic death rate, but there was a complete surface kill in only three or four days. If the plastic were rolled out for four or five days, then rolled up and stored away from direct sunlight (e.g., under black plastic) for two weeks, and rolled out again, it could be used to kill sprouts as they emerge. By storing the plastic out of the sun, it could probably be used for an entire season.

Literature Cited

Boone, J.L., C.E. Furbish, and K. Turner. 1987. Control of Phragmites communis. Results of burning, cutting, and covering with plastic in a North Carolina salt marsh. Institute of Ecology, University of Georgia, CPSU Technical Report No. 42.