by Sally Colby

Organic farmers will say that weed management is one of their biggest challenges. A variety of weed management methods are acceptable for organic certification – some more effective than others. Many organic farmers use cover crops to add organic matter and keep weeds down, and if managed properly, cover crops can eliminate the need for tillage.

Natalie Lounsbury, doctoral candidate in agroecology at the University of New Hampshire, said the practice of tarping has gotten a lot of attention recently and shows promise. However, the method isn’t foolproof, and must be done correctly for a good end result.

The process involves establishing a strong cover crop stand, terminating it at the appropriate time with a roller crimper and planting the cash crop into the residue. Organic rotational no-till using a roller crimper was developed in Brazil and has been adopted by many farmers. “It’s organic rotational no-till, not continuous no-till,” she said, “but the key is to till prior to establishing cover crops and not the actual crop. We’re using the cover crops to optimize the growth of cash crops.”

The cover crop itself can be the limitation. You need a high biomass of 8,000 kg/ha and the timing of termination can be tricky, especially with rye/vetch because they mature at different times, said Lounsbury. “The other limitation is that roller crimpers are an expensive and specialized piece of equipment, and most farmers can’t afford equipment that does only one job in the field.”

By adding tarps to a cover crop rotation, growers can develop a cover crop-based organic rotational no-till system. The issue of cover crops being difficult to terminate and incorporate goes away, and while tarps facilitate using cover crops, they involve the use of plastic.

The practice begins with a good cover crop stand at the right time of the growing season. For overwintered cover crops, spring and early summer are critical management times. “We seeded rye and vetch in mid to late September,” said Lounsbury, describing a tarping trial. “You can seed it earlier and get good results, but this is a realistic time.”

Although rye and vetch work well together, Lounsbury noted differences in how the two plants mature. Rye dies off as it approaches maturity, while vetch continues to grow, resulting in a significant increase in biomass in the last 10 days of growth. Waiting until vetch flowers is critical for increased biomass gains.

“Vetch needs heat units,” said Lounsbury. “It accumulates about 400 to 500 kilograms per hectare of biomass per every 100 growing degree days. Translated to Fahrenheit, that’s about 180 growing degree days. If the average daily temperature is 57º, it takes about 10 days. If the average daily temperature is 67º, it takes about six days. This amount of biomass at 4% nitrogen is the equivalent of about 14 to 18 pounds of nitrogen per acre.” Lounsbury compared this gain in nitrogen to the cost of organic fertilizer, which averages about $8 to $10 per pound of nitrogen – a substantial nitrogen gain for just a few days of extra growth.

The overall carbon to nitrogen ratio (C:N) doesn’t change much over the 20-day period. “You will not be getting available nitrogen with just rye,” said Lounsbury. “That’s one reason people like to terminate a rye cover crop early.” Lounsbury added that vetch remains the same in a low C:N of 10:12 for the entire period. “When you combine the fact that you’re getting a lot of biomass at the end of the period from vetch and it maintained a low C:N, you’re able to maintain a relatively low C:N overall cover crop biomass,” she said.

During tarping, nitrogen increases. Rye is not growing during this period and is not taking up nitrogen, and overall, the amount of nitrogen the rye took up prior to dying back is not that high. “The vetch is the nitrogen powerhouse,” said Lounsbury. “When vetch is putting on biomass, it’s also fixing nitrogen. If you wait for the vetch to grow, you’re going to end up with a lot more nitrogen. This is biologically fixed nitrogen, so in essence, it’s free.”

Tarp considerations include thickness, price and lifespan. “Billboard tarps are popular because they’re very heavy,” said Lounsbury, “but they’re not allowed under the National Organic Program. I would never recommend a plastic thinner than five millimeters. Lifespan is three to five or more years, depending on factors such as storage, rodents and field stubble.” Tarps are heavy and shipping can be expensive, so it’s worth checking local sources.

Silage tarps and black-on-black ag plastic are both impermeable. Landscape fabric (woven geotextile) is semi-permeable. Silage tarps are usually black-on-white or black-on-black. “Most people are using the black side up,” said Lounsbury, “so I don’t see a big difference.” When selecting tarps, determine the goals – killing weeds or killing cover crops. Perennial weeds have to be covered longer and may not be completely effective.

Black tarps block light and prevent photosynthesis, and sometimes heat the soil underneath. Clear tarps either cook everything because they capture solar gain or encourage photosynthesis and create mini-greenhouses. Whether the tarp is impermeable or not influences water dynamics such as runoff or water accumulation in undesirable areas.

Lounsbury said the weed seed bank is an important consideration because the cover crop stand and biomass must be heavy and uniform enough to cover the entire area to be tarped. “Soil texture and tilth matter,” she said. “This is much easier to implement on sandy soils. If you have heavy clay soil, no-till can be challenging.”

There are other benefits to tarping, including temporarily restricting nitrate leaching while plastic is in place. Tarping when temperatures are high can result in more total heat units in a short timeframe. The main drawback is that the practice requires plastic, which is already an issue in vegetable production. It’s also important to use a roller on the cover crop rather than mow it because stubble from mowing can break through the plastic.

“If we let cover crops grow longer, the benefits are worthwhile,” said Lounsbury. “And while tarping probably isn’t a one-shot solution to weeds, it can prevent a significant number of weeds from reaching maturity and going to seed, and eventually reduce overall weed pressure.”