More mixed vegetables growers are experimenting with tarping as a way to control weeds. It’s a simple concept – stretching large swaths of silage tarps across the soil prior to planting.
“We tarp everything with the exception of any beds that we’re going to put into black plastic for fruit crops. We tarp all of our beds as part of our bed prep routine,” said Hilary Martin, co-owner of Diggers’ Mirth, a collective farm in Burlington, VT.
Martin and Amanda Andrews of Tamarack Hollow Farm in Plainfield, VT, talked about tarping on their organic vegetable farms as part of a soil health series sponsored by the Maine Organic Farmers and Gardeners.
Field Prep & Timing
Diggers’ Mirth has about 12 acres in cultivation. The soil is silty loam and mostly flat. During the growing season, about two-thirds of it is in vegetables with the rest in cover crops. They primarily wholesale with half of their income coming from mixed salad greens.
In spring, they subsoil their ground using a root lifter, which lofts the soil. Then they spread amendments. Next, they run through with a six-foot disc harrow, followed by a field cultivator, which smooths and flattens the beds.
“At this point, if it’s not going to rain, we’ll water our empty beds. Ideally, we’ll let those beds sit for a week, if not two or three, and we let a full flush of weeds come in. Then we’ll lay tarps down for two or three weeks, and then we’ll lift up the tarps and plant directly into it the day we remove the tarp,” explained Martin.
Andrews, who has about seven acres under cultivation, agreed that moisture is critical to successful tarping, otherwise weeds won’t germinate under the plastic. She uses overhead irrigation and then leaves the irrigation in place with the tarps over it.
Andrews also pointed out that the decision of when to tarp should be based on the weed species a grower wants to control. For example, she tarped some ground in the early spring and then planted in July. Some of the predominant weed species she was attempting to control (galinsoga and ragweed) didn’t have warm enough conditions to germinate.
“That’s something I think about for our tarping rotation moving forward – what you’re tarping versus what your planting timing is and what needs suppressing,” she said.
Martin noted that chickweed and purslane also must be given adequate time to germinate prior to tarping.
The beds at Diggers’ Mirth are 160 – 200 feet long, and they plant salad greens in four-bed blocks. Eventually, they settled on cutting the large silage tarps into 100-by-20-foot pieces, enough to cover half the entire width of four beds and half of the length. It takes two tarps to cover the 100-by-20-foot sections. They also have some six- and eight-bed tarps.
They accordion a tarp at each end of the section they want to cover. It takes two people to cover four beds, and it’s as simple as grabbing the tarps and moving down the pathways toward the middle of the rows.
“We learned that the more we can make sure that there’s air between the tarp and the soil, the lighter the tarp will feel because there’s less friction. Running with your arms up in the air will get the tarp aloft, and if it drags, shaking it to get air underneath will help it move,” Martin said.
It’s important to have a good amount of overlap where the two tarps meet – at least a foot. Andrews said that since new tarps will shrink in the sun, she leaves 18 inches of overlap with new tarps. Diggers’ Mirth tried using soil to hold down the edges at first, but although it was easier to apply, the weight of the soil and excess water made tarp removal nearly impossible.
Both farms now use sandbags to hold down the tarps, placed every two or three feet. The more sandbags the better, especially where two tarps overlap. Removing slack, keeping prevailing winds in mind and scrunching extra tarp (rather than folding it) are additional tricks to keep in mind.
Moving & Removing
To remove tarps, Martin first picks up the sandbags, placing them directly into a tractor bucket or truck. Then they go to the middle of the field, grab a tarp and run back toward the end of the bed. At removal time, the tarps are usually wet and dirty. “Doing that shaking move where you’re trying to get air under there is helpful. More people are helpful when there are a lot of puddles and water,” Martin said.
Once they reach the end, they bunch the tarp into an accordion shape, which makes it easier to unfurl the next time. If the tarp doesn’t need to move far, bear-hugging and dragging can get it where it needs to go. For a farther move, Martin puts a strap or chain around the tarps and drags them with a truck or tractor. The most important part of moving tarps is to keep them in the accordion configuration.
Andrews prefers folding tarps rather than running them off the fields. First, she tries to remove as much water as possible, and then folds them with a method she calls “hot dog, hot dog, infinite hamburgers.” First, she folds the tarp vertically a few times and then in lots of hamburger folds (horizontally). They palletize the tarps because they are usually moving them to a different field.
“We’ve gone to the folding system because the surface tension that Hilary is talking about is a real deal, and if you’re fighting water and mud on the top and surface tension on the soil side – in our case fighting slopes – it’s just too many fights. We went to a neat fold system and we’re liking it,” Andrews said.
Both farms use a color-coded system to store the tarps over winter. Diggers’ Mirth leaves them in the field where they’re most likely to go in spring. Tamarack Hollow places their folded tarps in pallet bins and stores them in a greenhouse, so they’re not frozen in spring.
Tarping provides weed control, but it’s not a silver bullet. Martin said, “Tarps have not replaced weed control on our farm. We use flame weeding. We cultivate with a Tuff-bilt tractor, and we hand weed. But I will say that tarping has significantly reduced the amount of time that we’re on our hands and knees hand weeding.”
Both growers said there are unexplained benefits to tarping. Martin said she’s noticed a consistent increase in yields and quality of their greens. A University of Vermont researcher conducting some trials at the farm collected evidence to support Martin’s observations.
Andrews agreed that there is something else going on beyond weed control. “It’s pretty minimal temperature increases that you’re getting under these black silage tarps we’re using, but it’s doing something microbially that’s doing great things for the crops,” she said. “And there are researchers at every Northeast ag school working on it right now figuring out what the special sauce of tarping is.”
by Sonja Heyck-Merlin