Learning a small-scale organic method to reduce tillage with less weeds
by Ryan Maher and Brian Caldwell
Baseball fans know tarps are critical for keeping fields clean. Heavy rain falling on the diamond can quickly spoil a game. When you’re running for cover, turn your eyes to the field and you will see a crew working in unison to unroll a tarp over two-thirds of an acre. Hopefully the delay is short, less than an hour, and fine grained infield soils are protected. Once removed, all that water moves to grass edges, and credit to that massive grounds crew, baseball resumes without a puddle. This is just one of the luxuries of modern baseball. Small-scale organic vegetable farmers don’t have it so easy, but increasingly they too are using tarps. Growers are using tarps to manage weeds and improve soil conditions but, it’s taking longer than the line for peanuts in a 3rd inning cloudburst.
Tarping practices are taking many different forms, but the primary goal is usually the same. Organic farmers want to kill weeds. Often, beds have been prepared: tilled, shaped and fertilized as usual. But instead of planting right away, black, impermeable plastic tarps, 100ft long and multiple beds wide, are secured until the time is right to pull off and plant. In this way, tarps are another take on creating a “stale seed bed”. Both tillage and warmer temperatures under black plastic combine to wake up weed seeds. With adequate moisture, weeds germinate but eventually die without light. When tarps are removed weeks later, if the soil is not disturbed for planting, or perhaps just enough not to bring up new weed seeds from below, the tarp application can reduce weed pressure for the crop further into the season. Sometimes sourced as silage bunker covers, agricultural tarps are durable 6 mil plastic that can then be rolled or folded and reused multiple times over several years.
The use of plastic for organic weed control is not new. Black plastic mulch is common practice. The key difference in this approach is that tarps are completely removed before planting, as opposed to being left in place and planted into. There is clearly a wave of interest among small-scale organic vegetable farmers in tarping practices. However, many questions remain. Good weed management is more complicated and tarping clearly drives other changes in soils beyond weeds, like temperature, soil biology, fertility and the flow of water. And how can tarps be used with less tillage?
Reducing tillage with tarps
Not all farmers are using tarps after their planting beds have been tilled and prepared. Sometimes tarps are being used to reduce or even substitute for tillage. We have shared the story and no-till practices of Seeds of Solidarity Farm in MA in a previous SFQ article (Fall 2016). Building off this and other farmer experience, research at Cornell University and University of Maine has been looking at the tillage-like benefits tarps provide and what kind of “legacy” they leave to subsequent crops. Standard organic farming practices rely heavily on tillage to control weeds, but also for other goals like killing cover crops, breaking down crop residue, and creating a warm, fertile seed bed for planting. Tarping, if used as a reduced tillage practice, needs to provide some or all of these tillage benefits.
Tarping has significant effects on soils. It does not solarize soils at extreme high temperatures like carefully sealed, clear, transparent plastic, but soil temperatures do rise a few degrees on average, even when a tarp is placed over straw mulch. We know that soil microbial activity responds to soil temperature, so tarps can help convert organic nitrogen into plant-available forms. Meanwhile, rainfall does not infiltrate tarped soils and instead pools on the tarp surface or sheds off the sides. This can restrict nitrate leaching and conserve otherwise mobile nutrients in place, at least for a time. While it is not totally clear how these two processes balance out, crops can inherit a flush of nitrogen fertility at planting without any soil disturbance. We have found that overwintered tarps can build up over 4 times more soil nitrate in spring when compared to untarped, conventionally tilled soils. They may also improve the availability of nitrogen from slow release fertility sources, like compost. Finally, in spring, since tarped beds do not absorb runoff from snow and storms, they are not waterlogged and can be ready to plant earlier than untarped beds.
Tarps can help control weeds without tillage. There are many ways to create a stale seed bed and timing is often the big challenge. Using a flame weeder, for example, requires finding the right time to burn off weeds and not young crop seedlings. Timing is still a question for tarps and weeds. How do they best fit in an intensive vegetable rotation and how long do they need to stay on? It can depend on the time of year and the type of job. Plan for at least 3 weeks but don’t expect to control all weeds or kill perennials, particularly in early spring. Tarping in summer can give more heat units in less time. We have found tarping for 3 weeks in spring to effectively kill all emerged annual weeds and leave a weed-free bed for planting. However, tarping hasn’t always reduced our time spent hand weeding for the following crop. But timely tarping, where we cover beds that are otherwise not getting the attention they deserve, also keeps weeds from maturing and going to seed, reducing weed pressure over time.
Reduced tillage practices take many forms. This story is part of a series featuring reduced tillage practices for organic vegetable growers on the way to greater farm sustainability. Growers at diverse scales are tackling weeds, managing rotations, and integrating cover crops while minimizing soil disturbance. Look for past and future SFQ issues to learn the practices that are helping these growers build better soils. Visit this link or contact Ryan Maher of the Cornell SFP for more information on this project.
Surprisingly, we have seen little, if any, decrease of surface residues under tarps. We have been tarping for May and June plantings, taking advantage of the longer tarp time windows in winter and early spring. We have laid them over winter-killed cover crops, young winter rye, and bare soil from late harvested crops. Some farmers report faster decomposition of crop residues, perhaps due to increased earthworm activity, but we have not observed this in our trials. These remaining residues can interfere with direct seeding and could harbor pathogens. So it is important to consider tarp applications in a rotation and the soil conditions that are necessary for planting the following crop. For example, choose low residue beds for direct seeding, or when residue is high, use transplants. Prepare beds beforehand, so they need only limited or no disturbance after tarp removal. Build and level beds before cover cropping and finely chop and evenly distribute cover crops or crop residue with a flail mower before tarping.
Overall, we have found tarping can substitute for tillage by producing warm, fertile seedbeds that are weed-free for planting and moist but not too wet. Often when conditions are too wet or cool for tillage, tarps can still go down or come off. Meanwhile, no heavy equipment has compacted soils and soil organic matter and structure are not disturbed.
What about labor? Laying and moving tarps does not require a field crew but does require lugging 50 to 75 pounds of plastic, plus sand bags. The process has some similarity to applying floating row covers, but there is no crop to worry about underneath. Choose less windy days or work with the wind on your back and check tarps occasionally on windy sites. They collect water and can get messy. Don’t try to use tarps that are too big. Tarps that are greater than 30 by 100 feet require more hands and displace more water. When looking to tarp multiple plantings, several smaller tarps provide more options for synchronizing with different planting dates. If possible, direct any surface water toward perennial alleyways. Tarps are attractive because they can offer timeliness, add flexibility without adding field passes, and save time when time is most valuable. Beds are ready to go when it is time for the second seeding or whenever transplants are grown out.
More to learn
There are many questions to explore. Of great interest is how to best combine tarps with cover crops. Ideally, they should complement cover crops rather than substituting for them. We need cover crop benefits, like live roots, organic matter additions and legume-fixed nitrogen. Tarps may provide a way to kill cover crops without tillage, independent of cover crop maturity. We know that tarps create warm, humid conditions, but we don’t know how they affect worms, soil microorganisms, fungi, or soil-borne disease. How reliable are tarps for degrading plant residues? What methods are critical to achieving the best weed control for the following crop? And can we use more permeable materials, like landscape fabric, and get comparable benefits with less water issues? Research and farmer experience will help us fine-tune this powerful tool. We’d like to think baseball has some lessons to share too, but we’ll have to remember to pack the poncho and peanuts and watch more carefully at the next game.