Tarping for Vegetable Crops

What is tarping?

Tarping is a technique where opaque plastic sheets¹ are used in order to kill weeds or cover crops^2,3. Tarps are different from plastic mulch in that tarps are thicker, usually between 5-6 mils (0.005-0.006 inches), and can be reused for many years, generally around six^1,2. Tarps are used for bed preparation and are removed before a crop is planted, as opposed to plastic mulch, which stays on the bed throughout the cropping season. Tarps are usually made from polyethylene, and are approved for use in organic production. Old billboard plastics have sometimes been recycled as tarps, but they are made from PVC, which is not allowed in organic production⁴. Silage tarps are easy to find and purchase, and so they are the most commonly used tarp material². Typically, silage tarps are used white-side-out for silage storage, but they are more effective black-side-up when used to tarp vegetable beds. Sometimes, woven landscape fabric is used. This material repels water but is more permeable than other tarping materials.

Benefits of tarping

The primary benefit of tarping is killing weeds or terminating a cover crop. Plants are killed by blocking all sunlight, which is called occultation². In addition to weed control, tarping can have added bed preparation benefits.

• Soil warming. If there is good tarp-soil contact and the tarp is dark-colored, the soil beneath will warm up faster than untarped soil in the spring.
• Regulate soil moisture. Tarps can also help regulate soil water content². They are impermeable and can keep rainfall off beds, which helps during a wet spring. But they also prevent evaporation, which preserves moisture for seedbeds^1,3.
• Available soil nitrate. Tarps also increase nitrate availability when first removed, with greater nitrate accumulation when tarps are in place for longer¹. The specific mechanism for increased nitrate availability is still unclear, but may be due to increased microbial mineralization in warmer soils with consistent moisture, less leaching from rainfall, less nitrate uptake from germinating plants, or some combination of these factors^2,5.
• Small-scale production. You do not need a tractor to use a tarp, so there is a low barrier to adoption². It is also easy to apply tarps with 1-2 people, so they can be used on small scale farms with minimal labor availability².
• Organic reduced tillage. Tarping is a weed management tool that does not require tillage or herbicides, so it could be a critical component of a farm looking to produce reduced- or no-till organic vegetables^3,5.

How to tarp

Tarping has been under-researched and most experimentation has come from innovative small farms². There are probably many ways to use a tarp that have not yet been optimized. Do not be afraid to try out a new use of a tarp, keeping these basic tarping steps in mind:

1. Have an appropriately sized tarp.
   • Tarps are usually 25 lbs per 1,000 ft². Generally, one person can comfortably handle a 2,000 ft² tarp (50 lbs), so it is recommended to use sections this size or smaller².
   • Tarps can be cut to fit one bed or multiple beds. Think about the tarping rotation you want to use on your own farm and size appropriately.
   • Make sure tarps are slightly larger than your desired tarping area so that there is some overhang to weigh down the tarp and to accommodate raised beds⁴.
2. Lay the tarp over the desired tarping area and secure.
   • Place tarps black-side-up for the most soil warming benefit.
   • Usually sandbags are used to secure the tarp, but growers have also used cement cinderblocks, old tires, or staples. Staples are most useful if using woven landscape fabric because otherwise they will create permanent holes that will speed up the degradation of the tarp⁴.
   • Use more weight if keeping the tarp in place longer or if strong winds are expected.
3. Leave the tarp on the soil surface for at least 3 weeks and until you are ready to plant^1,2.
   • The weed-free period after removing the tarp is about 3 weeks³. To gain the most benefit from tarping, do not remove it until you are ready to plant.
   • To kill all plants, it is generally advised to leave the tarp in place for 3 weeks, but some farmers have found that weeds are killed faster during hot weather⁴.
   • For perennial weeds, tarps may need to be in place for much longer than 3 weeks. More research in this area is needed, but some perennials may need to be tarped for several months, a year, or more to be killed².
   • Some farmers advise against tarping over the winter due to the potential for rodents to damage the tarp. However, removing the tarp over the winter may negate some of the soil moisture regulation benefits. Think about your goals and what is most beneficial for your own farm.
4. Remove and properly store the tarp when ready to plant.

• Improper storage can lead to rodent damage and shorten the life of your tarp.

Tarping has been used most often in conjunction with a stale seedbed or with no-till cover cropping. When used with a stale seedbed, first till and prepare a bed² [Fig. 1]. Tillage in this situation is important to stimulate the germination of weed seeds². Tillage stimulates seed germination by exposing seeds to light, providing more oxygen, and warming the soil⁶. You want weed seeds to germinate so you can kill them with occultation and remove them from the soil seedbank. After preparing the bed, apply the tarp to kill any weeds that germinate. The planting beds should be 95-100% weed-free when you remove the tarp^1,2,7. Be sure to reduce all soil disturbance after removal so that no new weed seeds are stimulated to germinate. The weed-free planting bed will give your crop time to establish without weed competition. This is a critical period to control weeds because once your crop is established, it will be much less susceptible to weed competition⁸. If you have time, and want to further reduce the soil seedbank, you can remove the tarp and till or irrigate a second time to further stimulate more weed seed germination. Kill these newly emerged weeds with additional tarping, flaming, or herbicides². Research so far has not found a significant decrease in the weed seedbank after tarping, but more work in this area is needed¹.

Cover crops bring a variety of benefits to a farm, but they can be difficult to terminate in an organic, no-till situation. Tarps can be very useful because of their ability to terminate a cover crop without tillage or herbicides [Fig. 2]. Before applying a tarp, often the cover crop is mowed or compressed. Mowing will distribute the cover crop unevenly across the field⁴, which makes the residue less useful as a mulch after removing the tarp. Compressing the cover crop creates a more even mulch layer but requires specialized equipment. Compression can be done with a roller-crimper or with small-scale equipment such as a lawn roller³. Put the tarp directly over the compressed cover crop to terminate it. This method will not warm the soil in the spring because the cover crop acts as an insulative barrier between the tarp and the soil³. Once you are ready to plant, you can remove the tarp. Large-seeded and transplanted crops can be directly planted into the cover crop residue. In this case, the cover crop residue will act as a mulch during the growing season and will suppress some weeds. The greater the cover crop biomass, the better the weed suppression. If you are planting small-seeded crops, you may need to rake off the cover crop biomass from the bed before planting. In this case, there will be little residual weed control from the cover crop.