The use of silage tarps for weed management has increased dramatically in recent years, including in our own research in the Horticulture Crops Weed Science Lab at Purdue University. This article answers five common questions about tarping for weed management.
What are silage tarps?
Silage tarps are reusable, opaque plastic sheets that are black on one side and white on the other side (Figure 1). They have historically been used to cover silage used to feed dairy herds- often with the white side of the tarp facing upward. Compared to plasticulture mulch, which averages 1 mil in thickness, silage tarps are usually 5 mil thick. Silage tarps are commonly applied to the soil surface between crops and removed when it is time to plant a cash crop. This practice, increasingly adopted by small and organic farmers, aims to reduce the need for tillage.
Figure 1. Silage tarps (left) and repurposed greenhouse plastic (right) placed on the ground and secured with sandbags at Purdue Student Farm (Photo by J. Cerritos).
How does tarping work?
- Silage tarps operate based on the principles of occultation. Sunlight cannot pass through silage tarps, which prevents plants underneath the tarp to from photosynthesizing light into energy. Without sunlight, plants under the tarp eventually die.
- This is different from solarization, which involves the use of clear plastic to trap solar energy and greatly increase soil temperature. Solarization is best done during the hottest part of the year, whereas tarping can be done year-round
How can silage tarps be used?
There are many uses for silage tarps beyond weed control. For example, silage tarps can be placed onto soils with ample soil moisture to prevent soils from drying out. They can be placed onto fertilized beds that are not yet ready to plant to prevent the loss of fertility from leaching. But here, we will focus on the ways tarps can be used for weed management:
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Creating stale seedbeds
In one of our trials conducted last year at the Purdue Student Farm, we aimed to assess the effectiveness of silage tarps in managing weeds for small-scale onion production. Onions are not particularly competitive against weeds, so we were keen to explore how silage tarps could be used to create a stale seedbed.
A stale seedbed practice involves promoting weed germination and eliminating emerging weeds before planting the crop. One advantage of silage tarps is the potential reduction in tillage passes (Figure 2). During our trial, we observed that tarps provided similar weed control to tilling twice prior to onion transplanting. This suggests that tarps could effectively manage weeds early in the season, helping transplanted onions compete against early-season weed growth.
Figure 2. The first bed was tarped for 5 weeks before transplanting, while the next bed was tilled before transplanting onions. Note the greater soil moisture in the tarped row compared to the tilled row. (Photo by J. Cerritos.)
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Early season weed control on slow-to-emerge crops
Some small farms have used silage tarps to provide an optimal environment for carrot seed germination and emergence while simultaneously providing weed control. We explored a similar strategy with potatoes, which can take several weeks to emerge after planting. We planted seed potatoes and either immediately tarped plots or sprayed them with Dual Magnum, a pre-emergence herbicide. The goal was to control weeds that would typically germinate after potato planting but before potato emergence. The tarps were removed when 50% of the potatoes started emerging (Figure 3). We found that tarped beds did not adversely affect potato emergence but did effectively controlled weeds for the first 3 to 4 weeks after planting. This method also reduced end-of-season above-ground weed biomass significantly compared to the herbicide treatment.
Figure 3. Potato plants emerge from a plot that was tarped for 2 weeks (Photo by J. Cerritos).
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Cover crop termination
Silage tarps are a valuable tool for terminating cover crops, such as annual rye or vetch, which are often planted in the fall and can sometimes be difficult to terminate by other methods. Mowing or roller-crimping the cover crop before tarping will ensure that the tarp is lower to the ground and stays in place better. The mulch left behind by the cover crop residue improves soil moisture retention, suppresses weeds, and reduces erosion. In 2023, at the Purdue Student Farm, we demonstrated how tarping could terminate buckwheat compared to mowed and roller-crimped plots (Figure 4). This year, we are expanding our trials to evaluate the effectiveness of silage tarps for terminating cowpeas alongside traditional mowing and roller-crimping methods.
Figure 4. Silage tarps are placed Laying black plastic on top of flail-mowed buckwheat (Photo by J. Cerritos).
What type of weeds do silage tarps control?
In trials conducted on potatoes and onions, we observed that tarping effectively controlled annual weeds but was ineffective at managing perennial weeds, such as Canada thistle (Figure 5), because these weeds have energy reserves in their extensive root system. We have also observed that stale seedbed tarping may be ineffective at controlling weeds that prefer light to germinate, such as redroot pigweed. For these weeds, it may be best to wait until they have emerged, then place the tarp over them to kill them. Alternatively, we have seen great initial success by placing clear plastic over a prepared bed to encourage weed seed germination, followed by three weeks of silage tarping to kill the emerged weeds. This works especially well in the early spring by increasing soil temperatures to promote weed seed germination and emergence.
Figure 5. Canada thistle persists after removing a silage tarp (Photo by J. Cerritos).
What are the drawbacks and limitations of silage tarps?
While silage tarps show promise as a valuable tool for weed management on small farms, they also present certain drawbacks and limitations that we have observed. We use sandbags to hold our tarps in place. One significant limitation is the logistical challenge of handling silage tarps (and their sandbags), especially when moving them in windy conditions. This task can be particularly challenging with limited personnel.
Additionally, in one of our trials at the Meigs Horticulture Farm, we encountered another issue after lifting the tarps: a notable increase in voles and mice (Figure 6). Furthermore, while tarps are often reusable, they are made of plastic. Some farmers hesitate to use large quantities of plastic on their farms due to concerns about disposal and potential contamination issues.
Figure 6. A mouse is uncovered while removing a silage tarp (Photo by J. Cerritos).
Take home message
While silage tarps hold potential as a valuable tool for weed management on small farms, tarping alone often will not provide complete, season-long weed control. It should be combined with other weed control practices for optimal results. As with all production practices, the adoption of silage tarping on your farm should be gradual until you understand if and how the practice fits into your production system.