Exploring alternatives to plastic mulch
UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu
As our farmer to farmer soil health for vegetables series has unfolded, plastic mulch has come up in every conversation. There are so many benefits to using it, including moisture retention, weed control, and soil warming, but the environmental impact is hard to ignore. It's estimated that in the US alone, farmers use around 1 billion pounds of plastic annually. In this article, I'll review some alternatives and share up-to-date research and grower feedback.
Our final farmer to farmer soil health gathering is Wednesday January 20th at 7pm. We'll feature Rodrigo Cala of Cala Farm, who has been experimenting with cover crops for years. He'll discuss his efforts to integrate sheep for terminating cover crops, as well as his efforts with summer cover crops and species mixtures. Sign up at z.umn.edu/soilhealthchats.
|Plastic is a mainstay on most vegetable farms,
but farmers are working hard to find good alternatives.
Alternative plastic mulch-like products
1. Biodegradable plastic mulch
If a grower wanted to swap black plastic mulch with something nearly identical, but more environmentally friendly, biodegradable plastic mulch may be the best option. These plastics perform similarly to regular plastic mulch, but rather than removing your mulch at the end of the season, it's tilled into the soil where it decomposes.
Many growers have questions about the soil health impacts of biodegradable mulches. Unfortunately at this point, the research is fairly unclear. Some studies show impacts to soil microbial communities, and others do not. There is also substantial variation between products. Overall, biodegradable mulch impacts the soil in two ways. 1. Just like regular plastic mulch, covering the soil surface with a relatively impermeable black plastic changes the dynamics of the soil underneath by reducing light infiltration, increasing heat, and reducing water infiltration, evaporation, and gas exchange. All of these changes impact microbial communities, root development, and nitrogen use efficiency. Impacts vary depending on the soil conditions and climate in which mulches are used. In cool climates, plastic mulches may increase microbial activity by warming the soil, whereas in warm climates, mulches may warm the soil beyond the optimal temperature range for many microbes. In general, the use of plastic mulches increases nitrogen use efficiency (Bandopadhyay et al., 2018). 2. Following incorporation, biodegradable mulches add carbon, microorganisms, and other materials such as adherent chemicals and dyes. While the breakdown products of biodegradable mulches are generally considered non-toxic, there is limited research on the long-term effects of using these materials.
Since no current models of biodegradable mulch meet the criteria of the national organic program, these mulches can only be used in organic fields if they are removed at the end of the season.
One of the main reasons growers have not quickly adopted the use of biodegradable mulch is that it can look quite messy when it begins to degrade, and pieces can stick to produce, especially produce that touches the ground such as melons. For growers who have customers on-site, the small pieces of black plastic scattered around the farm may be considered unsightly.
It is worth noting that some initial research suggests that after tilling biomulch into the soil, the decomposition process may tie up nitrogen in the soil for a period of weeks. As such, it is likely advisable to avoid planting a second crop immediately after tilling your mulch under (just like you would wait a few weeks to plant after you've terminated a cover crop).
Penn State Extension educators published a series of case studies with grower reviews and tips for using biodegradable plastic mulch.
2. Paper mulch
Paper mulches are made of cellulose-based materials, whereas other biomulches tend to be made of vegetable starches and polymers. Paper mulches can be applied using the same methods (i.e. mulch layers), but there are some reports that the edges are more likely to tear during application if the disks are not set up at the right angle. Some researchers have opted to install paper mulch by hand due to the degree of tearing. Others recommend re-burying the edges at least once or twice during the season, as loose edges can catch in the wind or on equipment and tear. Paper mulches also tend to develop more tears and holes throughout the season than standard plastic mulch. There seem to be fewer studies on paper mulch than on other biodegradable mulches, but many show that it keeps soil consistently cooler than plastic mulch. This is likely in part due to the lighter color of plastic mulches, which tend to be tan to brown, but there are newer black paper mulches entering the market. As such, if you're considering trying paper mulch, try it first on cooler season crops.
I personally do not know of any MN farmers who use paper mulch - if you use it, I'd love to hear your thoughts about it. I plan to use some this summer to get a better feel for it.
|Paper mulch. Image: Johnnys|
3. Developments in new technologies
Researchers in Morris have been working with AURI to develop a bio-based spray-on mulch made from agricultural residues. They are still refining and testing the product, but it's something to look forward to in the years to come.
Straw is one of the most universally-used organic mulches, and for good reason. It achieves many of the same benefits as plastic mulch: weed suppression, reducing fertilizer leaching, and moisture retention. It also helps to reduce the incidence of splash-dispersed pathogens, and initial research shows that it can help to reduce Alternaria pressure in Brassicas. Straw mulch has also been cited as an Integrated Pest Management strategy for some pests including onion thrips and potato beetles, because it can interfere with pupation, and can support communities of beneficial insects.
Straw does keep the soil cool, which for some crops, can have negative yield impacts (but for cool season crops is ideal). Fields mulched with straw can also cause problems in squash and pumpkins if a fall frost occurs; we tend to see more damage in straw mulched plots than bare soil.
Make sure to purchase high quality, weed free straw, or you may end up with more weeds than you started with. Many growers prefer to run at least one cultivation pass before mulching to eliminate the first flush of weeds before laying the straw.
|Sprouting hay bale / a reminder to purchase
weed free, high quality straw. Photo NH
2. Strip tilling or direct seeding into a rolled cover crop
I'm seeing more and more vegetable growers experimenting with strip tilling, or direct planting into a field of rolled winter rye. There's still a lot to figure out in these systems, but they are promising for a few main reasons. 1. Rather than importing straw and spreading it, you're essentially creating straw in place with a cover crop. 2. With this system, you're also able to keep living roots in the soil over the winter. Dr. Ajay Nair's team is leading a lot of this research in Iowa, and you can read more about some of their trials here. In Minnesota, rolling and crimping a winter cereal often does not successfully terminate it. If conditions are too wet, rolling and crimping may not be sufficient to break the stems, and instead may simply push them over. Some growers who use this system first terminate their cover crop with an herbicide. Others use a mower to cut it down right before the cover crop flowers. There are pumpkin growers who direct seed into mowed or rolled rye - this works best for larger seeded crops. For smaller seeded crops, strip tillage, the practice of tilling the strips where you want to plant and leaving the rows in between untilled, is a good compromise. Read more about strip till trials from Dr. Nair's lab in the link above. Keep in mind that soil covered in straw does not get as warm as soils covered in plastic; using row cover early in the year can help offset the cooler temperatures.
|Farmers seeding pumpkins into a
terminated rye cover crop. In this case there was limited
rye establishment, so the weed control benefit was limited
Photo: Annie Klodd
3. Deep compost mulch
Deep compost mulching is simply the practice of adding a thick layer weed-free compost on top of your soil, essentially burying weed seeds. Depending on how much compost you're able to generate on-site, this system can be prohibitively expensive. This can work well for farmers who connect with local schools, hospitals, or other institutions who compost large quantities of food-waste. Keep in mind that it is possible to add too much compost, especially if it is high in phosphorous. Test your soil regularly to make sure you're not over-applying certain nutrients.
Woodchips are an excellent source of organic matter, but they should only be used in rows. Wood chips are best suited to systems with fairly wide bed spacing to avoid ending up with woodchips under your beds. Woodchips have a very high C:N ratio, and so they will pull nitrogen away from your crops if placed too close to the rooting zone. Over time, woodchips can add a substantial amount of organic matter and are excellent for absorbing and retaining moisture. Many growers are able to obtain woodchips for free by working with local arborists. Check with your certifier before using woodchips if you are an organic grower, as you may not be able to trace the source.
5. Wool mulch (woolch)
Wool mulch was researched extensively in the early 2000's. It is based on the byproduct wool from various industries in Minnesota, and showed great promise. Unfortunately, the product was never fully commercialized. You can read more about it here. While this is not a commercially available product at this time, I am mentioning it to spark ideas among growers who may have sheep farmer friends and neighbors.
There are many other materials that can be used as mulch in small-scale systems - leaf litter, bark, etc. but these materials tend to be less abundantly available for larger-scale farms.
1. Between rows
White clover has been the go-to between-row cover crop for many growers. It establishes well, can provide habitat for beneficial insects, and fixes nitrogen. There are a few drawbacks to using it, primarily that it tends to spread quickly, and so keeping it out of beds can be a challenge. White clover responds well to mowing and should be mowed prior to flowering in order to keep a well-established stand. Make sure that your rows are wide enough to accommodate a mower if you go this route.
In addition to pure white clover stands, many growers choose to incorporate a grass such as winter rye or a fescue. There are many reasons to integrate grasses and legumes, including improved winter hardiness, improved soil coverage, and tolerance to disturbance. Seed when there is rain in the forecast, or provide irrigation for good establishment (frost seeding is also an option).
While clover is the go-to for this purpose, there are many other options. Researchers in Morris, MN have been exploring alternative living row covers in recent years, with a focus on day neutral strawberries. While they are currently relying on white on black plastic for beds, they have explored a variety of living row covers as a substitute for landscape fabric (or herbicides / cultivation between rows). In 2019, they tested winter camelina, winter canola, and winter rye in both Morris (silty clay loam) and Farmington (sandy loam). Canola and rye both provided good weed suppression, though the canola needed to be mowed (canola is also a Brassica, which is important to consider for crop rotation purposes). All treatments yielded slightly less than plastic landscape fabric between rows. Read more about the trial here.
|Cornercopia student farm has been using white clover
between rows for years. Photo from their blog.
2. Within rows
One of the primary challenges to incorporating cover crops in vegetables is the short window of time between harvesting late summer and fall crops, and the first freeze. One approach to getting a cover crop in early while providing some weed control is to underseed your cover crop directly into your main crop. This practice is more commonly discussed in the context of field crops, but vegetable farmers are beginning to experiment with it more and more. There are a number of caveats to consider with this practice: you need to plant your cover crop late enough for your primary crop to become well established, or the cover crop can out-compete your primary crop. Typically seeding is recommended after your final cultivation pass, or right before the canopy begins to fill in. However, this means that your cover crop could become shaded-out by your primary crop if it forms a dense canopy. Therefore, this approach is likely best suited to upright crops with a fairly slim canopy such as peppers or staked tomatoes, which are less likely to shade out an understory crop than a crop like pumpkins or melons. Keep in mind that a cover crop growing underneath your primary crop will compete for nutrients, so it must be fertilized. This system can also increase the overall canopy humidity, which may impact disease management; if possible, choose a low-growing cover crop.
While this type of system is attractive for many reasons, it takes trial and error to figure out the best approach for your farm. Projects like this are great candidates for on-farm research grants such as the MDA Sustainable Ag Demonstration Grant or the SARE Farmer Rancher Grant. Our team can provide support and feedback to farmers who want to apply to these programs.
Plastic mulch recycling services
For growers who are sticking with plastic, there may be recycling opportunities available in your area. All of the farmers who have attended our farmer to farmer gatherings who also recycle their on-farm plastic use Revolution Plastics. The Recycling Association of MN also lists a few sites that accept agricultural plastics.
In our farmer to farmer gatherings so far, we've highlighted Racing Heart Farm, Featherstone Farm, and Tiffany LaShae - all of whom are using innovative strategies for improving soil health in their vegetable farming practices. At each event, we've asked them to share some resources that have informed their practice. Here are the books they recommended:
- The no-till organic vegetable farm by Daniel Mays
- The organic no till farming revolution by Andrew Mefferd
- No-till intensive vegetable culture by Bryan O'Hara
- The new harvest by Calestous Juma
- Original instructions: Indigenous teachings for a sustainable future by Melissa Nelson
- Managing Cover Crops Profitably - SARE handbook (free online)
- Google Group: Climate Resilient Vegetable Production. This is a listserv where growers post questions related to reduced tillage and other climate resilience practices. If you'd like to join, you can reach out to Rue Genger at email@example.com