Over the last 10 years or so, the use of black silage tarps on small farms has become more and more common. This growing popularity of using tarps can largely be attributed to Jean-Martin Fortier from his book, The Market Gardener. JM was, and is, a huge proponent of the benefits of tarping when it comes to weed management and no-till soil preparation for new plots. Occultation is the term for this kind of tarping with non-transparent plastic.
Another tarping method that isn’t quite as common is called solarization. This method refers to using a clear tarp, typically greenhouse plastic, rather than a black non-transparent silage tarp. In this article, we will be looking into the differences between occultation and solarization in the market garden context, and the different situations where these two approaches can be beneficial. I’ve had experience using silage tarps as well as clear tarps at previous farms I’ve managed and have seen the amazing benefits of both when used in the right context.
Occultation refers to the process of blocking something from the light. When heavy-duty UV-resistant 5 mil silage tarps are laid over a field block, any above-ground vegetation begins to die. This process has multiple effects:
- Light deprivation
- Heat retention
- Moisture retention
- Accelerated decomposition of vegetation
- Creating habitat for worms and other microorganisms
When weeds and native grasses in a field are deprived of their ability to photosynthesize during occultation, they immediately begin to die. The black plastic absorbs heat from the sun, while also preventing any moisture from evaporating as it normally would. This combination of heat, moisture, and dying organic material creates the perfect conditions for the composting process to begin. Worms and other beneficial microorganisms are attracted to this ideal environment and begin to feast on the dying vegetation. It’s amazing how the simple act of laying a tarp down can begin such a domino effect of natural processes.
Time of Year
The time of year, ambient temperature, and moisture level all play a major role in the speed and efficiency of occultation. In general, tarping during the warmer months will create the desired results much faster. The soil biology is much more active in higher temperatures, working harder and faster than it would during the dormancy of winter.
Moisture levels also play a big role in the effectiveness of occultation. Even during the warm season, if a field is bone dry the decomposition process will be extremely slow. In order for the microorganisms to thrive and do their work, moisture is a crucial ingredient. In the same way you would need to water a compost pile to maintain moisture levels, it’s recommended to water a field in a dry climate prior to tarping.
Different Uses for Silage Tarps
Occultation with silage tarps is primarily used for three purposes on a small farm:
- To prepare a new growing area to create future beds
- To create weed-free “stale beds” after they’ve been prepared for a new planting
- To protect open field fallow beds from erosion during the winter
Preparing A New Site
Silage tarps are an amazing tool when it comes to preparing a new piece of ground for farming. If the site doesn’t need to be leveled for drainage, a tarp could be the only thing required to prepare the space. When such is the case, it’s recommended to tarp a new field block anywhere from six months to a year in advance to thoroughly kill off and decompose all of the native grasses. While all farmers may not have the luxury of waiting so long to prepare a space, it’s definitely the cheapest and least disruptive approach. In this scenario, it is advised to mow the space before tarping to help kick-start the natural process.
In situations where there is pressure to begin growing and selling produce as soon as possible, or where the ground needs to be leveled or adjusted, tilling and/or earth moving will likely be required. However, many no-till farmers will even concede to an initial till being a necessary step when it comes to preparing a new grow site.
If such is the case, tarping afterward is still extremely beneficial. When a site is graded or tilled, all of the grasses will be chopped up and mixed in with the soil. Watering and tarping a field after this has taken place will harness all of the nitrogen and carbon that would otherwise escape into the atmosphere, and instead, turn it back into nutrient-rich organic material. In addition, any weed seed that was brought up to the surface from the tilling will germinate, sprout, and die. While the initial soil disturbance will inevitably kill off a lot of beneficial bacterial and fungal microorganisms, tarping the space with optimal moisture and temperature levels will recreate an environment where these organisms can begin to repopulate quickly.
Some grasses are more persistent and harder to kill, such as Bermuda grass. Bermuda grass has rhizome roots that can grow and spread in the absence of light. These kinds of roots will require being dug out by hand–even after tarping.
Creating Stale Beds
Occultation with silage tarps is also used in between plantings on established beds. In this context, farmers will finish cropping out a bed, and then remove old crop residue to be taken to compost. If it won’t interfere with the coming crop to be planted, leaving the root mass in the ground whenever possible is highly recommended. This material is incredibly nutritious for the soil when it decomposes.
Next, the beds will be broadforked to aerate and break up compaction, then sprinkled with soil amendments (as needed), and often followed by a light application of compost. Then the beds can be smoothed and shaped, mixing in the compost and creating an optimal seedbed for either direct seeding or transplanting. The mixing/shaping step can be accomplished either with a rake, a drill-powered tilther, a power harrow, or a tiller with a depth roller (to only cultivate the top 2” or less).
At this point, the beds would be watered and then tarped. Any weed seed in the top layer of the soil would then be induced to germinate, sprout and die, creating a weed-free bed, also referred to as a “stale bed.” This process can happen in as soon as two weeks in the warm season but may take longer in the cooler months. Checking regularly under the tarp to make sure you aren’t rushing the process is key.
Stale beds are especially helpful when direct seeding a crop, as opposed to transplants which already have a head start on the weeds. Watching your crops uniformly germinate in a clean bed with zero competition from weed seeds is incredibly gratifying!
Another nice thing about silage tarps is that they can be custom-cut to cover one bed at a time, three beds at a time, or left larger to cover a whole field block at a time. Whatever your scale is, you can still incorporate the strategy.
Protecting Fallow Beds
The third most common application for silage tarps is protecting beds that are intentionally being left fallow. Many farmers with more land will practice cover cropping to protect their beds from erosion during the winter, or put them out of production for a season to focus on replenishing the soil. However, most small-scale growers don’t have the space to practice cover cropping, which requires seeding in the late summer. With limited space, most small-scale growers will need to be growing cash crops in their beds as long as possible, even into the late fall or early winter.
When such is the case, using silage tarps is a viable alternative. When the last of the fall crops have been harvested, beds can be prepped and then tarped to stay protected over the winter. Not only does this protect the soil from the elements, but it also gives the farmer a head start on spring plantings in the coming year. Beds will be weed free and dry when it comes time to pull the tarps and plant during the optimal window of time.
Solarization is the process of using clear plastic tarps to produce a greenhouse effect on the surface of the soil for killing off weeds, old crop residue, and even soil pathogens. While the practice of occultation with non-transparent tarps does heat up the soil (especially with heat-absorbing black plastic), solarization with clear tarps can heat up the soil exponentially more. With light being allowed to pass through the clear plastic, and the heat and moisture being subsequently trapped, temperatures rise drastically higher.
When to Solarize?
In my experience, solarizing is most beneficial for quickly flipping beds when succession planting quick turnover salad crops. Unlike occultation with black silage tarps, clear tarps do not create a hospitable environment for microorganisms where composting takes place. Think of it more like passive flame weeding using the power of the sun instead of your propane tank. It can turn a carpet of green residue into unrecognizable dried-up carbon in less than a week. In addition, it plays the same role of killing off any weed seed on the surface, as it still traps moisture to induce germination.
While solarization is killing off organic material on the surface, and surely some microorganisms along with it (similar to flame weeding), the benefit is that with the moisture being retained, the majority of the microbiology just below the surface should remain unharmed.
Solarizing At Wise Earth Farm
I first experimented with solarizing while working as Field Manager at Wise Earth Farm in Kelowna, B.C. in the spring of 2021. We had great success using it in between plantings of direct sown arugula, and field-grown microgreens (sunflower shoots and pea shoots).
After harvesting five 50’ beds of baby arugula, I would run the power harrow over the beds to chop up the plants and mix them into the top two inches of soil. We would then pull a clear tarp over all five beds. In our context, it would only take 6 days of solarizing before the entire section was ready to be replanted with the next succession of baby arugula. Just before seeding with the JP-5 Jang Seeder, we would sprinkle the beds with worm castings-based compost to reinvigorate the soil’s surface. The next crops came up healthy and lush in weed-free beds!
CLICK HERE to see a visual where I describe the process on our Modern Grower Instagram account.
The alternative to solarizing after cropping out baby arugula would be:
- Running over all the beds with a wheel hoe to undercut the old plants
- Raking up all of the crop residue
- Gathering all of the old plants in buckets or wheelbarrows
- Transporting the material to a compost pile
- Tarping the beds with a silage tarp for 2-3 times longer until weed seed had germinated and died (or irrigating until weeds emerged, followed by flame weeding).
As you can see, solarizing has the potential to save a LOT of time and labor. Not only that, by reducing the time between plantings to less than a week (as opposed to 2-3 weeks), you can fit in more successions during the course of your growing season, which ultimately means more profit!
CLICK HERE to see an example from my time at Wise Earth Farm where I was solarizing one bed at a time in a small rotation of weekly planted field-grown sunflower shoots and pea shoots. Tip: Landscape pins work well to hold down the edges of the plastic when doing one bed at a time.
Whether it’s occultation with black silage tarps or solarization with clear greenhouse plastic, I believe both methods can serve unique purposes on a small-scale farm, ultimately saving you time and labor! If you have any personal experiences with polarizing that you’d be willing to share or successful strategies with silage tarps, shoot me an email. I’d love to hear from you!
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