Occultation: Using Darkness as a Cultivation Tool

You may have put down cardboard to kill weeds, remove a cover crop, or turn lawn to vegetables. The cardboard kills unwanted plants by depriving them of the light they need to live. That is occultation. 

Recently, farmers have adapted it for faster use on larger areas. At Quail, we use occultation to clear the ground for our summer crops. By putting down an opaque cover in spring, we will have weed-free, plantable soil in time for summer transplants.

As in all of life, there are trade-offs: you are able to eliminate the labor and soil disturbance of tilling, weeding, or spading, at the cost of the material for covering and a month of occultation time. You don't have the fuel or carbon loss of tillage, but you are using plastic. Much like drip irrigation or hoop house coverings, the plastic tarps or sheeting for occultation can be re-used, but they are still plastic. And occultation uses time as a tool--you do have to wait for the biology of the plants to do the work. Very large areas tarped for very long can suffer from oxygen deprivation; that does not apply to the small areas, light tarps, and short occultation time we use or recommend. It does ease the need for strong backs and long hours digging or hoeing. Of the four family members involved in Quail, two of us are elders, two have disabilities of some sort, and one works two jobs besides Quail. I spend most days in the office filling orders. This technique allows us to have a modest but useful amount of space for food, seed, herbs, and trials in spite of those limitations.

On our growing beds, we use a mix of methods--winter mulches and cover crops to prevent weeds; hoeing to prepare for spring crops, and occultation in spring for summer crops. Farmers who use occultation to prepare the soil for early spring crops put down tarps in the fall. We prefer to have growing roots in the ground during winter, and to let rain soak in. We tarp the beds about the time the rains taper off, so our tarps help hold the winter's moisture for summer crops, and keep worms and other soil life active near the surface. 

By letting the weeds or cover crops grow lush, then cutting them and leaving them on the soil surface,  our technique provides food for worms and other soil life. They incorporate the plant material into the soil without us having to till it in or bury it. By the time we take the tarps off in May, the plant matter is almost entirely gone, turned into soil.

Occultation Step by Step

1) Here we are in spring, with plum blossoms and a lush growth of weeds and cover crops. We are on an old pasture, so tough perennial clumping grasses seed into the beds among the cover crops.

When there are winter crops we want to keep, like these green onions, we either position the tarps so that the plants are on an edge, or we move the plants. The hand sickle is a great tool for attacking tough stems or weeding around areas too delicate for mowing. We disconnect and coil up the drip irrigation, if it hasn't been done already.

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2) We mow the garden area with a string trimmer, leaving the cut material on the ground. Notice the plants are tall and lush, but not yet stringy or setting seed. This is the sweet spot for all cover crops--whether they are cultivated ones or random weeds, they should be cut while juicy and green. To speed breakdown of the cut plants, we water the whole area after mowing and before spreading the tarp. It's one of the few times we use a sprinkler instead of drip or hand watering.

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3) Next, we roll out the covering material. It needs to be lightproof, so corrugated cardboard works, if you can find enough of it. Don't use tarps from the hardware store that are meant to keep your woodpile, boat, or car dry. Lightproof tarps are now sold by farm supply stores and mail-order outlets. Sometimes they're called "blackout tarps." Other very heavy ones are called "silage tarps." 

We use ordinary black plastic sheeting, sold by the roll at the lumberyard or hardware store. It's easy to handle, cheap, folds or rolls for storage after use, and lasts at least 5 years. We cut it into 20 x 25 ft pieces, which each cover 3 of our planting beds and adjacent pathways. It comes folded to only 3 ft wide on the roll. Once rolled out in place, we can unfold it to its full width. Here's the start of the process. We've lined up the first edge on a straight line at right angles to the main path. Next we will unfold it.

4) Here it is after unfolding in place. Note we've weighted the edge with steel fenceposts to keep the wind from lifting it as we work. It's important to line up plenty of weights before you start--scrap lumber, sandbags, rocks, or rebar all work. The wind is able to shift heavier items than you'd think, so count on using many small-to mid-size weights. 

5) Here you see the entire area covered. This area took four  20 x 25' pieces. We try for at least 2 to 3 feet of overlap on the pieces, and weight them down well. If you have a prevailing wind direction, make sure that you start downwind and work upwind so that the edges don't scoop wind underneath. 

The drip tubing is light but wide, so it really helps to keep the plastic flat and in contact with the ground. It's a good way to keep track of the driplines for each bed too, so they are at hand when it's time to plant.

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6) Now it's 1-2 months later, and the land is hot and dry. The plastic is dusty and a bit wrinkled but still in place. It's time to peel it back, fold it up, and store it for another year.

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7) When we lift the tarp, it's amazing how little vegetation is left. This little heap of dead weeds is all we raked up from a 200 sq ft area, and we'll use it for mulch. Some plant matter composted in place. Worms and beetles dragged much underground. It's all been digested into good soil.  We'll just water, spread compost, and plant.

Total area occulted: 1100 square feet. (Eight beds 22' x 4', with 2' wide paths.) Total time removing irrigation, mowing, and tarping: four hours. Total time raking afterward: about 20 minutes.

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Here is one of our main reasons for using occultation. This is a random clod of dirt from the area after occultation. The soil texture is much improved. Organic matter is distributed throughout the top 8" of soil. There are many worm tunnels, and millions of pores that harbor oxygen and allow easy penetration by water and roots.

 Here's another reason we like the tarps better than tilling and disrupting the area. This little lizard was able to take shelter under the tarp, happily eating bugs and hiding from the cat. Looks like there was an earlier run-in with cat, raven, or some other predator--notice that her tail is in the process of re-growing.

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