The Garden Decoder: What Is ‘Occultation’?

Spring is here—and so starts the gardening season in earnest. There are many tasks to complete in order to have a successful growing season. What if I were to tell you that there’s something relatively simple you can do now to reduce the number of hours you’ll have to devote to weeding? And what if I were to tell you this method involves no herbicides? Interested? Let’s find out more about occultation.

Featured photograph by Emily Murphy, from Ask the Expert: Regenerative Organic Gardener Emily Murphy on How to Rewild Your Landscape.

What is occultation?

No, it’s not a cult. It’s not about Wicca. Nor do you need to join a coven. In simplest terms, “to occult” means to hide or to cut off light from a star. In our case, the star is our Sun. Occultation in gardening involves placing opaque tarps on top of the soil to block sunlight (the method is also known as “tarping”). Occulting is an important tool in organic gardening and farming. It kills weeds and cover crops, warms up the soil, and traps moisture. It’s also an important tool in no-till gardening. Occultation won’t eliminate all weeds in a single growing season, but implemented over several years, it will greatly reduce them.

How does occultation work?

When implemented very early in the growing season, before planting, occultation via black tarps causes the soil to heat up. The warmth and moisture it generates causes the weed seeds to sprout. But because the tarps block the sunlight, the weed sprouts wither and die. Its success rate is dependent on when the tarps are laid down, what the weather and temperature are for the time the tarps are in place, and the types of weeds in the beds. Occulting early in the season helps cut back the early weeds as the tarps will trigger the first flush of weeds to sprout. If the weeds are annuals, you may have a better outcome than if they are perennials, since perennials have a rootstock that would be unaffected by occultation. Sorry, you may need to hand pull those.

What do you need to start occultation in the garden?

Good-quality opaque tarps. Black is the best color, but any opaque color will work. This is something you want to invest in quality materials because you can reuse the tarps year to year. You’ll need something to hold down the tarps, such as rocks or heavy logs. You will need to water the soil, as the weed seeds need water to germinate. Give the soil a deep drink. Plan to leave the tarps on for about a month before you start your first plantings. You can also use the method in between crops if you have gaps in plantings and have the space.

How does occultation differ from solarization?

Both solarization and occultation involve covering soil with tarps. In solarization, clear tarps are used to allow the sun’s rays to “cook” the undesirable plants and seeds, and, if left long enough, undesirable insects. Think of solarization as a greenhouse with no venting. The soil temperature can get to 125 degrees F. Solarization generally takes less time to work—a couple weeks to decimate weeds—but this scorched-earth approach can also kill the good insects and microbes. Occultation, on the other hand, takes usually three to six weeks to work, and while studies are not complete, occultation doesn’t seem to kill beneficial soil microbes because the process doesn’t generate as much heat as solarization.

Occultation is a useful tool in organic and IPM (individual pest management) gardening and farming. But it is not the only tool you need. It works best in conjunction with other organic and IPM tools and depends on your climate, the plants you want to grow and the weeds you have.

See also:

• The Garden Decoder: What Is ‘Hugelkultur’?
• The Garden Decoder: What Is a ‘Cover Crop’?
• Weed Wisdom: What 10 Common Weeds Are Trying to Tell You