By Marilyn Homewood, Master Gardener
Solarization and occultation are chemical free methods of killing vegetation and some soil borne pests in garden beds using plastic tarps. While both use tarps, their mode of action is different.
Solarization
This is the process of placing a clear plastic tarp over the area to be treated in order to heat up the soil beneath the tarp. It uses the power of the sun to create a greenhouse like effect on the area. The plastic traps heat and moisture which initially encourages seed germination and plant growth. The plastic causes the temperature of the soil to rise as well as blocking additional precipitation. This results in the death of vegetation beneath the tarp.
Takes at least 6 weeks in high summer. To be effective the sides of the tarp must be buried to prevent heat loss.
This method may also be used for the treatment of soil amendments prior to addition to the garden where weed seeds or some soil borne pests are of concern. Material is spread in a layer on a tarp and then covered with clear plastic sheeting. The edges of the clear plastic must be tucked under tarp securely. As an example, a minimum of 3 days at 104 degree F will kill Asian jumping worms and their cocoons.
Occultation
This method uses opaque coverings that block both precipitation and sunlight (preventing photosynthesis) thereby killing the vegetation by starving it. The longer the tarp is left in place, the more effective it is. The time taken depends upon what problem is being addressed. These tarps are held down by heavy objects such as bricks, stone or in my case, pieces of split wood. Some perennial weeds such as quack grass (twitch grass) with long rhizomatous roots or those with long taproots survive prolonged deprivation from sunlight and will still need to be removed by digging.
Type of Tarp
For solarizing the ground, clear greenhouse plastic (new or used) is ideal. It is thicker and less likely to tear. Occultation requires an opaque tarp (preferably black, UV treated). Black polyethylene sheeting in rolls (4ml thick) can be used; tarps from the hardware store and my personal favourite, silage tarps. These are large, heavy duty tarps used in agriculture that may be reused many times.
Variations on a Theme
Woven landscape material has been used but is generally considered less effective as the woven texture allows water to penetrate and heat to escape, however many have used it provided they make allowances such as leaving it in place for longer periods.
Cardboard has also been used but is even less effective as it quickly becomes sodden and begins to break down.
Personal Experience
I have used occultation many times in the past and this summer as I was enlarging my nursery bed on 3 sides, I used it again. On two ends, I placed tradition black tarps however on the third I used commercial grade woven landscape material overlapped by about 1 foot and pinned to the ground at frequent intervals. I put these down the second week of April (before anything had started to grow) and kept the covers on until the second week of September. I was covering “the lawn” which at my house is a mixture of grasses and weeds such as quack grass, dandelions, plantain and creeping bellflower.
Upon removing the tarps, no growth was showing. Under the landscape material, it was mostly bare but not entirely. From past experience, I know that there may be perennial weed root left so I usually broad fork the treated areas in order to remove them. Surprisingly, there were no roots found in the areas that had been under waterproof tarp. Under the landscape material the only roots found were those of the creeping bellflower which I removed by hand.
All things considered, it was a successful treatment and the area is now home to new cutting peonies and narcissus.
Resources
https://www.uvm.edu/extension/news/tarping-solarization-and-occultation
