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Using the sun to kill weeds and prepare garden plots

Quick facts

  • Solarization is the process of placing a clear plastic tarp over an area to heat up the soil underneath.
  • Occultation uses opaque coverings instead of clear. Occultation takes longer.
  • The plastic traps heat and moisture, which encourages seed germination and plant growth.
  • By blocking access to water and heating up the soil, the process eventually kills the vegetation underneath.

Late summer is a great time to prepare garden space to start a new garden or bee lawn. Solarization and occultation are simple methods to remove existing vegetation and get ahead of weeds.

What is solarization?

Solarization is the process of placing a clear plastic tarp over a field, garden bed or lawn to heat up the soil underneath. The intention of solarization is to kill weeds or grass, though it can have added benefits of reducing pathogen populations in the soil.

The plastic covering produces a greenhouse effect:

  • The plastic traps heat and moisture, which encourages seed germination and plant growth.
  • By blocking access to water and heating up the soil, the solarization process eventually kills the vegetation underneath.

Clear plastic tarps from your local hardware store (2-6 mil) are sufficient for polarization. The process typically takes around 2 to 3 weeks during hot summer months. The process is complete when the vegetation underneath the tarp is dead.

Clear plastic covering over a large section of a field with two uncovered dirt plots and a barn in the background.
Solarizing an organic alfalfa plot
Large field plot of dirt with sparse patches of plant life with a barn in the background.
Plot after solarization

What is occultation?

Black tarp over a large section of a grassy field with sandbags holding it down and rainwater in the corners.
Black tarp covering a field for occultation

Occultation is similar to solarization, but opaque coverings are used instead of clear. While it may sound counterintuitive, fields covered in clear plastic become hotter than fields covered in black plastic.

Black plastic actually absorbs light, whereas clear plastic allows light and heat to pass through. So occultation takes longer.

Occultation typically requires at least four weeks to be effective. The longer you keep the covering in place, the more effective it will be, up to about six weeks, at which point efficacy begins to level off.

Common materials for occultation on a small scale include common tarps and cardboard. On a larger scale, you can use silage tarps and old billboards. Since these materials tend to be relatively heavy, you can use sandbags, bricks or other heavy objects to weigh down the edges.

Why choose occultation over solarization?

While occultation takes longer, there are a few benefits.

  • The types of tarps used in occultation tend to be re-usable.
  • Solarization tarps are thin and more prone to tears.
  • Because opaque tarps are multi-functional, you may already have some at home.

New fields and lawns

If you’re using solarization and occultation to start a new garden in a space with grass or other vegetation, there are two approaches.

  • The first is to till the soil first, and then add the tarp to suppress weed seeds brought to the surface through tillage as well as remaining grass.
  • The second is to use the tarp to kill the grass. Mow the grass as short as possible before adding your tarp.

Tilling prior to solarizing or using occultation will speed up the process and can have the added benefit of aerating soil that has been compacted. 

Existing beds: prepare the seedbed

Solarization and occultation can be used for weed management at any point in the season (most often in the spring) in existing garden beds and fields. These strategies allow you to eliminate the first flush of weed seeds before planting vegetables and flowers. 

In order for weed weeds to germinate, they need warmth, good seed to soil contact, moisture and, in some cases, light. By preparing a seedbed prior to solarizing or using occultation, you increase the likelihood that weed seeds will germinate under your tarps compared to a field that has not been prepared.

To prepare the seedbed:

  • Aerate compact soil with a tool such as a broadfork or a pitchfork. 
  • Work in compost, manure or other fertilizers. 
  • Smooth the soil surface with a tilther or a rake. 
  • Water the soil before covering with the tarp. 

Water

Moisture is a key ingredient for weed seed germination, so watering before installing your tarps will improve performance. Water until the soil is moist down to about 12 inches before adding tarps. 

One key principle of solarization and occultation is preventing water from reaching the soil after the initial irrigation. So these methods will be less effective in low spots that receive significant drainage, and where water can enter underneath the tarp from the edges. 

Staking tarps

Tarps blow away easily and need to be held down.

  • For solarization, it’s best to bury the edges to form a tight seal.
  • If this is not possible, you can use landscape staples to hold the edges down, but take care to avoid tearing the plastic.
  • You can weigh down the tarp with bricks or sandbags on the corners for extra support, especially in very windy areas.
  • For heavier tarps, placing heavy objects around the edges and across the top should be sufficient.

Soil health impacts

Graph showing soil temperature each week for six weeks. Yellow line represents soil temperature under the solarized treatment, black line represents soil temperature under the occultation treatment, and blue line is control. Yellow line is consistently highest, followed by black line, with the blue line consistently the lowest.
Soil temperature reaches its highest during solarization. Source: Grace Smith, Sonja Birthisel and Eric Gallandt. University of Maine 2017.

Heating the soil can have significant impacts on soil biology, but these changes are not necessarily negative. There are few studies about the long-term impacts of soil solarization on microbial communities, but it is an emerging area of research.

One recent study of solarization and its impacts on microbial communities reported a decrease in overall species richness and abundance (Kanaan et al., 2018), and another reported a decrease in soil microbial activity after solarization (Smith et al., 2017). However, the Kanaan study showed increased yields in both wheat and eggplant following soil solarization.

As this topic becomes more commonly studied, we will learn more about the longer-term impacts of these practices.

Author: Natalie Hoidal, Extension educator, local foods and vegetable production

Reviewed in 2021

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