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Use the sun's energy to kill weeds and establish new gardens and bee lawns

August 20, 2020

Are you planning to start a new garden or bee lawn? Late summer is a great time to prepare your new space. Two tools, in particular, solarization and occultation, are simple methods to remove existing vegetation and get ahead of weeds.

What is solarization?

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 during solarization, occultation, and control treatment, in a 2017 University of Maine study by Grace Smith, Sonja Birthisel and Eric Gallandt.

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, 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. 

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 plantlife with a barn in the background.
Plot after solarization

What is 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 would someone choose occultation over solarization? While occultation takes longer, there are a few benefits. For one, the types of tarps used in occultation tend to be re-usable, whereas solarization tarps are thin and more prone to tears. Because opaque tarps are multi-functional, you may already have some at home.

Black tarp over a grassy field with sandbags holding it down.
Black tarp covering a field for occultation

Tips for success

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. If using the latter approach, 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 your main 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.

Preparation improves the success of solarization and occultation. Preparation includes:

  • Aerating compact soil with a tool such as a broadfork or pitchfork. 
  • Working in compost, manure or other fertilizers. 
  • Smoothing the soil surface with a tilther or a rake. 
  • Watering. 

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 in the soil profile before adding tarps. 

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

Staking

Tarps are like sails in the wind. They blow away easily and need to be held down. For solarization, it’s actually best to bury the edges to form a tight seal.

If this is not possible, some gardeners use landscape staples to hold the edges down, but take care to avoid tearing the plastic.

You can also use objects like 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. 

Natalie Hoidal, Extension educator, local foods and vegetable production

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