How to Solarize Soil in 13 Easy Steps

Soil solarization is an easy way to kill off weeds and pests.

New plastic sheeting weed barrier in vegetable garden

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Overview
  • Working Time: 1 - 2 hours
  • Total Time: 6 - 8 weeks
  • Skill Level: Beginner
  • Estimated Cost: $2.50

Soil solarization is a simple and inexpensive way to use the sun to prepare a garden for planting. Solarization involves watering a patch of soil, covering it with clear plastic, then allowing trapped heat from the sun to bake the soil to kill off weeds and pests.

A better term for this method might be soil sterilization because you're not only killing weeds and pests; rather, you're killing all organic life in the soil, even the good stuff like earthworms, mycorrhizal fungi, and bacteria that attack pests and break down nutrients to make them available to plant roots.

Fortunately, beneficial organisms are usually quick to recolonize the sterilized soil, but you can also speed up the process by re-introducing mycorrhizal spores and beneficial nematodes, available at many garden centers. You can also rebuild all that organic matter with ample applications of compost and composted manure.

Before Getting Started

Before you start solarizing your soil, consider the pros and cons of soil solarization, and weigh the cost and effort involved in the different methods of weed and pest control.

  • Solarization only works in an area that receives a full day's sun for an extended period of time: 6-8 weeks. Insufficient sunlight will not build up enough heat underneath the plastic.
  • It is better for clay soils that retain water than sandy ones that drain easily.
  • The process involves large sheets of non-recyclable plastic, which eventually need to be disposed of.
  • Solarization kills off soil-borne fungi and bacteria that can harm plants, but it will not affect those that are airborne.
  • While it is better at killing off weed seeds that are close to the surface, solarization is not as effective on root systems that go deeper than 8 inches into the soil.
  • Solarization should begin in late spring before most seeds have had a chance to germinate and larvae have had a chance to emerge, and the sheets should be kept on through the hottest part of the summer.
  • The process uses ample amounts of water, which may not be environmentally friendly if you live in an arid climate or need to conserve water.

Alternatives to Solarization

  • Weed seeds will be blowing into your new garden soil, after all, so your garden will never remain weed-free. Is the weed problem small enough that regular hand-weeding will keep the weeds at bay?
  • The best weed control is a healthy garden with few bare spots where weeds can take hold. You may not need to kill off all the weed seeds before planting a lush garden.
  • Pests will always find your garden as well. If you have an infestation, are there other, natural ways to control pests that might be just as effective? For example, milky spore is a bacterium that gets rid of Japanese beetle grubs in the soil. It is harmless to other animals and to all plants. Introducing predatory insects like ladybugs might also control your pest problem.

What You'll Need

Tools

  • 1 garden fork
  • 1 hoe or garden rake
  • 1 garden hose

Materials

  • 1 sheet clear plastic
  • 1 soaker hose (optional)
  • fabric pins/staples
  • duct tape

Instructions

If the benefits of solarization outweigh the disadvantages, you have soil that is appropriate for solarization, and if you're prepared to replenish your soil with compost, here are the simple steps involved in solarizing your soil.

  1. Clear Away Debris

    Clear the soil of debris and plants.

  2. Break Up Soil

    Break up large clods of soil so that the garden bed is friable (crumbly) and ready for planting.

  3. Level and Smooth Soil

    Use a hoe or garden rake to level and smooth out the soil so that you have a relatively smooth, even surface.

  4. Water

    Soak the soil with a garden hose to roughly a one-foot depth. The soil should neither pool on the surface nor drain too easily. Healthy soil acts like a sponge.

    Optional: Lay a soaker hose over the soil so that it can be occasionally watered.

  5. Cover Surface

    Cover the surface with a sheet of greenhouse plastic or other clear (not dark) plastic.

  6. Pin Down Plastic Sheet

    Secure the plastic sheet in place with landscape fabric pins/staples. Pin the plastic as tightly to the soil as possible to retain heat.

  7. Add Soil

    Cover the ends and sides of the sheeting with a few inches of soil to prevent blowing or tearing.

  8. Check for Tears

    Check the plastic sheet for tears or holes and repair with duct tape.

  9. Wait 6 to 8 Weeks

    Longer is better.

  10. Remove Plastic

    Find a way to re-use or recycle the plastic, keeping in mind that you may need to repeat the solarization process every few years.

  11. Add Compost

    Use a garden fork to amend your now-sterile soil with compost, being careful not to turn over the soil and bring weed seeds up to the surface.

  12. Water

    Water in the compost.

  13. Begin Planting

    Your garden area is now ready for planting. Since your growing season is most likely over, consider planting a cover crop to keep weeds at bay.