On Soil Solarization

Wendy S. Delmater
By Wendy S. Delmater on Wed, Jun 18, 2014 - 5:16pm

Some of our raised beds have had problems with bacterial wilts, either verticicillium wilt or fusarium wilt. The only way to kill these organisms is Soil Solarization. 

Soil solarization is a practice used to manage weeds, nematodes, diseases, and insects in soil. The soil surface is covered with clear plastic, which allows sunlight to pass through and heat up the soil to temperatures that are lethal to most diseases and garden pests..

The best time for soil solarization is the hot summer months: June, July, August. Solarization can be done on any soil type but for best results, it should be done in open, unshaded areas. Shade is not your friend for this process.

The area you're to solarize must first be cleared of weeds, sticks, old roots, and other debris . They should be removed so things do not poke through the plastic. Water helps to conduct heat, so if soil is moist but not waterlogged it will "cook" more thoroughly. In sandy soils, solarize right after rain or irrigation, preferably the day before plastic is applied - not the same day so the soil is not muddy. The plastic has to stay clear, not muddy! Note: We find the heavier gauge plastics are not only resuable, but easier to take up than thin ones that disintegrate.

We're solarizing four raised beds this year, but it works well on flat ground, too. We bought rolls of heavy-duty plastic painting tarps from a big-box store, and saved a bunch of them for future projects; they're good for things like protecting the wood pile or emergency window or roof coverings.  It must be clear: black plastic heat up at the surface but does not heat the soil, and translucent (whitish) plastic does not let enough sunlight through to do a good job.

A clear plastic sheet or strip has to be stretched out over the area you're treating. The plastic should be a little bigger than the area you're treating because the edges will need to be weighted down or buried in the soil.  The plastic should be stretched tight and the edges sealed completely by burying in soil or weighting them down, hard. If the edges are not completely sealed, heat will leak out and problems may result in these cooler areas.

Solarize for at least six weeks. It's like you locked the germs and pests in your car with the windows rolled up in the heat of summer! You may have to repeat this process every so often, as the results can last as little as 3 to 4 months, but you've get in a good autumn crop in clean soil. Speaking of clean - to avoid losing months in your garden solarizing again CLEAN your gardening tools in-between working in different areas/raised beds. Diseases like  bacterial wilts can be transmitted by dirty tools. I keep a cannister of Clorox wipes in my gardening tote for just that purpose.

1 Comment

BSV's picture
Status: Silver Member (Offline)
Joined: Jan 26 2009
Posts: 170
Cleaning garden tools

Wendy makes an excellent point about sterilizing garden tools. Purchase a bottle or two of 91% denatured alcohol (you may need to search for this, for 91% is needed, not 50% or so) and spray your cutting tools before and after using them. 

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