Soil solarization in raised beds
Garden of Eatin', How-To

Soil Solarization in Raised Beds

For a while I was lucky with my raised bed garden, and had few problems with pests and diseases. But this summer, a vicious case of wilt (I’m guessing Fusarium wilt) weakened or stunted several of my plants (mostly peppers) and a hungry colony of flea beetles had taken up residence in my tomatillo crop.

Combined with the drought in California, a particularly hot season, and a month-long vacation looming, I decided to put all those problems to bed — under a sheet of plastic for the remainder of the summer.

Soil solarization is a highly effective, nonchemical method for controlling soilborne diseases. While it’s commonly used on commercial farms, it’s not as prevalent in home gardens because it does require part or all of the soil to lay fallow during peak summer. In a home garden where space is often limited, it’s hard to give up a raised bed for the four to six weeks it takes to treat the soil.

But if you plan ahead (or, like me, you know you’ll be away from the garden for an extended period), soil solarization is an ideal solution for killing weed seeds, controlling nematodes and pests, eliminating soilborne plant pathogens, and improving tilth and soil biology. Think of it as a solar oven in the garden, baking everything underneath it — and what comes out is sterile soil, free of the problems that used to plague your plants.

The Benefits of Soil Solarization

Simply by using the power of the sun, solarization can rid the soil of most weed seeds, especially those from annual weeds (some perennial weeds, like Johnson grass, may have deep roots or rhizomes that the heat won’t reach).

The intense heat also speeds up the decomposition of organic material in the soil, thereby releasing soluble nutrients such as nitrogen, potassium, calcium, and magnesium and making them more available to plants.

Solarization disinfests the soil of the fungal and bacterial pathogens that cause Fusarium wilt, Verticillium wilt, potato scab, Southern blight, early blight, tomato canker, club root, crown gall, and damping off. (Note that it doesn’t work on airborne diseases, like garlic rust or late blight.) Certain fungi, such as the spores that cause Fusarium wilt, can live for many years in the soil and even in the soil clinging to your garden tools, so in these cases crop rotation is not an effective means of control.

Solarization also reduces populations of nematodes (like root knot and dagger) and destroys the eggs, larvae, and pupae of destructive pests (like cucumber beetles and squash vine borers).

But What About the Good Guys?

It’s believed that earthworms simply burrow deeper into the soil to escape the heat, and come back to the surface when conditions are ideal. As for beneficial soil organisms, many of them are able to either survive solarization (such as mycorrhizal fungi, which are highly heat tolerant) or rapidly recolonize the soil (such as the bacteria and fungi that parasitize plant pathogens and stimulate plant growth).

In fact, the increased numbers of beneficial microbes can make the soil more resistant to pathogens after solarization (as opposed to nonsolarized or fumigated soil). In turn, plants grow faster, get stronger, and stay healthier.

Timing Is Everything

Soil solarization works by trapping radiant energy from the sun under a thin plastic tarp to heat the soil at temperatures high enough to kill soil organisms.

Soil temperatures of at least 99°F, held steady for about four weeks, will prevent the emergence of many annual weeds and mesophilic fungi, which are the majority of common plant pathogens. That means soil solarization works best in the hottest month of the year when days are long and skies are clear, which for many zones is around the summer solstice in June or July.

Out on the Southern California coast where I live, our hottest months are September and October, when Santa Ana winds blow in from the high desert and bring us sweltering Indian summers (as well as notorious California wildfires).

Even with a relatively mild high of 75°F in early September, before the heat wave’s hit, the first 3 inches of soil in my raised bed (that gets full sun for most of the day) heats up to 116°F. In just a few weeks, that temperature will steadily rise and cook whatever’s still lingering under the plastic.

Soil solarization is less effective in the spring, even if your garden is bathed in sunshine all day. The mild weather does little to control soilborne diseases, so it’s not worth the effort to try early in the year before you start your planting.

For best results, determine the hottest four- to six-week window in your climate and plan to solarize your soil in that time.

Preparing the Soil

Before you begin, remove all the plants and mulch from the bed. If they were affected by disease, bag and trash them.

For successful solarization, the soil should be smooth and flat to allow the plastic to lay snug against it. Till or turn over the soil and remove or break up any clods, rocks, weeds, and plant debris. You don’t want anything in the soil that could potentially tear or puncture the plastic. I typically dig to a depth of a fork tine, but for heavy clay soils or soils that haven’t been cultivated in a while, going at least 1 foot deep is recommended.

Turn over the soil and remove all debris

Rake the soil smooth and flat

Studies have shown that adding organic animal- or plant-based amendments (such as aged animal manure or cover crop residues, especially Brassicaceae cover crops like mustard) before solarizing improves its efficacy. The amendments not only increase the rate of heat generation in the soil, but also its heat-carrying capacity. And when it comes to soil solarization, more heat is always a good thing.

So, if you have some composted chicken manure, worm castings, bat guano, green manure, or well-aged kitchen compost, go ahead and incorporate that into the top 3 to 4 inches of soil, and rake it in well.

Water deeply before solarizing

Once your soil is smoothed over, set your garden hose or irrigation system over the bed and water deeply. You want the top 12 inches of soil to be moist. The moisture conducts heat faster and deeper into the soil, while making soil pathogens more sensitive to the heat.

Choosing the Plastic

Surprisingly, clear plastic is actually more effective than black plastic at heating up soil. The sun’s rays easily pass through clear plastic, only to be trapped inside to heat the soil. By contrast, black plastic tends to absorb and deflect part of the heat.

And though it might seem counterintuitive, the thinner the plastic, the better it captures and traps the heat. Very thin plastic (1 mil) is the best option, but can be susceptible to damage from birds, critters, or the elements. A good compromise is 1.5 mil to 2 mil plastic with weatherproofing or UV resistance, as it will last long enough until you remove it from your bed.

2 mm plastic dropcloth

I use a 2 mil plastic dropcloth (found at my local hardware store), but for larger gardens, you can find bulk rolls of polyethylene from farm supply stores or online.

Covering Your Raised Bed

Cut the plastic to fit, leaving at least 8 inches of overhang on all sides. There are a few ways to secure the plastic:

  • You can dig a trench along the inside perimeter of the raised bed and then bury the edges of the plastic 6 to 8 inches deep.
  • You can hold the plastic down with heavy objects (like 2x4s, pipes, planks, paving stones, or whatever you have around the house) placed around the perimeter of the bed, making sure they’re tucked tightly against the walls.
  • You can staple gun the plastic to the bed itself if the walls are made of wood.

Staple gunning the plastic is the way I do it, and I simply wrap the corners around the bed as if I was wrapping a package. Staples go in every 6 to 8 inches to ensure a secure seal, and then a couple of weights are laid on top of the plastic to keep it stable under any winds. (You don’t want your tarp to become a sail during a summer storm!)

Lay the plastic over the bed

Lay the plastic flat against the soil

Secure the plastic over the raised bed

Wrap the plastic tightly around the bed

The weights can be bricks, beams, sandbags, stones, or even dirt (but make sure you remove this dirt before you take the plastic off, as you don’t want to contaminate your newly sterile soil with it). You also want to be careful with sharp or rough-edged weights that could cause pressure points, leading to tears.

Raised bed wrapped in plastic

Raised bed being solarized

The idea is to keep your plastic as flat against the soil as possible, with little to no flaps or openings that could let in outside air (and decrease the temperature under the plastic). Patch up any holes in the plastic immediately with duct tape, and keep an eye on your beds throughout the four- to six-week period in case the plastic needs patching. (One morning, I found a few holes in one of my tarps that had apparently been caused by a raccoon running through the garden.)

If your daytime temperatures are cooler than normal, you can increase the amount of heat generated in your raised beds by adding a second layer of plastic over the first layer. Separate the layers slightly with PVC pipes, plastic bottles, or other smooth objects that can run the full length of the bed; that small pocket of air can increase the heat in the soil by as much as 10°F.

Maintaining the Proper Temperature

Soil solarization is most effective when the top 6 inches of soil is maintained at or above a daily temperature of at least 110°F for four to six weeks, as most pathogens reside in this upper layer. If you don’t have a soil thermometer to test this, a meat thermometer works well. I like to get a reading in the middle of the day right in the middle of the bed; afterward, I simply patch the hole with duct tape.

Condensation under plastic

See all this condensation under the plastic? When I rest my hand on the surface, I can feel how how it is under there. That’s what you want, every single day.

Post-Solarization

After removing the plastic, you can sow seeds or plant transplants like normal. To avoid bringing any surviving weed seeds to the surface, stick with shallow plantings.

There’s no need to further cultivate the soil. You’re already starting fresh with soil that’s gained additional nutrients from solarization, so fertilizer can wait until midway through the season (simply do a side dressing, soil drench, or foliar spray as needed).

To improve your chances of not reinfecting the soil, always start with clean pots and new soil for seed starting and transplanting, and wash away the old (possibly contaminated) dirt from your garden tools, gloves, and other accessories that frequently come in contact with your plants.

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  • Opal

    Good article.

    However, a mm (millimeter) is 1/1000 of a meter, and a mil, which is a standard unit for plastic measurements, is 1/1000 of an inch. The units (mm) in the article are incorrect.

    1 mm = 39 mil

    2 mm plastic is very thick; 2 mil plastic is not

    • Thank you for catching that; the article has been corrected. Not sure how I mistyped it when the picture of the packaging very clearly says 2 mil! 🙂

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  • Patti Szczygiel Jennings

    I’m preparing to solarize an area of bed. I have a river rock/stone that is square. Measures about 25 inches in width and length and about 4 to 5 inches thick. I’d like to put it back in it’s original position in this bed after solarizing is complete. How can I clean the rock (or can it be solarized) to clear any root rot nematodes/poisons from it’s side that will sit in the soil. The rock is my mothers and it came from her family farm homestead and she will not part with it. She wants it back in the original position in that part of the soil when the solarizing is complete. Is this possible? I thought of placing it on top of the solarizing tarp with side up that was in the soil and then placing a second tarp layer over that so that the rock would get it’s own ‘solarizing wammy’. Would doing that solarize the rock or must I use a bleach mixture to scrub and cleanse the rock????? Till I receive an answer I will keep the rock out of site on a part of non-traffic area sidewalk.

    • I would recommend scrubbing the rock with a very dilute bleach solution and then leaving it out in the sun, the same way you’d clean your garden tools.

      • Patti Szczygiel Jennings

        Thank you. I will do that.

  • Dawn

    I’m in a community garden with about 30 raised beds. I have early blight in my tomato plants (Brandywine and Super Sweet 100) and so does virtually every other bed in this garden. I live in Chicago, and it’s been a horribly wet and cold couple of months. Even the farmers at the local farmer’s market who come from Illinois, Wisconsin and Michigan are telling me they’re not sure they’ll have field tomatoes this year. A couple told me they have terrible blight due to all the rain. As of mid-July, even if I ripped out all the plants, including the things that are doing well like my cucumber plant, I’m not convinced it would get hot enough the rest of the summer for solarization to work. What are my options? Give up on the raised bed? If everyone else’s bed is contaminated with blight spores, I may be SOL because I believe bugs spread it. I live in an apartment so I have no other option. It’s either this community garden or nothing. Switching to another bed won’t solve the problem if the other beds have blight as well.

    • With early blight, the spores are spread by splashing water, insects, tools, and human contact. Any nearby plants are easily afflicted so if your entire community garden has it, and the weather is consistently rainy, there’s not much you can do. The spores continue to live in the soil and will only dissipate through crop rotation (at least 3-4 years) or, as a last resort, application of a fungicide. I recommend trying a blight-resistant tomato variety next season and mulching heavily to ensure water does not splash up from the soil to the leaves. Also, be sure to trash your tomato plants (do not compost them) and disinfect any tools, gloves, boots, etc. that came in contact with the soil to prevent reinfection the following year. Good luck!

      • Dawn

        Thank you, Linda. I appreciate your follow-up. I will think about what to do next year. With every gardener planting tomatoes in our space, it’s going to be tough to avoid blight in the future, too. Your suggestions are good ones and I’ll certainly spread the word!

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  • Tavia Lee Wagner

    What about any toxins from the plastic leaching into the soil?

    • No cooperative extension site or ag resource site I’ve searched makes any mention of plastic compounds leaching into the soil. As long as you remove the plastic after 6 weeks, before it starts to break down into the soil, I think you’ll be fine. It’s only meant to be a temporary cover, not a permanent mulch.

  • Very well written with plenty of detail. This is a great technique for also warming your soil up right before you plant some seeds also.

  • I just saw this technique being implemented on El Camino Real–they were wearing white suits. I assume you didn’t need the white outfit for your beds. I may give this a shot with a section of our garden…I think we are invaded by ants, however…they will be back even with soil solarization.

  • April

    Also what do you do to keep your pathways weed free?

    • I apply at least 6 inches of wood chips as a mulch for my pathways. Since wood chips break down over time, we “refill” the paths once a year.

      • April

        I do this too but I have a terrible time with grass and weeds growing in the wood chips. I’m thinking of hitting it with vinegar.

        • If you’re smothering the weeds with a very thick layer of mulch but they’re still popping up, I’m guessing you have seeds blowing in from elsewhere and taking root. Or, there might be weed seeds traveling with your mulch.

  • April

    Will this help with thistle, grass and morning glory? The last two years I’ve not planted all of my raised beds because of a very difficult pregnancy and a rough start for baby, so I’m suddenly seeing morning glory in places I never have. And orchard grass is showing up everywhere. Same with thistles. I’m surrounded by farmland and lots of wind, so it blows in. Maddening! Also, this is the year I refresh the wood chips in my beds. Should I add them before I cover? For morning glory and grass would it be better to use black plastic to starve them out? I have enough beds that I can do most this year and the rest next year.

    • You would need to dig up and pull out all traces of those weeds (like the rhizomes from the thistles as well as the long root systems from the morning glory — I assume you mean wild morning glory, i.e. bindweed here) in your beds before solarizing. Then, the heat will kill off any seeds that have dropped.

      As for the wood chips IN your beds… I’m not sure if you mean mulch? If you are going to solarize a bed, it has to be completely empty. The only thing you should be adding in, before covering with plastic, is organic fertilizer. Once the plastic is removed, you can plant and mulch like normal.

      • April

        I was afraid you would say that. Since most of what I’ve read about killing morning glory says keep pulling it so it starves the root system, I was hoping that black plastic would accomplish the same thing. I’ve been digging up thistles and half the time not getting that very tip of the root. So irritating. The morning glory is also showing up in my asparagus bed. That one is what I’m most worried about. Asparagus is sacred. I never dealt with these types of weeds where we used to live, so I’m new to this particular problem.

        • Persistent pulling is the only method I’ve heard of for eradicating bindweed. Their roots go down several feet, and they creep much like mint does, sprouting new shoots very easily. But each year that you pull them up, you’ll weaken the plants more and more. Just remember to remove the vines right away and never let them flower.

  • Karen

    Very useful info! I will also be doing this, as soon as I make more beds. Thanks for sharing!

  • April Mae

    Great post! Super informative and I will be doing this, I got wilt on my zucchini and crane melon this season. Boo.

    • If you’ve got the space and the time, I highly recommend it.

  • Interesting. I had not heard of this technique before. We have a problem with squash vine borer and tomato blight every year, but I don’t know if I am willing to give up any of my raised beds for so long during Iowa’s already short growing season. I will keep it in mind if we ever plan to be gone a good chunk of the summer. By the way, your garden looks a lot like mine – a series of rectangular raised beds surrounded by wood chips. Of course, mine is minus the tropical plants and has a six-foot-high fence around everything to keep the deer out! 🙂

    • If you’re able to rotate your crops on a 3-year basis, that should take care of many soilborne pests and diseases. But I definitely recommend solarizing if those problems seriously persist every season. Sometimes it’s just better to sacrifice part of a growing season and work on prevention (especially if you plan ahead and start your plants in pots while waiting) than to constantly deal with treatment.

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