One day, while perusing Rodale’s Garden Problem Solver to look up a tomato plant disease, I came across a brief explanation of solarizing. This is essentially a natural means of pasteurizing the soil to kill off many of the things that cause so many problems.
Once upon a time, it was thought that certain plant diseases simply could not be addressed once they took hold in the soil. At some point in the…1980’s I think, this technique was developed in Israel and has been tested enough times over the years that it is now a generally accepted technique for conditioning the soil to kill off those diseases, plus weed seeds and certain pests. The funny thing to me is that, even though I can do a search for “solarizing” and get a whole slew of hits, it’s still not really a household name so not universally known. There are lots of sources out there which can likely provide better detail, but here’s a decent primer.
The best time to solarize the soil is during the hottest period of the summer—so, depending on where you are, can be anywhere from July through early September. July here was surprisingly cool, so it’s a good thing that I decided to start at the beginning of August, right when the temperatures have started to spike again. Here’s the basic process:
- Make sure your bed is completely cleaned of plants and weeds.
- Loosen up the top foot or so of soil with a pitchfork, or tiller.
- Water the soil heavily so that it is completely soaked. Much wetter than a regular watering. Basically, you want to make mud. Then let the bed sit overnight.
- The next day, cover the bed with clear plastic sheeting—anywhere from 3 to 6 mils thick. Do not use black plastic because it will not produce the necessary greenhouse effect. I used a 4 mil sheet.
- In most gardens, the final step would be sealing the plastic sheeting by covering the perimeter with soil. However, that’s impractical for our landscaped garden beds, so I seal the edges with mason block caps.
Then, for my part, I moved the butternut squash vines back to their earlier place, across the now-prepped bed and into the peppers. I can’t leave them where they are, because I need to walk on that area to tend to the tomatoes. I also can’t just lay them down on the plastic because they will shade the soil and make this whole effort pointless.
The plastic will stay in place for four to six weeks. During that period it will rain, so I will need to use a broom to sweep the water off of the plastic, otherwise the pooling water will reduce the greenhouse effect.
With the high humidity under the plastic, combined with direct sunlight, the soil temperature can get as hot as 140 degrees, which will kill off pathogens, shallow weed seeds and destructive nematodes. One of the funny things about this process is that it does not appear to wipe out certain beneficial organisms, which means the bed will produce healthier, more robust plants. The jury is still out on how long this treatment lasts, but some studies appear to suggest that a properly solarized bed can stay free of pathogens for up to five years. Because we rotate our crops, and because the onions and garlic are harvested right around the time solarization is necessary, my plan is to solarize one bed every year, and then plant the tomatoes in that bed the following year because those are the plants that have the most trouble with disease. We’ll see how it goes.