"What exactly is solarizing?" That was a question that was sent to me recently by a reader of this column. Because the subject is somewhat unusual and the results are very effective, I thought I would share the answer with you today.
"Solarizing" might sound like a cosmic phenomenon or something you'd do to the windows in your greenhouse or conservatory. However, the real definition is, quite literally, much more "down to earth."
A simple definition of solarizing could be the act of trapping the heat from the sun and concentrating it on garden soil in an enclosed environment.
Most often, solarizing is used to kill off weed seeds in the soil without having to revert to chemicals. Many landscapers and gardeners also use solarizing to kill off harmful organisms and fungi that are otherwise quite difficult to get at.
Solarizing is quite an easy project for even the most "horticulturally-challenged" person! And it's also quite fun. All you really need are some sheets of clear plastic, preferably from 1 to 4 mil thick. You'll need to get twice the square footage of the area you intend to cover. Most major hardware chains and garden centers sell it in rolls or by the foot and you can usually find it in various widths. You'll also need some objects to use as "spacers." More on those in a moment.
Start by tilling the soil, either manually or using a rented rotary tiller, then roughly rake the surface so it's fairly level. Then give it a good watering. I mean a REAL soaking, so the water goes down about 12 inches or so.
Now cover the entire area with half of the plastic sheeting. You're going to add a second layer of plastic sheeting but you're going to want to create some air pockets between the two layers which is where the spacers come in. You can use bricks, smooth rocks (without any sharp edges that might tear the sheeting) or empty 12-ounce beverage cans.
Invite a couple of friends over to help you "empty" the beverage cans. Or if you worked up a particular thirst with all that tilling and raking, you can drain the beverages from the cans all by yourself. Tell your spouse that this is an essential step in ridding the soil of weed seeds and fungi. It's worth a try.
Spread the spacers evenly on the surface of the first sheet of plastic, maybe around 18" apart. If you're using bricks or cans, lay them on their sides, not upright. Now place the second sheet of plastic over the first sheet and your rows of spacers. It doesn't matter if the two sheets touch each other in places between the spacers, but you do want to create a fairly sizable air-trap between the sheets.
Now all you have to do is seal the two sheets of plastic to the ground. Scoop soil up and over the perimeter of the sheeting. You can augment the soil with some bricks or smooth stones if you think the plastic sheeting is in danger of being blown away by strong wind gusts.
Leave the "solarizer" in place for a month or two (the longer the better) and when you finally remove it, your soil should be free of weed seeds, fungi and soil-borne pests such as nomatodes.
Because this process only works by using the sun's heat to raise the soil temperature enough to kill off all the bad stuff, this generally is NOT a project for the winter months, depending on your location. After all, if the plastic sheeting is covered by frost or snow, the sun's rays can't do their job.
However, the winter is a good time to plan ahead. In December and January you can decide which areas of your landscape would benefit from a solarizing treatment in the spring or summer. You can also start work on emptying those beverage cans you are going to need as spacers!
Hey, I told you that this was a fun project, didn't I?