Some plants, like peas and lettuce, are happy enough with cool air and cold feet, but others insist that their roots be kept warm. This creates problems in areas such as interior Alaska where the ground is frozen so deeply that it may be well into fall before the soil is warm enough to satisfy corn, tomatoes, beans or squash outdoors.
It is possible, of course, to attack the problem with brute force. Build a greenhouse, or use heating cables in the soil. Mounding the soil also helps. So do raised beds. But all of these together are barely enough in the Fairbanks area.
Clear plastic allows the soil to retain the heat supplied by the sun. My own experience is that it also provides a perfect environment for weeds to grow under the plastic. Maybe they cook in warmer climates, but here in Fairbanks clear plastic can be pushed right up by rampant (and very healthy-looking) pigweed and lambs’ quarters.
Black plastic or landscape fabric? They stop weeds, and you’d think they would absorb sunlight and warm the soil. Nope. They’re not in good thermal contact with the soil, and while the black covers themselves may warm, they do not transfer that heat to the soil. Black covers have the net effect of shading the soil, lowering its temperature.
Luckily, it is possible to combine the two.
Ever seen a rainbow? Or the breaking of white light into colors by a glass prism? Then you are aware that sunlight is actually a mixture of light of different colors. What you may not know is that our eyes are sensitive to only part of those colors, and that growing plants need mostly the same colors that we can see. But only about half of the energy of sunlight is in these colors. A little bit is in the ultraviolet, the part of the solar spectrum that tans and sunburns our skin. That’s only a small fraction, and most of that is stopped by the ozone in the atmosphere. A much larger part of the invisible energy is in the near infrared.
Most of this near infrared energy passes pretty freely though most substances (such as air) that we consider clear. It is a large part of what makes sunlight feel warm. But plants cannot use it to photosynthesize, so if they get only near infrared light they cannot grow. No weeds!
IRT stand for infrared transparent, and IRT plastic allows the near infrared radiation through to warm the soil, but blocks the visible radiation that would allow weeds to grow. I’ve been using IRT ground covers to grow squash and beans for years, even though I grow them in raised beds.
It does have one problem: it’s waterproof. At one time, I could find it with microscopic holes that let rainwater drain through to the soil underneath, but all I can find now allows rainwater to puddle on the surface and the soil to dry out underneath. Until this year, I had to carefully shape the soil so that the plants (and the holes for them in the plastic) were in low spots. This year I’m trying something new. What? I’ll tell you when I know whether it works, but if you look, it’s visible in the photo.