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Killer Rays from the Sun

Monday, August 22nd, 2011

Thinking of replacing your lawn but not sure how to kill the grass?  Just as we use the sun’s rays to power our houses, heat water and run our cars, we can use the sun to kill lawns as well.

The process, called solarization, uses the heat of the sun’s rays to literally cook plants, weed seeds, nematodes, and pathogens (the “bad guys” that cause plant diseases) in the uppermost layers of soil.

Summer is the best time of year to solarize. The air is warm, but more importantly, the sun has its greatest heating potential.  Solarize for six weeks or so and your lawn will be gone with a minimum investment of time, energy, money, and best of all – no herbicides!

Solarization comes to us from the clever folks in Israel where resources are limited but demand is great.

Steps to solarization

1. Cut your lawn very, very short.  Make the surface as smooth and even as possible.

2. Irrigate to saturate the soil one to two feet deep

3. Cover the lawn with 2 to 4 ml sheets of clear plastic sheet (available in the paint section of the hardware store.  This is the most environmentally ‘unfriendly’ part).  Spread the plastic so it is in contact with the soil surface, leaving as little air space as possible

4. Extend the plastic six to eight inches beyond the edges of the grass.  The edges of don’t heat as well as the center so extending the plastic assures even heating throughout.

5. If the lawn is large for several sheets of plastic, overlap the seams

6. Anchor the edges of the plastic with rocks, bricks, pieces of wood or mounds of soil.

7. Turn the irrigation off (imagine what would happen if the water went on with plastic covering sprinkler heads!)

8. Wait six to eight weeks.

9. Remove the plastic carefully.  If the plastic doesn’t have UV inhibitor (nice but not necessary) it will likely fall apart by the time the lawn dies.

To make the process even more effective, spread a second layer of plastic over the first.  Use two-by-fours or bricks to create a few-inch air gap between them.  Research shows that the second plastic layer can raise soil temperature another two to ten degrees.

Since the plastic is clear, you can watch the lawn turn from green to yellow, then to straw brown.  Once that happens, let the plastic sit another week or two, just to be sure.

Once the lawn is dead, you have a few options.  Clear away dead grass where you plan to put walkways or otherwise need an even surface.   Where the lawn is to become planting bed, just treat the dead stuff as compost.  Leave it in place and plant into, or mound soil atop it.  Eventually, it will disintegrate either way.

Since solarization works best in the upper foot or so of soil, don’t rototill or spade after you are done (actually, its best not to rototill ever).  Rototilling, or turning the soil deeply, brings seeds and pathogens to the surface where they will again proliferate.

Solarization Q and A

Why irrigate first?

Wet soil heats more quickly than dry soil.

Why clear plastic?

I’m often asked if black plastic works as well as clear.  The answer is a definite “NO!”  It’s a matter of physics, but rather than give you a complex explanation, here’s an example from our everyday experiences to clarify the concept.

Imagine a hot summer day.  Park your car in a sunny parking lot.  Roll the windows up, close the doors and leave for several hours.  When you return, open the door.  What’s your first thought?  “Boy its hot in there!” We all know from experience, that the air inside a closed up car gets far hotter than the outside air.

Now, have your car windows tinted.  Repeat the process and compare the air inside the car to the air outside.

What happens? Even though the outside surface of the car gets just as hot, the air inside stays cooler when the windows are tinted (like black plastic) than when windows are clear (similar to clear plastic).

This is the “Greenhouse Effect.” Clear glass and clear plastic trap the heating power of the sun’s rays. That’s why soil beneath clear plastic heats up more than soil beneath black plastic. Adding an air gap and a second layer of plastic heats the soil even more.

Can I solarize other areas of my garden too?

Certainly!  Raised beds, perennial beds, weed infested fields, even slopes can be solarized to kill weeds, pests and pathogens.  Just make the surface flat enough for the plastic to lie tight against the soil. Keep in mind though, that solarization will kill all plants under the plastic so if there are some you want to keep, dig them out first.