Saturday, June 15, 2013

Solarizing the lawn

For want of another name, I've been calling the 20'X25' patch of grass and weeds at the back of the house "the lawn." The weeds, burr clovers, dandelions, thistles and bermuda grass were mown and watered and from a distance provided a green patch where garden designers would say you could "rest' your eye. On closer inspection it did nothing but cause me embarrassment. Something had to be done.

The options were to get someone in a re-sod the entire thing. Though I was pretty sure they would use chemicals  to kill the weeds and I've never used herbicide in the garden (once in the driveway a couple of years ago and that didn't work!) And its expensive!

I'm a garden coach and a Master Gardener. Deep down I knew I had to tackle this myself.

So I can either kill all the weeds and resow the lawn seed, or design a beautiful sitting area with a gazebo/pergola covered in vines with a dry creek bed and some interesting yard art. The reseeding of the lawn would be cheaper but the latter so much more interesting.

Step one for either plan: Kill the Weeds

This can be accomplished by using cardboard and mulch. I've used this to good effect in the front yard but... well, I was feeling like a one trick pony since I always seem to be advocating that (probably because it is so effective.)

Recently I attended a Master Gardener talk on soil solarization. It's used in the Central Valley and Southern California where the heat of the sun is employed to sterilize soil from both weeds and pathogens. The speaker seemed dubious as to how well it would work in the Bay Area since you need really high temperatures, but I am willing to experiment.

Basically you use clear plastic sheeting to trap the heat of the sun to super heat the soil. It is best done in the summer months June - August, where the days are long and hot. Needless to say - Do not try this in Ireland!

It can only be done where you get full sun. Shade areas will not build up the necessary temperature.

For more details on the theory behind it check out the note on it on UC Davis site.

There are 4 easy steps. (Okay, I use the term "easy" very loosely.)

1) Roto-till the soil.
That wasn't as easy as I thought it would be. My wonderful neighbor, Al "lent" me his roto-tiller and then had to show me how to use it. I wasn't very good at it so he just did it for me. I recommend saturating the soil well a day or two before hand since we had a hard time getting the blades into the hard dry soil.
 I'd turned off the water last winter so the ground was really dry.

2) Flatten the soil with a lawn roller.
I hired one from A Rental Center for $12 for the day. I did however flood the lawn the night before (having learned my lesson the hard way - literally!) so that the soil would actually shift and move. It worked well, apart from the fact that the bung for the roller broke (the roller is basically a steel drum that you fill with water.) Thank goodness for pink duct tape!

3) Saturate the soil with water.
The water will conduct the heat from the sun down into the soil - that and the compaction from the roller helps to transmit the heat energy deeper down. I really soaked the area.

4) Cover the area with clear plastic for six weeks. 
I used the stuff you find in the paint section of the DIY store. The thinner it is the better. It needs to be tight to the ground - it helped to sprinkle a wee little tiny bit of water over it to weigh it down. Then you add a second layer. The gap between the two layers provides some insulation at night to stop too much heat being lost - though I am a little worried that the water between the sheets will lessen this effect. Di I mention that it wasn't much water? We'll see in six weeks. (Black plastic would radiate the heat out at night.)

Hopefully in six weeks time I'll take off the plastic and have weed free, disease free soil.

At that stage I can either resow a lawn or move forward with a nice shade area...but since we didn't come here for the grass what do you think we'll do?

Byddi Lee


  1. Nice post and great information. Did you seal the edges of the plastic with earth all around to keep the moisture and heat underneath? I remember that as one of the steps. We tried this at Nine Palms but didn't dig, water or seal well. Just laid plastic over damp Bermuda grass which, of course, just kept going under the plastic! Laughed in our faces, it did.

  2. I used planks to seal the edges. I hope they work! Its really hot already!

  3. How did your project come out? I have zoysia grass that I am trying to get rid of too.

    1. Hi - I'd have to say that I had no more success than if I'd used cardboard and mulch to remove the lawn. The bur clover continues to germinate so evidently the solarization did not kill the seeds. These seeds are robust and are not blown in on the wind so must be germinating from whatever is left in the soil but hand pulling once a week keeps those at bay. I didn't get Bermuda grass coming back so that was at least a success.

      It simply did not get hot enough here to warrant using this as standard practice. It takes too long and the garden is "out of commission" for the entire summer. Block light using cardboard and mulch and you can get as good a result.

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  5. I live in San Jose CA and am trying to kill our Bermuda grass area with solarization via following steps: mowed the bermuda as short as possible, put fertilizer over it to encourage growth, watered heavily and then covered with 4 mil plastic. It has been under plastic for 3 weeks; will leave it covered for at least 8 weeks. We can still see the moisture accumulation/spots thru the clear plastic (may have watered too heavily before putting the plastic down). I put in garden staples to anchor the plastic and have some bricks around most of the edges but the bermuda is growing and extending out around the perimeter in many areas. I've removed the lush Bermuda creeping out at various points on the perimeter and note that the soil is wet.
    At 12:30 pm on 7/30/16, the soil temp read is 100 degrees 5 inches down. Believe it needs to be btw 110 - 125? Will retest temp today at 4 pm. Outside temp today is 92 degrees but we have had temps in the 80s a couple weeks ago. Any suggestions on how to improve the outcome?

    1. Hi Barb

      Best of luck with your project.

      I found the following year that the solarizing wasn't as effective as I'd hoped -

      Do considering laying cardboard under the mulch even though you have solarized. THat way any weeds that may have survided will be smothered... A bit of a belt and braces approach.