Zilker Neighborhood Gardens

Commentary by a National Wildlife Steward who helps Zilker neighbors establish native plant gardens

Monthly Archives: June 2011

Soil Solarization – Killing More Grass (without chemicals)

Well, I implemented the newspaper and mulch method to kill grass/weeds on part of my front yard and my back yard, which I described in an earlier post. The backyard worked pretty well requiring only a little weeding here and there – mostly because squirrels buried oak acorns into the ground, and they are popping up.

The front yard has been more of a challenge. As I wrote about earlier, some grass is popping through my newspaper and mulch area in the front yard. I’m a bit disappointed with this. The method has killed the St. Augustine grass that was below it. But what seems to be growing is some bermuda grass, one of 24 most invasive plants by the City of Austin. I have decided to remove the mulch and newspaper, dig up the dead grass, till compost in the soil, then plant, then put mulch back on. This will be a chore, but I am certain I will be much happier with the planting bed. At this point, I’ve only planted trees in the beds, and I’m still having a hard time deciding on my plant choices.

I have a semi-formal plan for my front yard and the garden bed I prepared with newspaper and mulch is only HALF of the plan. So I still have another half of the yard to kill grass on. I have decided to try soil solarization on the other half. One of the speakers in our Habitat Steward training discussed this method, and I know many of you have used it.

The point of soil solarization is to get the sun to heat up the top 6″ or deeper of soil and kill all the weeds (or grass for my purposes). Contrary to what many people think, you should use clear plastic that is 1-6 mil in thickness. Some sources say 1-2 mil is better as it lets more of the sun through. In our climate right now, soil solarization is the perfect way to go since it is so hot and the sun so direct. (In cooler climates, one can use black plastic but I won’t go into that here for Texas. Use clear plastic, especially in a sunny area in the summer.) Might as well take advantage of our heat wave!

BENEFITS

Solarization during the hot summer months can increase soil temperature to levels that kill many disease causing organisms (pathogens), nematodes, and weed seeds and seedlings. It leaves no toxic residues and can be easily used on a small or large scale garden or farm. Soil solarization also speeds up the breakdown of organic material in the soil, often resulting in the added benefit of release of soluble nutrients such as nitrogen (N03, NH4+), calcium (Ca++), magnesium (Mg++), potassium (K+), and fulvic acid, making them more available to plants.

Plants often grow faster and produce both higher and better quality yields when grown in solarized soil. This can be attributed to improved disease and weed control, the increase in soluble nutrients, and relatively greater proportions of helpful soil microorganisms. (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74145.html)

Before you put the plastic down, the ground should be soaked with water, as wet soil conducts heat better than dry soil. Having the soil moist seems important, and you might to wet it during the process (life up and edge and spray water in) – sources indicate this might reduce the temperature for a time, but overall is important for the solarization to occur.

One of the keys to success is getting the plastic as tight as possible to the ground. So I plan to mow the current grass as low as possible in the area I want to solarize. I will also edge the area deeply so I can secure the plastic tight down the ground. You can also use bricks or rocks etc. to keep the edges down. It will be hotter if there’s less billowy air pockets inside. We do get a fair amount of wind, and you don’t want your plastic to set sail.

Then you wait 5-6 weeks. Have a few cool drinks and know that you’ll get to the gardening when it is cooler outside!

An excellent resource on soil solarization can be found from the University of California. They have further, very detailed downloadable packets of information here.

The Natural Gardener also has a short description of soil solarization. They have slightly different advice on the preferred thickness and length of time to keep the plastic on than the UC source.

I welcome comments regarding this approach – have you done it? How effective was it? Any challenges? Personally, I sure hope this works as the alternative of digging the grass out by hand is not optimal.

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