Soils - Solarization
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  (September) You spoke about solarization a while back, and it caused some questions among me and my Friends. First, why should clear plastic be used instead of black ? Second, if there is a bad weed infestation, should Round‑up be used before laying the plastic? Finally, should the ground be tilled before the plastic is laid?

  Clear plastic is preferred in the summer months because it actually generates more heat than black plastic. If you are simply trying to smother out weeds, then black plastic is fine. If you are just now getting started, you would have to opt for black plastic. Using clear plastic in cooler times actually serves as a little greenhouse, helping plants grow. For true solarization, you must do it in the hottest summer months. At that time, till the soil deeply, water thoroughly and then cover with clear plastic in July and August -- leaving the soil covered for 6 to 8 weeks. Using Round‑up before solarization is fine. If done properly, and at the right time, however, it shouldn't be necessary.

  (December) Earlier this summer, you mentioned sterilizing flower beds using plastic. I have an area in my yard that I would like to start a new flower bed in. It is full of grass and weeds, and I want to know how to prepare it now so it will be weed free next spring. I think you said to use clear plastic and leave it down for a few weeks. Is that right? Any other pointers would be greatly appreciated.

  Soil solarization is a process we do in the summer, using clear plastic to build up high heat to kill weed seeds, disease organisms, etc. It won't work in the winter, because we can't generate enough heat. At this time of year, you can scrape the soil of grass and weeds, till the soil and cover with black plastic. This will help to smother out any living weeds or grass, but won't prevent weed seeds from germinating next year once the plastic is removed. Using clear plastic now would be like having a little greenhouse in the yard -- the weeds and grass could continue to grow. You could use clear plastic in the spring to encourage weeds and grass to germinate earlier, and then kill them with a non-selective herbicide such as Round-up and then plant and mulch. Don't re-till the soil after you kill the surface weeds, or you simply bring more weed seeds to the surface, and will have more problems.

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