At our Native Plant School sessions, the topic of weed control often comes up. When establishing new beds or planting buffalograss, eradicating weeds prior to planting is critical for success.  Hand weeding can be time consuming on these larger areas. Often we first use chemicals to control difficult weeds in our landscapes and garden areas without thinking about other options or ramifications of the chemicals we use. Solarization is another technique you can use without reaching for the chemicals to control problem weeds.  

What is solarization?

Solarization is the process of covering an area with clear plastic to heat the soil and kill weeds and seeds in the top six inches of soil. If done properly, the use of chemicals to control weeds is not necessary. 

What weeds can be controlled? 

Solarization can be an effective method of controlling many weeds such as bermudagrass, bindweed and other annual weeds. Keep in mind that some of these weeds have extensive root systems and many re-sprout, even after being subjected to super high temperatures.  It may take several solarization attempts to completely eliminate them from the area.  In the end, it may take a few more months before you are ready to plant, but you have not used chemicals to control these problem weeds. 

Steps to Solarization

• It is best to use this method during the longest, hottest days of summer.  The goal is to get soil temperatures under the plastic above 140 degrees.  It is easier to reach these temperatures in June through August. 
• This is a process that will last for a couple months.  Plan ahead in your planting schedule so solarization has enough time to work.  Some of the more aggressive weeds will not be eliminated in just a few weeks. 
• It is best to remove existing growth and lightly till the entire area.
• Remove stalks and debris that will puncture the plastic.
• Rake the area smooth.  It is critical that the area is completely flat so plastic lays right on the soil with no air pockets. 
• Irrigate the entire area so it conducts heat better.  The soil should be moist to 12 inches deep, but not muddy.  This is a real trick in clay soils.  This is a critical step in the process, because it is not recommended to re-irrigate after the solarization process has started.
• Dig a 8-12 inch trench around the solarization area.   
• Lay one entire piece of plastic over the area and tuck the edges into the trench you just dug. 
• Cover the edges of plastic in the trench with soil, pulling plastic tight as you move across the whole area.  This makes a good seal around the entire site.       

Solarization incorporates the same principles of a hot compost pile to kill weed seeds and break down organic matter.  We have used this technique in smaller areas here at the Arboretum from time to time with mixed results.  Some have been very successful, but others have not completely eliminated some of the target weeds.  Smaller areas have had better results than larger areas especially when dealing with aggressive weeds like Bermudagrass. 

In the end, I think solarization should have a place in your weed control options. It is a nice alternative to using chemicals. Give it a try sometime.