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 Summer Heat – Solarize Pesty Soil

7/24/2016

8 Comments

 
Picture
     While bemoaning the extreme heat we’ve had and undoubtedly will continue to have, we can consider it the ideal time to solarize portions of the garden that have been infested with assorted pests – weeds and weed seeds, diseases, nematodes, and insects.  Solarization utilizes the beating heat of the sun to eradicate these pests.  The more direct hot sun, the more effective. 
     It’s a simple process, but must be left in place for a minimum of 6 weeks and preferably longer during the hottest time of year.  In cooler areas like the coast, a longer period may be necessary.
     In my Altadena garden years ago, I eradicated a 6’ X 5’ patch of Bermuda grass by leaving a padded black tarp on for 6 months, from May through November.   It was my Thanksgiving and Christmas presents to myself that “overdoing” the light- and water-exclusion technique resulted in nary a blade of Bermuda ever plaguing my garden again.
     The University of California has since developed its own technique that can be used in any scale garden or farm.  For a full discussion, go to http://ipm.ucanr.edu/PMG/PESTNOTES/pn74145.html
    
Here’re the basics
  1.   Choose a time of year when days are long, air temperatures are high, skies are clear, and there is no wind.  Generally, in inland California, this will be June through August; along the coast, this will be August through September.
  2. Using a clear plastic tarp, the top 6 inches of the soil will heat up to as high as 140°F, and the heat trapped down to 12-18 inches will kill a wide range of those problems.  The greatest effect is at the surface of the soil, and it decreases at deeper soil depths.  Control is best in the upper 6 inches.
  3. Solarization can also improve soil structure availability of nitrogen and other essential nutrients for growing healthy plants.
  4. Many beneficial soil organisms survive solarization or recolonize the soil quickly afterwards.
 
Here’s the method
  1. Cultivate and remove plant matter, adding amendments since disturbing the soil after solarization may bring up new viable weed seeds and pathogens.
  2. Level and smooth the soil bed so plastic will lie flat with few air pockets.
  3. Irrigate the soil to at least 12 inches deep just before laying down the plastic.
  4. Choose plastic:  clear or black?  Clear plastic will enable more heat to pass through into the soil.  Black plastic will absorb and deflect part of the heat; but when air temperatures are too low to kill weeds, the black’s light-exclusion may kill them.
  5. Choose plastic thickness.  Use 1-mil thickness for greater heating, but it’ll more easily tear or break down more quickly in sunlight.  Use 2- or 4-mil in windy areas or for longer periods of time.  In cooler areas, use double layers with air spaces created by plastic bottles or PVC pipes between layers to raise soil temperatures.
  6. Lay plastic close to the soil, and anchor so there will be no “sailing” during breeze gusts.
  7. Leave in place for at least 6 weeks during the warmest time of year, so daily maximum temperatures in the top 6 inches of soil are at or above 110-125°F.  Leave in place longer to assure more success.
  8. Remove plastic, and cultivate the soil as little as possible (less than 2 inches deep) to avoid bringing up viable weed seed and remaining pathogens.
 


8 Comments
Joanne P link
7/25/2016 08:49:29 am

Solarizing soil kills the soil biota. The news coming out of last year's climate talks in Paris emphasized that we need to move in completely the opposite direction: carbon sequestration depends upon us building up the organic matter in our soils, and cultivating a rich and diverse population of soil organisms. Every time you solarize your soil, you take things in the opposite direction!

Weeds are a foundation species. They move in as Nature attempts to establish a functional ecosystem. When you wipe the slate clean with solarization, you are opening the door to even more foundational species as the ecosystem attempts to begin yet again. The solution is to nudge your mini-ecosystem beyond "foundation" into more advanced stages of complexity - through the use of rich deposits of organic compost, shaded from the sun with mulch.

In my experience, solarization techniques (and lasagne layering techniques) do little for Bermuda grass, which is usually the prime reason people turn to them. Bermuda grass spreads through stolons, and the stolons will stay alive, deep within the soil where solarization has no impact, or beneath nearby sidewalks or foundations, just eagerly waiting or you to clear out the competition with solarization, so that Bermuda - a foundation species - can come in and take over without competition.

In my experience, the best solution to Bermuda grass in a year-round, frost-free environment, is to agressively build up organic soil, with huge applications of compost and mulch, making the soil too rich for Bermuda's linking. Then it is quite easy to dig out the scrawny Bermuda. You have to dig every bit, every piece of stolon (they will all resprout), but it is a permanent, chemical-free removal that launches your garden toward much greater richness.

Reply
Yvonne
7/25/2016 11:13:24 am

Thanks, Joanne!
As assured by University research and my own experience, the beneficials recouperate more quickly than the pests that were mostly rid by the solarization process. As for the Bermuda grass, our own relying on massive application of compost resulted in even greater spreading and intense growth and health of the grass, literally proliferating and worsening the problem. This is why I urge longer covering of the Bermuda-infected area (including the conjunction of sidewalks) several feet beyond where its greenery is observed, and for months longer than the University-recommended 2 months, and using black plastic for light-exclusion.

Reply
Joshua Siskin link
8/9/2016 06:41:22 am

Mary Beth Fiedler, gardening in mid-Wilshire, converted her previous landscape, which consisted primarily of Bermuda grass, into a garden of natives. To begin, she spread newspapers, six pages deep, with a total thickness of approximately one quarter of an inch, over her Bermuda lawn and then saturated the newspapers with water. The newspapers were then covered with composted mulch, courtesy of the Los Angeles Department of Sanitation. The mulch was free even though there was some expense associated with her gardener hauling and spreading it. Once the mulch was in place, she let everything sit for one year. She thought she would have a problem with the Bermuda grass growing back but, after several years, she has yet to see this happen. (as reported in LA Daily News, 4-20-13.)
I do not know if the Bermuda has grown back since then. Keeping a 3-inch layer of mulch consistently will certainly depress its growth. However, based on my experience, Bermuda always comes back eventually, but if you are vigilant, you can pull out new growth without much effort and keep it under control. Remember that with any kind of insect pest, plant fungus, or weed, the operative word is "control" as opposed to "eradication," the latter being an unrealistic goal.

Reply
Yvonne
8/9/2016 08:45:25 am

Thanks, Joshua! Bermuda is indeed persistent, requiring a gardener's attentiveness in keeping an eye on its potential reappearance. Definitely deal with "pests" - whether insect, disease, or larger critters - with a perspective of "control" relative to your level of tolerance. They too are always part of our world; it's just a matter of balance!

Reply
Jackie link
1/27/2022 12:51:17 pm

Great article! Thank you for sharing this informative post, and looking forward to the latest one.

Reply
Mary Jane link
1/27/2022 12:53:50 pm

This is a very informative—edifying article to all. Thanks a lot! Continue to post!

Reply
Michelle link
2/25/2022 03:59:59 am

Great Article! Thank you for sharing this is very informative post, and looking forward to the latest one.

Reply
Zoe link
7/6/2022 05:49:13 am

What an exquisite article! Your post is very helpful right now. Thank you for sharing this informative one.

Reply



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