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My failed Solarizing Experiment

Posted by dottyinduncan z8b coastal BC (My Page) on
Sun, Sep 22, 13 at 12:27

This hot summer, I covered an area of my veggie garden with clear plastic to see if it would kill weed seeds and the bind weed that persists in this area. In early September, I removed the plastic and found that under the plastic there was dried grass and weeds. A little rain has fallen since then and the weeds are growing once again. They haven't missed a beat! Obviously, it didn't get hot enough under the plastic to kill anything other than the surface weed growth. We have had a very hot summer, for this area, with temperatures in the high 70's and lots of sun. The plastic has been down for 2 months. Maybe it just doesn't get hot enough here to solarize or I missed a step?


Follow-Up Postings:

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RE: My failed Solarizing Experiment

For solarization to be effective, the site has to be prepared properly. It's a process. If leveled, watered carefully, and sealed tightly, soils should be able to reach killing temperatures within six weeks in a hot and sunny summer. How deep the heat goes is largely related by soil type.

But the most important thing to know is that solarization, even at its very best, cannot control many of our stubborn perennial weeds. Bindweed won't be killed by the treatment. I'm sorry that you were led to believe otherwise.


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RE: My failed Solarizing Experiment

Yeah.. Sorry about that. It has been reported that bind weed can have tap roots up to more than 20 feet. When the drought came through the last few years. Creeping charlie and bind weed took over!


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RE: My failed Solarizing Experiment

If 70F weather and plastic cover killed seeds ability to germinate,Southern states would be desert after a 100+F summer. If the soil is prepared for planting,wet and covered to sprout weeds 2 or three times,there will be fewer weeds in the bed until the soil is desturbed again. IMO the trade off for harm done to soil viability makes the value of solarizing a push at best. However,I expect like all things in life there are exceptions. A lot can be said for simply turning weeds under and mowing before they go to seed on ground being rested.


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RE: My failed Solarizing Experiment

I gave up on clear plastic, as like the OP all I got was more growing weeds.
Now I use Black Plastic. I first clean the area, add my amendments, till the area, level and water the area, then cover with the Black, tuck in the edges to prevent the wind getting underneath it. It will stay like that till spring, at which time I remove and plant.
No weeds, until I disturb the soil.
Contrary to other opinions, it does no harm to the bacteria, plants grow just as well as should be, in fact better.
I have done this now every year since 2006, and won't change my methods.
I do have a friend who did the same thing but on straight grass, an area he wanted to expand into. Clean as a whistle, all he had to do was till it up.
I think the seeds start, but with no light die off before getting rooted.

This post was edited by beeman_gardener on Sun, Sep 22, 13 at 16:18


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RE: My failed Solarizing Experiment

A local arboretum tried solarizing several years ago. Their conclusion was that we don't get hot enough for it to work. My gut feeling is that you need to be somewhere that overnight lows are usually in the 70s for it to reliably work.


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RE: My failed Solarizing Experiment

Here's a clue why solarization doesn't work in the NW: The process was developed in Israel.


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RE: My failed Solarizing Experiment

Very interesting comments. Beeman, your experiment with black plastic is interesting. Are you saying that you prepare the beds in the autumn, cover with black plastic and then plant in the spring? If you do this, does the black plastic keep the soil warm so that you can plant earlier?


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RE: My failed Solarizing Experiment

For soil solarization to be effective the soil temperature needs to get into the 120 degree F range and whether any soil will do that depends on many factors beginning with the angle of the sun to the soil. The further north one is the less likely the angle of the sun, even in mid summer, will be correct for this to work. Daytime air temperatures have little affect on soil solarization if preparation is done properly.
All of the good research I have seen on soil solarization says the soil surface must be tilled and wet down and then covered with two layers of clear plastic. In the southern tier of the United States it takes from 4 to 6 weeks for this to work. Further north it can take 8 to 10 weeks, if the angle of the sun to the soil is optimal. Cornell (New York) and Michigan State University have reservations about solarizing soil in their areas of the world, so I would guess that soil solarizing in British Columbia would be even more difficult.


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RE: My failed Solarizing Experiment

"dottyinduncan" I have to be honest, I haven't noticed any improvement regarding temp changes. That would require two sowings of the same thing, one under the black and one not covered.
I do know it doesn't make a scrap of difference regarding germination etc. It certainly helps with compaction. I pull it back in the spring and the ground is just like I left it in the fall, no compaction from rain or snow, which is a major benefit.


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RE: My failed Solarizing Experiment

Soil underneath black plastic stays in very nice tilth if it doesn't get dried out.

Where growers are serious about killing off fungals and such, they fumigate with serious poisons...not what I would want to do or could do.


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RE: My failed Solarizing Experiment

I live in Central FL and solarized this past summer. It worked very well for me. Clear plastic over a very well-watered bed and sealed all edges for 2 months. We had a really wet summer with lots of rain and I was a bit worried that the temp wouldn't be high enough to solarize and wouldn't work....it did, and I am very happy with the results. I did this in order to kill of soil disease as it was a problem last growing season. I have planted kale, bush beans, tomatoes, broccoli, and beets already.


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RE: My failed Solarizing Experiment

The difference in latitude from central Florida and British Columbia can explain the differences is experiences in solarizing soils. The angle of the sun to the earth in central Florida is enough that it may only take 4 weeks of covering of soil for solarization to work while that angle in BC is low enough the even 12 weeks may not be enough time for solarization to work.
Air temperatures have little to do with how much solar energy is trapped by the plastic covering the soil. Black plastic, opaque, does not allow the UV rays from the sun to reach the soil and in all of the research I have seen is not as effective as two layers of clear plastic.

This post was edited by kimmsr on Thu, Sep 26, 13 at 7:22


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RE: My failed Solarizing Experiment

I have never understood the sense in solarizing soil to kill weed seeds. To get the soil hot enough, you also kill off the soil organisms, and when you use the soil again, the bad bugs appear first to create food sources for the good guys. Then as nature takes its course, you have weed seeds being deposited on that same ground from day one. What is the point?


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RE: My failed Solarizing Experiment

I have never understood the sense in solarizing soil to kill weed seeds. To get the soil hot enough, you also kill off the soil organisms, and when you use the soil again, the bad bugs appear first to create food sources for the good guys. Then as nature takes its course, you have weed seeds being deposited on that same ground from day one. What is the point?


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RE: My failed Solarizing Experiment

dottyinduncan - sounds as if your climate is a bit like mine. If I spread transparent plastic on the ground I get MORE weeds under it than outside it. It acts like a greenhouse and provides a lovely warm, damp, wind free environment for seedlings. Never gets hot enough to kill anything.


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RE: My failed Solarizing Experiment

If one simply lays a sheet of plastic on the soil and does not properly prep the soil that sheet of plastic may well act as a greenhouse. Also, initially, plants may well grow in a properly prepared soil under a sheet of plastic with well sealed edges if the soil temperatures do not reach those needed for solarization.

Here is a link that might be useful: soil solarization how to


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RE: My failed Solarizing Experiment

From kimmsr's link: "In areas that don’t get very warm or don’t get much sun, solarization may not work and can even create a greenhouse like setting that actually stimulates weed growth." Exactly what happens in my climate, and seemingly in dottyinduncan's.


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RE: My failed Solarizing Experiment

I have used black plastic for weed control also. It works great. Water well, cover with black plastic for two or three weeks, then pull the plastic and plant. The soil will be full of earth worms and predatory bugs of all kinds.

I would not expect this to help with bacterial or fungal pathogens, but it does help with weeds.

The conditions under the black plastic are good for weed seed germination, but the sprouts die for lack of sun. The black plastic is totally opaque.

Logically, I think black plastic will eventually kill any plant that relies on photosynthesis, as long as you keep it fully covered. Of course, some plants can grow quite a bit from stored energy in the roots, and it might be challenging to keep them covered long enough for them to die off. (They would be constantly growing out from under the plastic or lifting it up or poking through it or whatever).

I have never tried solarization, but as many others have noted, you do need to get the area good and moist before you cover it. The fact that you had "dry grass" at the end tells me you didn't do that part as well as you could have. A big part of the reason for the moisture is to make sure the heat spreads out.

--McKenzie


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