Heating garden soil in spring....black plastic vs. glass?

squirrellypete(z7b AL)November 2, 2012

I thought I might experiment with laying out some black plastic in the planting beds this year to help the soil warm up before Spring planting (toms, peppers, watermelon, etc...), I know alot of folks do this. I know black absorbs heat thus it's better than white plastic. But I was curious about hypothetically putting glass over the planting bed instead. I've got loads of extra windows, shower doors, etc....just lying around that I could lay over top of the area, maybe up a foot or so from the soil on concrete blocks, essentially turning it into a big cold frame I guess to warm the soil.

I know greenhouses and even house window areas can get uncomfortably hot even on cold winter days so I wondered about this method. Any thoughts as to how it would compare to the black plastic or is this just a really lame idea? I know it would also be much more labor intensive than just rolling out plastic but if the soil warmed up considerably more or quicker it might be worth it. Often times our last frost date isn't so much the deciding factor for planting as much as the soil temps. After danger of frost has past, my tomatoes always seem to limp along for a long while in the Spring until the soil gets nice and toasty.


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fruitnut Z7 4500ft SW TX


Each year I warm my watermelon planting area by putting down black weed barrier and covering that with clear poly, plastic to many. This warms the soil by a lot. I've measured as high as 130F two inches deep in March. I get watermelons up in 5-10 days. I have done it in as little as 3-4 days by covering the beds with 3 inches of insulation at night. This only works during sunny weather of which we have a lot all year.

I don't even need to cover ahead of time but a week would help. Then I leave the black after emergence to control weeds.

Basically I cover with black, cut slits to plant the seeds, water, and cover with the clear poly. The soil doesn't dry out before emergence this way. If the coverings are elevated off the soil surface it will dry the seed zone quickly. The clear can be elevated after emergence if wanted to form a mini greenhouse over the plants. Some ventilation will be needed in the clear to avoid over heating.

This year I planted in late March, started harvest in late June, and am still eating watermelon from the same plants. About 130 days of watermelon harvest with ~ 190 days frost free.

    Bookmark   November 2, 2012 at 11:48PM
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squirrellypete(z7b AL)

fruitnut, thank you for the description. I'm in a similar temp zone to you, winters usually on the warmer and/or sunny side with some occassional cold snaps. I wasn't sure how far in advance I'd have to put plastic down to get substantial soil warmth for either planting seeds or seedlings. Sounds like it doesn't take that long at all if the sun is out. I was thinking I'd possibly need a month in advance lol.

    Bookmark   November 3, 2012 at 12:41AM
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Clear plastic will get your soil up significantly higher than black. Clear plastic acts as a greenhouse, with the sun heating the soil directly, and the plastic keeping the heat in. Black plastic works by radiating heat from the dark plastic to the soil. Since the plastic sheet is very thin, it has very little heat to transfer. I warm my soil before planting with clear, and then pull it up and put down a layer of black for the rest of the season. When you use black plastic, the ground should be very level, so that the plastic rests right on the soil, and is not sitting above it.

    Bookmark   November 9, 2012 at 2:48PM
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fruitnut Z7 4500ft SW TX


Clear over black works even better. Try it sometime. I do agree that the black is best right down on the soil. This keeps the moisture from distilling from one area and running down the underside to another area.

    Bookmark   November 10, 2012 at 1:19PM
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squirrellypete(z7b AL)

Thanks jonfrum, I didn't realize that but it does make sense, same principle as the glass I was talking about but obviously less heavy lugging involved. Will have to give it some thought and decide which method or methods to try.


    Bookmark   November 10, 2012 at 11:13PM
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Anyone have problems keeping plants watered/moist enough with the black plastic? I am unable to use a soaker hose and when I have tried black plastic in the past it seemed to cook the plants. Am I still getting a benefit by heating with clear plastic for approx 4 weeks, taking it off and planting my tomatoes and peppers (with no black plastic)?

    Bookmark   May 4, 2014 at 1:48PM
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fruitnut Z7 4500ft SW TX

I use the black weed barrier. Shaw Fabric Products Lumite or similar. Water passes right thru. For very early emergence or to warm soil I cover that with clear. Then use the clear as a mini greenhouse. Doing that I sometimes have melons on the vine at the time of last frost.

In ME with many more clouds the advantage of an all out effort might only be a week or two instead of a month. But with your shorter season any gain can be critical to getting a crop.

    Bookmark   May 4, 2014 at 4:51PM
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sand_mueller(z 7a, oklahoma)

The earth and I both hate plastic...the black and the clear. Its funny how the environment always suffers when we try to get more dollars or tomatoes. Vice president Wallace was dumped in 1944 because he was a conservationist and the bomb factory/energy magnates wanted to make sure the "green revolution" would change agriculture from biology first to chemistry first. Who is going to stand up against this insanity?

    Bookmark   May 7, 2014 at 6:16PM
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