Black Plastic mulch

ulasaskinApril 9, 2011

This year, I planted my Tomato seedlings a bit early, so I am trying to get them into the ground asap. I heard about black plastic mulch. I started to prepare a bed by placing soaker hose down and then black plastic 1.5mil over it and tightly tucked it into the soil. My only question is, how is it that this does not kill the beneficial microorganisms in the soil? I cannot find any information about this in searching the net. All I find is information about the benefits of black plastic sheeting mulch....higher yields, earlier transplant, etc. Anyone have any advice? Can anyone tell me about what happens to microorganisms?


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Dan Staley

More importantly, if it is going to kill microorganisms in the soil, it surely will kill tomato roots at the same time.


    Bookmark   April 9, 2011 at 11:13PM
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Belgianpup(Wa/Zone 7b)

Soil micro organisms function best at soil temperatures of 77F to 95F, and in the top 6.5" of soil.

Tomatoes grow best in soil temperatures of 58F and higher, but not over 90F, so temps that are good for the microherd are good for the tomatoes, and temperature extremes are bad for both.

If you invest in a small soil thermometer ($6-8, or $5 at Harbor Freight) that has a 6" probe and a dial face, it's handy to just stick in the soil where you want to know how cool/warm/hot the soil is, it's great for educating yourself so you don't have to guess. I got tired of guessing wrong.

If you have cool springs and warm summers, you will probably have to remove the plastic when the weather warms up, or it will cook the tomatoes. Replace it with organic mulch (leaves, straw, etc), which will help keep the soil moist without the bare soil too-wet/too-dry stresses, which torment my tomatoes in my sandy soil. Too much watering flushes nutrients out of the top 6-7" (most-active level) of soil, where most of the feeder roots are.

I use black plastic sometimes, but I don't bury the edges, I just anchor the plastic with some larger rocks (got plenty!) Burying the edges seems more useful for clear-plastic solarizing of the soil, where you WANT to kill (practically) everything. I also think a little air exchange from the edges might be good for the soil.

Since I've got "rock issues", I often forget the plastic, esp when I got the tomatoes in later, and just surround the tomato stems with larger rocks. The rocks collect heat during the day and release it at night, useful here in the maritime PNW. You don't have to remove them in summer here.


    Bookmark   April 9, 2011 at 11:30PM
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luke_oh(zone 5 NE Ohio)

I use the black plastic under my tomatoes and peppers and have for many years. I don't remove the plastic after the soil heats up and lay heavy straw under the plants. I don't stake my tomatoes and let them lie on the straw. I do place more straw down during the season so that the tomatoes are on dry straw to prevent rotting. This method also eliminates a lot of weed competion during the summer.


    Bookmark   April 10, 2011 at 11:07AM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

My only question is, how is it that this does not kill the beneficial microorganisms in the soil?

The simple answer is that is does. Unfortunately many people don't understand that or aren't soil micro educated or worst of all, don't care. I'm glad you recognize the problems.

Lots of great info posted by Sue above and I totally agree. While those in the cooler zones 'might' be able to get away with it with only minimal soil microherd damage, especially if they use the lightweight perforated landscape fabric rather than actual plastic, those of us in the warmer zones like you and I simply can't.

I put down the lightweight black landscape fabric to pre-warm the soil for planting but once the soil temps reach 70-75 degrees it comes off and is replaced with organic mulches. Used that way it is good for 3-4 years.


    Bookmark   April 10, 2011 at 12:44PM
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luke_oh(zone 5 NE Ohio)

Ok, I'm listening. How does this kill the microorganisms in the soil? I have plenty of worms under the plastic. Of course I can't see the microorganisms, but I'd like to know how they die under the plastic? I guess that I'm not so soil micro organism educated, I'm listening.


    Bookmark   April 10, 2011 at 4:32PM
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Belgianpup(Wa/Zone 7b)

Luke, the earthworms can migrate deeper into the cooler soil or sideways to a cooler (shaded) area.

Most micro-organisms are found in the top 6-7" of the soil, they can't move far or fast, so they are far more susceptible to overheating. When the actual soil temperature goes higher than 95F, they croak.

I once stuck my temperature probe into the damp soil in my Mom's Las Vegas, NV backyard (full sun), and the temp was about 103F (air temp was 114F). The temperature in a 2-gallon black plastic pot (full sun) was about 105F, and she wondered why most of her potted plants were dying.


    Bookmark   April 10, 2011 at 5:47PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Appreciate your willingness to listen Luke. I linked a great FAQ from the Composting forum on the Microherd and all that they do for us in the garden. And no, you don't need to be hot and heavy into composting to derive benefits from them assuming you garden sol has the normal amount of organic matter in it.

The culprits are lack of oxygen and the heat. As Sue said the soil temps are surprisingly high under that black plastic, especially in the warmer zones, and the oxygen levels are markedly reduced as well. And long before the soil temps and low O2 cook and kill the beasties, the temps have rendered them dormant, inactive, so all those benefits we derive from them are lost.

So soil temp monitoring is very informative, both for planting times and for keeping your own garden herd happy and productive. Not to mention the benefits to the plant roots.

If you are far north zone 5 you may be able to use the landscaping fabric longer than many of us can but several factors like soil composition, sun exposure, and watering regimen can affect it quickly so soil temp monitoring pays off in increased production and over-all healthier plants.

I hope this info is helpful to you.


Here is a link that might be useful: Who and what is the 'Microherd'?

    Bookmark   April 10, 2011 at 6:25PM
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luke_oh(zone 5 NE Ohio)

Thanks for the information and I do think that our geographic locations gives us a different outlook on the use of black plastic mulch. Mostly I do use the perforated plastic that allows water and air to penetrate and when the temps rise after the plants get a good start I cover with a heavy layer of wheat straw. This insulates the soil under the plastic. If the straw is laid down too early the soil does not heat up and so no microbial activity, I guess.
I have never experienced gardening in a hot climate and can certainly understand cooking the soil. Over 60 years ago I started gardening with my grandfather and he always said don't hurry the garden, you have to wait until the bugs start working the ground. I guess that he was talking about micro organisms even though he had never heard the word before. Thanks again, luke

    Bookmark   April 10, 2011 at 8:14PM
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When using black plastic mulches, the plants are a significant consideration. I use Lumite woven black plastic to grow watermelons, cucumbers, gourds, etc. I have used it for tomatoes. As you might guess, it works very well with the vines like watermelon but not so good with tomatoes and peppers. The reason is that watermelon vines spread to completely cover the plastic at which point it is no longer a cause of soil heating.

Never use plastic mulches with squash family plants. They have insect pests that love to hide under the mulch and crawl out at night to feast on the plants. Mixta, maxima, and pepo are all affected in my experience.


    Bookmark   April 11, 2011 at 12:16AM
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Coming from a similar locale as Luke (just a bit warmer with 6A), I've used black plastic for a number of years for peppers, tomatoes, and eggplants. Last year, I put it down on my tomatoes, but not for my peppers and eggplants due to time. I had my best pepper and eggplant crop ever. Although not empirical by any means, I'm going to skip the plastic in the tomato bed this year and see what happens.

    Bookmark   April 11, 2011 at 10:04AM
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Thanks for the information. I used the black landscaping plastic last summer on almost everything. Had a very productive garden and could control the weeds. Our garden is new tilled from a former pasture so you can imagine the weed problem I would have had without the plastic. I did not realize the problems that could happen using the plastic so I will monitor the temperature this year. It does get pretty hot here in the summer.

    Bookmark   April 11, 2011 at 10:45AM
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vegangirl(z6 VA)

We've used black plastic for tomatoes, eggplant, peppers, okra, beans, etc for decades. I'll have to read that thread on the microherd! We can't get okra or eggplant to mature unless they are planted in black plastic.

    Bookmark   April 12, 2011 at 2:08PM
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Can you use black plastic fabric under melon vines? I don't want to burn the vines.

    Bookmark   July 23, 2014 at 6:00PM
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seysonn(8a WA/HZ 1)

Black Plastic is used early in the season to keep the soil RELATIVELY warm. It does not heat up the soil to such an extent and depth to kill micro organism. White plastic possibly can do that because it works on GREENHOUSE EFFECT. black plastic ONLY absorbs solar heat and get warm and then warms up the soil that it is touching. But its major advantage is that, being black, it will not radiate heat into the space at night also prevent cooling the soil by air movement over it. Also, in the spring time rain water can also cool down the soil. So BP can fend off that too.

I had covered my beds with Black Plastic weeks before planting but I took them off in June.
The bottom line is that keeping black plastic or black fabric can contribute to warming up the soil, which is not desirable during the summer months.

    Bookmark   July 24, 2014 at 5:06PM
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grandad_2003(9A/sunset 28)

Fbabrams1959 - i use black plastic for melons in our south Louisiana zone 9a area. It works great for me. However, most everything else gets leaf mulch as black plastic tends to heat the soil beyond their tolerance levels.

    Bookmark   July 26, 2014 at 11:28AM
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