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julieann_grow

Solarize to get rid of weed seeds

julieann_grow
13 years ago

I have a raised bed that I won't plant in until Spring.

To get rid of the weeds, would it be a good idea to cover the bed with clear plastic and 'solarize' it to get rid of the weed seeds.

Will I damage the good stuff (micro herd and good bugs)? Is it a waste of time since weed seeds may be too deep in the soil to get solaized?

Comments (36)

  • bpgreen
    13 years ago
    last modified: 9 years ago

    Solarizing will kill the weed seeds. It will also kill the microbes, so you'll probably want to replenish those after solarizing. Before covering with the plastic, water it well.

    Also, this will only work if you have a period of hot sunny weather. Since you're in zone 9, you may still have some of that coming, but most of us don't and depending on where the zone 9 is, it may be too cool already.

  • rhizo_1 (North AL) zone 7
    13 years ago
    last modified: 9 years ago

    I agree that this is not the time of year to be successful in solarizing an area. Gotta have the external heat, too, and plenty of it.

    Yes, proper solarizing DOES do damage to the soil flora and fauna but studies have shown that the soil very rapidly recovers.

  • dottyinduncan
    13 years ago
    last modified: 9 years ago

    I tried this a couple of months ago and am quite disappointed with the results. We had a period of hot sunny weather and it got quite hot under the plastic. However, once the weather turned, I took the plastic off and immediately, weeds started to grow obviously from seeds. Perhaps that's a good thing because now I can cultivate these weeds out and next spring it might be ok.

  • Kimmsr
    13 years ago
    last modified: 9 years ago

    As this article from the University of California indicates the best time to solarize soil, in California, is June and July, although it could be done as late as September. This indicates that about 6 weeks is sufficient, but I have also seen articles from others, North Carolina State University for one, that indicates you need a minimum of 12 weeks to properly solarize.

    Here is a link that might be useful: Soil Solarization

  • beeman_gardener
    13 years ago
    last modified: 9 years ago

    Interesting the different points of view!
    I 'solarize' but use 'black plastic'. The big difference is I add it to the ground as soon as the crops are out and leave it on till spring, after cleaning up the plot.
    Before I get jumped on, I do own a microscope and used it to determine any damage to the microbes. There is no damage to either the microbes or worms that I can determine.
    I have almost eliminated weeds, and haven't seen a potato bug in the last 5 years.
    Just my 2 cents worth.

  • rhizo_1 (North AL) zone 7
    13 years ago
    last modified: 9 years ago

    beeman, your technique is not solarization, which requires that sunlight penetrate the upper layers of the soil and with moisture, 'steam' treat the soil. The use of black plastic simply heats up the surface, as the plastic collects the heat from the sun, but does not allow any penetration of wave lengths.

    Just wanted to clear up that terminology, in case anyone gets confused.

    There would be no reason whatsoever to think that covering an area with black plastic in the winter would cause damage to soil micro or macro organisms. However, you are preventing the free exchange of soil waste gases (CO2) and atmospheric gases, so the oxygen breathing critters won't thrive. But unless you work hard at making your covering air tight, I'd guess that no harm can result from your method. Many things are dormant throughout the winter, anyway.

  • bpgreen
    13 years ago
    last modified: 9 years ago

    Beeman, Rhizo isn't saying that what you did won't work. He's just pointing out that it isn't solarization.

  • rhizo_1 (North AL) zone 7
    13 years ago
    last modified: 9 years ago

    That's right, bpgreen. I'm not criticizing beeman's process, but it should not be confused with solarization. What he is going is simply blocking out the light. That can be quite effective for certain things, but it can't raise soil temps high enough to kill most weed seeds or soil borne pathogens.

    A poorly managed solarization process WILL fail, beeman, you are right. Clear plastic, if not entirely sealed on all sides or applied at the wrong time of year, etc., will act like a greenhouse...one with a pretty low ceiling. ;-) But, if the plastic is installed properly (not impossible, by the way) and applied at the right time of year, the heat generated by the sun's rays will penetrate deeply, heating the soil to temperatures considerably over 100 d. F. The thicker the plastic, the better. And two layers of clear plastic are better than one.

  • beeman_gardener
    13 years ago
    last modified: 9 years ago

    OK rhizo_1.
    A Horse is a horse, of course. What's in a name. The principal end result is all that matters.
    We tried your version of 'solarization' using clear plastic and it didn't work, even though we took great care to 'get it right'.
    Bind weed and thistles still grew under the plastic. So we tried the black and that really fixed the problem. It still gets extremely hot under the black on a sunny day, works like a heat sink. The weeds start to germinate, then fail.
    So, as I said horses for courses. Use whatever turns your crank.....blacks better.

  • DrHorticulture_
    13 years ago
    last modified: 9 years ago

    OK, I'll try to explain rhizo's point again. There are two ways being discussed to kill weeds:

    1. Cook them (solarization) using clear plastic.
    2. Starve them via light deprivation using black plastic.

    You tried method 2 and it worked. That's great. It's what I did too, back in spring. It works better than #1 in cool climates with frequently cloudy weather. #1, in places where it works, is faster and more reliable. It also can kill dormant seeds, which #2 does not. So, it's important to use the right term for each method, as they are definitely not two subsets of the same method. Yes, we are nitpicky on Gardenweb. Don't take it the wrong way.

    On a slightly unrelated note,
    clear plastic heats up the soil because the soil heats up through radiation. Black plastic's only means of heating the soil is through conduction. As air is a poor conductor, and it's impossible to get 100% contact between soil and plastic, the black plastic is always burning hot but the soil is not.

  • julieann_grow
    Original Author
    13 years ago
    last modified: 9 years ago

    Thanks so much for such a complete answer! Good job. I guess I will have to wait, though, as it is a bit late in the season. Even here in the Deep South.

    I guess I'll do newspaper and mulch to try and keep them down. Although, maybe black plastic with our sunshine....hmmm?

  • dottyinduncan
    13 years ago
    last modified: 9 years ago

    OK Beeman, you got my attention when you said that black plastic kills bindweed and thistle??? Have you done this successfully and had the bindweed and thistle absolutely killed? I'd be delighted to give this a try on a section of the garden that is a constant struggle for me to keep b & t free. Unfortunately, here in the PNW we won't see much hot sun until next May.

  • scardanelli
    13 years ago
    last modified: 9 years ago

    Solarization definitely works if you live in the right climate and perform it at the right time of year. Here in zone 7, I solarized a large area of ground covered with bermuda grass. I ran a soaker hose under clear plastic, bured the sides of the plastic and left the plastic on for 8 weeks during june and july. Worked like a charm. Black plastic (smothering) may be better for cooler climates, but clear plastic (solarization) works wonderfully in warm climates.

    -Matt

  • tapla (mid-Michigan, USDA z5b-6a)
    13 years ago
    last modified: 9 years ago

    Beeman G has argued this point before in other threads, but the assertions don't hold water. Clarifying from the science side: Black plastic absorbs sunlight and turns it to heat on it's surface, where most of the heat is lost to the surrounding air - and not transferred to the soil. It effectively SHADES the soil. SOME heat is conducted through the plastic and directly to any soil it is in contact with, but any dead air spaces between the plastic and the soil greatly insulates/reduces this conductive process. Therefore, if you really want to increase solar heat gain in your soils, clear plastic stands head and shoulders above black as the preferred material. It [clear] allows excellent light transmission so that solar irradiance can be turned to heat AT the soils surface and then be conducted directly into the soil. The air spaces between the clear plastic and the soil then acts as a very effective insulating barrier, helping to trap the, allowing trapped air temperatures to build and further heat the soil. You maximize conduction AND relative heat gain while providing an insulating layer of air with clear plastic, while with black plastic, you reduce conduction and nearly eliminate direct-to-the-soil solar gain, though the insulating air space is the same.

    In our mind's eye, we can see evidence of this in another practical application by asking this question: Do the interior surfaces of a vehicle get hotter when the glass in the vehicle is clear, or when the glass is tinted. Obviously, the interior gets hotter if the glass is clear, as would the soil with clear vs black plastic.

    The tinted glass itself, would be much hotter than clear glass, but that is because the glass absorbs and turns the light to heat, then dissipates the heat into the air, thus eliminating about half of the solar heat gain allowed by clear glass. Because black plastic is nearly opaque, it would eliminate even more light transmittal than tinted windows, effectively reducing solar gain to a fraction of that realized through clear plastic.

    Do you feel warmer in the sun, or in the shade of a tree? Same principle - sun hits the leaves - turns light to heat on the surface of the leaves - heat is lost to the air - you're cool under the tree.

    Al

  • phebe_greenhouse
    13 years ago
    last modified: 9 years ago

    VERY interesting thread. I use a barrow system of vegetable gardening, with black plastic between the raised rows. I can and do put black plastic on top of fallow barrows, though being greedy I usually plant them pretty quickly. But the black plastic keeps weeds out in the interim. I have wondered at the killing power of the sun but never realized there's a whole school of thought on it.

    I want to try both ways and see what the difference is -- one barrow covered with black plastic and one with clear plastic.

    What I actually do to defend against weeds is use a generous topping of the black compost available at the county dump -- it's only $10 per truckload, great stuff, makes gardening on a large scale finally possible.

    However, I really like the idea of cooking the weed seeds. In my extremely fertile gardens (lots of homemade compost and manure) I am not concerned about microbes and worms and such: they'll recover.

  • rhizo_1 (North AL) zone 7
    13 years ago
    last modified: 9 years ago

    Please do a bit of research on the technique of solarization before 'experimenting' with it.

    It's more than a 'school of thought', which translates into 'theory'. It's much, much more than a theory.

    You need to decide just what you need to accomplish. If you need to kill deeply buried weed seeds, soil borne pathogens, parasitic nematodes, etc., then solarization might be the solution you are looking for.

    If you simply want to keep surface weed seeds from germinating, then black plastic is sufficient. But you need to take more care in the preparation for solarization than with the black plastic treatment.

    In your climate, you'd need to take advantage of the summer sun, for four to six weeks.

  • amy_harvestfaire_org
    12 years ago
    last modified: 9 years ago

    OK - if you are going to be talkin' some science, let's talk it correctly, shall we? Air spaces provide some insulation value, surface air provides very little - especially if there is a breeze. Plastic is useful for reducing air exchange, but has negligible insulation value aside from that - I can give you the numbers if you need them.
    So - the clear plastic does allow radiant heat to pass and then, sealed down, reduces heat loss from air movement adjacent to the soil by creating a dead air space. The clear plastic will, however, allow heat to go right back out as soon as the sun goes down - unless you cover the same area with a blanket of insulation each night. Failing to do that is why it is taking you 6 weeks and more.
    So the black plastic both blocks light and increases soil surface temperatures. The clear plastic increases soil temperature more but does not block light. Covering either/both at night will increase the effectiveness of the treatment, as will sealing down the edges.
    Soil is a heat sink, though - if you really wanted to sterlize the soil, you should put is on an insulation blanket, then install the clear plastic, sandwiching the soil - otherwise, the heat you are picking up is all dissipating downwards, which is why getting results takes so long and proves to be so shallow.
    So - heating the soil (by loads) in a solar kiln is faster and more effective than either method outlined earlier.

  • ms_minnamouse
    12 years ago
    last modified: 9 years ago

    I tried solarizing to kill weed seeds and it didn't do anything. In fact, some weeds grew light I set up a greenhouse for them, despite the really hot temps I imagine the clear plastic would have created.

  • bpgreen
    12 years ago
    last modified: 9 years ago

    ms_minnamouse--Did you thoroughly water the area before covering with clear plastic?

  • jolj
    12 years ago
    last modified: 9 years ago

    tapla, I got everything that has been said on these post, I think????
    My question is if black plastic dose NOT heat up the soil that much(??F degree??).
    Then why all the fuss about killing micro's in the soil with
    BP?

  • rhizo_1 (North AL) zone 7
    12 years ago
    last modified: 9 years ago

    What fuss? ;-) What do you mean, jolj?

    Good solarization techniques (clear plastic) can certainly do a number on beneficial soil microbes, but it has been proven that their populations recover very quickly. For those people who are struggling with pathogens or weeds, solarization is most likely well worth the trade-off.

  • jolj
    12 years ago
    last modified: 9 years ago

    All over this & other forum on Garden-web, people have been saying do not use Black Plastic because it will kill the good micro herd & soil food web.
    I was wondering if Al was saying that the black plastic would or would not get to a temperature that would kill the herd or web?
    I have killed weeds in the shade with black plastic, but that was blocking sun light, not heat.
    I am going to use clear plastic to kill off the weed when it gets hot(90-101F), then cover with black plastic to shade the soil for a week or so.
    Then uncover it ti kill anything that may still be there.
    Lot of trouble I know, but if half of what I hear about round-up is true, then the plastic is the best way.:-)

  • nothwehr
    12 years ago
    last modified: 9 years ago

    I am contemplating using solarization (i.e. with clear plastic) to get rid of wild violet which have a tuber-like root system about 4 inches below the surface of the soil. In my case I am less interesting in ungerminated seeds than I am the weeds themselves. The root system makes it very hard to kill mechanically. I would be interested to get other's opinions on whether this will work. Also I read some where that rototilling prior to watering and covering helps the process. Would that be an advantage over just clearing the soil of all vegetation?

  • Belgianpup
    12 years ago
    last modified: 9 years ago

    I've used both methods and black plastic is easier and more versatile, my favorite.

    Solarization with clear plastic requires:
    1) a well-moistened soil (not just the surface),
    2) heavy-duty clear plastic with no holes in it, with all the edges buried,
    3) a location in full sun,
    4) continued, maximum hours sunlight,
    5) no kids or pets running on it to make holes, and
    6) time, the length of which may depend on the weather, the latitude, and the hardiness of the weeds and seeds you want to kill.

    Some people seem to think that the heat kills the seeds with this method, and it may with some, but what it does best is cause the seeds to sprout and start growing, then the super-greenhouse effect turns the 'greenhouse' into a solar cooker and cooks the weeds to death. Fine.

    Black plastic (the heavier, the better) is my favorite because you can use it any time of year. I live in western WA, where both the winters and the summers are relatively moderate (although sometimes we wait all spring for some summers and spring goes right into fall!)

    Black plastic works differently from clear plastic solarization, but it really does work. Extreme heat is not the only thing that kills. Black plastic does warm the soil, but not as deeply as clear plastic. But deep enough to get the seeds to sprout. They get to the surface and... there's no light. Horrors! They can't photo-synthesize! So with the moisture and warmth, they rot.

    The advantage of black plastic is the lack of light. Some plants thrive on heat, and I'll bet the ones that do are the ones with deep, thick roots. They're the ones that are the hardest to kill, because you can pull them up or dig them up, and you'll never get all the parts; sometimes I swear if I listen closely, they're calling up to me, "I'll be back!" (AND with a heavy Austrian accent!). Got dandelions? KILL!

    Yes, it takes more time, but how much do you plant in winter, anyway? We have weeds that do fine in winter, like the evil Bittercress.

    I water the soil thoroughly with a sprinkler if it hasn't rained much lately. I chop the tops off any large or obnoxious-looking weeds and stomp them down. I spread my lovely black plastic and lay rocks, pavers or half-filled sandbags along the edges, not tightly. If it's in an area susceptible to winds, I'll toss some around the center, too.

    The black warms the soil when the sun shines, and prevents compaction when it rains. The earthworms come up and munch on the vegetation that is now kind of semi-rotten, and the microbes get an extension of warmer temps for a while. If any seeds think they want to sprout, they can... they'll die. The dandelions and other deep-rooted plants will grow to the surface on the strength of their roots, and struggle to find sunlight (*evil grin*) and die.

    I've got something new in my yard. I thought it was some kind of forget-me-not, but it appears to be a fleshy-rooted weed called Alkanet. One plant two years ago, twenty last summer. Gotta get out my black plastic.

    Sue

  • flora_uk
    12 years ago
    last modified: 9 years ago

    Sue - alkanet gives me a rash so be careful dealing with it. I am not a 'rashy' sort of person but alkanet has tiny bristles which get into your skin.

  • albert_135   39.17°N 119.76°W 4695ft.
    12 years ago
    last modified: 9 years ago

    Doesn't it depend upon you climate and your weeds? In the high deserts some weeds seem to like being covered with dark plastic for a few weeks.

  • astrohoof
    12 years ago
    last modified: 9 years ago

    A number of posts on this forum have discussed "black plastic" as another version of "solarizing". Actually "black plastic" is used as a form of "Mulching" which will impair survival of unwanted plants and allow the soil to be planted in the Spring with little or no need for tilling. It is also often left in place with holes cut where you place your plants. The pathways can then be covered with sawdust or shavings.

    Other types of mulch are generally organic and deteriorate over time. These include grass, leaves, bark or even Newspaper which have the advantage of adding bulk to your soil over time.

    In my opinion, I believe black plastic sheet mulching is a good way to depress the survival of weeds over the winter and provide a soil ready for planting in the Spring. I do not leave my plastic down and plant thru it because I feel unhindered aeration is essential for healthy soil during the growing season.

  • u2pia00
    11 years ago
    last modified: 9 years ago

    Ok, so i just moved into a new place. It looks like they had a thriving garden at one time. The soil looks great. the neighbors say that the fellow before me brought in compost and manure every year. Anyway its full of weeds now. Two years since last cultivated. My thoughts are to cover it with plastic for the entire year and do a straw bail garden on top of the plastic the first year. any suggestions?

  • dstack
    10 years ago
    last modified: 9 years ago

    I'm looking forward to doing this with clear plastic for my nematode infestation as well as weeds. But does anyone have a link to where I can purchase completely transparent plastic tarps for this? All I have are semi transparent (white) drop cloth plastic. I imagine our sun in S. Florida is intense enough to penetrate, but I'd rather go completely transparent. Anyone?

  • dottyinduncan
    10 years ago
    last modified: 9 years ago

    Go to a building supply store and buy the heaviest clear plastic you desire.

  • TXEB
    10 years ago
    last modified: 9 years ago

    Try the greenhouse film - 6 mil stuff made for high tunnels, etc. It should have the best combination of solar transparency and UV stability of the films generally available. If you Google greenhouse suppliers you'll find a number of options. A couple of personal favorites are greenhousemegastore and farmtek. Warning - not cheap.

  • Tracy Brisch
    8 years ago
    last modified: 8 years ago

    ***********SUCCESS******** I used some old unwanted shower doors that compressed the weeds during the afternoon heat of a 90 degree June day here in Colorado today and I AM VERY HAPPY WITH THE RESULTS! Moisture accumulated underneath (by itself) and in about 90 minutes the bindweed simply burnt up...WEED FREE - FAST! I was able to move the glass doors several times in a single day. I'll see how long it takes weeds to regrow but this worked and I will be looking for more shower doors to keep out of landfills. I am also going to try some wrought iron table tops that will compress weeds and should heat up pretty well on top of clear plastic tomorrow since I don't have enough shower doors yet and expect similar results to burn weeds reasonably quick with the SPEED OF SOLAR HEAT WITHOUT CHEMICALS! I love repurposing things and this jusr makes sense....since solar panels use glass don't they? Tie a rope to move them easily!

    NEXT DAY: Yes, with the weight of wrought iron on the clear plastic did work.

    Because the mower and the weed whacker still leave low growing weeds I wanted to eliminate them (kindof like removing hair instead of shaving - ha!) to simply buy me more time. I had covered these areas with 4" tree mulch in February and still weeds came up.

    Framed shower doors are ideal. A frameless one broke just trying to get

    it in my car and of course it beng tempered I was certainly glad.

  • tapla (mid-Michigan, USDA z5b-6a)
    8 years ago

    Check with your local glass company. Most regularly replace insulating glass units (IGUs) in doorwalls (patio doors) that have compromised seals that caused condensation between the lites of glass with which the IGUs are fabricated. The most popular size is 34x76. The IGUs can be cut apart with a utility knife so the lites of glass can be used individually. We always have a few old IGUs saved, and give them away for use in solarization or making cold frames. I'm guessing most other glass companies would be glad to make them available at N/C as well.

    Al

  • gumby_ct
    8 years ago

    I thought that solarization was done not only to kill existing weeds but to kill any weed seeds. So let's hope that has already happened. Avoid turning the soil over after or you will just bring up more weed seeds.


  • MaryMcP Zone 8b - Phx AZ
    8 years ago

    Great idea Tracy. I've used clear plastic with good results. But I think you need to leave the glass in place much longer to kill the roots as well. But it would be a good experiment if you left your current work as is and did it again, leaving the glass on for 30 to 45 days. (I did 90 days with the plastic.) Post back please.

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