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I have always used black tarps for this purpose, because weeds grow under the clear stuff for me. I may not have pinned it down close enough though. Others say clear is faster. Does this have anything to do with climate variation? Does one method work better in hot, sunny climates, and the other in a cooler, shadier place?

Here are some opinions from users on this site:

This is somewhat confusing. Have there been any 'scientific', unbiased studies showing that one is better than the other? I'm not really looking for personal opinions, although they may be helpful. I am looking for evidence from an authoritative source.

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You mean transparent and or coloured (like black)? –  BYJ Aug 12 at 15:59
    
@ondoteam Transparent, yes, to let through as much sun as possible, vs. opaque, to build heat without allowing light through. –  J. Musser Aug 12 at 16:02
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Good question. I'll take my coffee and then write a formal answer. –  BYJ Aug 12 at 16:04
    
Hey! @Laughing_Jack wrote in his comment on my answer an interesting point. Are you really asking for solarization or maybe for the best method to kill soil undesirables using tarps? –  BYJ Aug 14 at 0:01

2 Answers 2

up vote 3 down vote accepted

Solarization:

It is a practice that can kill most soil organisms, included the good ones. Solarization basically consists in the following:

  1. Release the soil.
  2. Remove big weeds and big rocks.
  3. Irrigate.
  4. Seal the soil with tarps.
  5. Wait a little bit (four to twelve weeks).
  6. Remove the tarp.
  7. Add compost to the solarizated soil in order to restore beneficial organisms.

In the documents linked at the bottom, you can read in detail about the entire procedure. The documents also include some advices for the tarp thickness

How it works?

Tarps allow short wave radation (infrared) to enter. Once light let in and it is reflected by soil: the wave change to a longer one and can not go out (heating the soil). Currently there are some tarps which also let in ultraviolet light (very good for sterilization too).

Transparent tarps or not?

Always transparent. This is because, for example, black tarps will absorb and radiate the solar light back to the air and not inside, so it will heat the soil too, but just by contact with the tarp.

*Thanks to @Laughing_Jack for the correction.


Literature:

Solarization - University of Florida

Solarization - University of California

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Technically speaking, black tarps don't reflect most light, but rather radiate their heat somewhat indiscriminately. IIRC, the theory for black or opaque being potentially better in a few cases is that opaque stops sunlight passage to the greenery in the weeds, stopping the weeds from photosynthesising. Actual soil heat is, in pretty much all cases, lower with opaque covering, but if you would benefit more from blocking out the sunlight than from actual temperature, opaque can be better. –  Laughing_Jack Aug 12 at 17:43
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@Laughing_Jack you are right, thank you, I was thinking in absorb and repel. By the way we are talking about solarization not "mulching" procedures and I think that use opaque tarps is more mulching related than solarization. –  BYJ Aug 12 at 18:00
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Thank you for the reply. –  J. Musser Aug 12 at 18:02
    
@ondoteam interesting that you separate mulching and solarisation. I'd always viewed solarisation as a somewhat extreme and highly targeted form of mulching (to kill soil undesirables) but realise that's mostly an over-simplified view of the two techniques. Thanks! –  Laughing_Jack Aug 13 at 23:24
    
@Laughing_Jack "kill soil undesirables" good point . Maybe J. Musser was asking about that and not specifically by solarization. I am going to ask :D –  BYJ Aug 13 at 23:57

Here's what you don't really want - an informed opinion without scientific references (for now, anyway). I think it depends what you mean by the term 'solarization' - it actually means using transparent plastic or tarpaulin or whatever, pegged over soil, in order that the heat from the sun will bake, fry, kill off and generally 'burn' out soil pathogens and weeds. This obviously works much better in places where there's a reliable amount of strong direct uv (sun) light on a daily basis - if you tried it in the UK, unless you struck lucky for a week or so in high summer, it wouldn't be that effective, or would take an awfully long time.

Much more commonly used here is thick black plastic or tarpaulin, tightly pegged down onto an area of soil for at least a year, in order that weed/plant roots are starved of air, water and light to kill them off. It probably also kills some pathogens because they're deprived of moisture, but won't be so effective in that arena as the heat generated by direct, strong uv light on clear plastic.

So, if you what you really want is solarization, then clear plastic/tarpaulin is the thing to use - it's faster for one thing, and more effective at clearing pathogens. But if you just want to get rid of plant roots, then black plastic pegged down tight will work.

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You make me laugh. :) Thank you for the reply. So in your experience, the most effective method depends on climate? –  J. Musser Aug 12 at 18:01
    
@J.Musser - not exactly - if you want solarization specifically, use clear plastic - if you just want to keep ground clear or kill off plant growth, use black to keep out light. Solarization will work down south in the UK, and probably further north if left long enough, just won't be that quick, but basically, the choice of black or clear is down to what you're trying to achieve. –  Bamboo Aug 13 at 9:57
    
@J.Musser: just done some checking, and solarization is not a recommended technique in most of the UK because UV is not strong enough for long enough. Black plastic all the way here, but presumably you've a choice for where you live. –  Bamboo Aug 13 at 10:27

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