Are clear or opaque tarps better for solarization?

Question

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:

• In comments to user923's answer to the question 'Is it possible to kill a large area of weeds organically/naturally?', user923, as well as Joe, recommend using black. In chris's answer to that question, he recommends using black.

• In trip0d199's answer to the question 'What are some natural ways of killing ground ivy (creeping charlie)?', he/she recommends using a dark tarp. In OrganicLawnDIY's comment to that answer, he recommends using clear.

• In Randy's answer to the question 'How do I get rid of Crab grass in an over grown garden that will be a veggie garden next season?', he recommends using a 'covering to hide the sunlight'

• In stormy's answer to the question 'What is a long-term solution to a goathead infestation?', she recommends using clear.

• In Robert Cartaino ♦'s answer to the question 'How do I kill a butterfly bush so it stays dead?' he recommends using black. In my comment on that answer, I (J. Musser) recommended using black.

• In a comment to the question 'How do I kill this salt cedar?', TeresaMcgH says both will work (sounds very reasonable to me).

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.

Answer 1

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

Answer 2

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.