reducing weeds: a 101 on soil solarization, with sonja birthisel

AN ARTICLE about soil solarization for weed control, the practice of covering beds or fields with plastic to keep down unwanted plants, caught my attention in the summer of 2018. It was published on the Cooperative Extension’s online home called eXtension.org and was written by University of Maine doctoral candidate, and she was my guest that winter on my radio show and podcast.

I know: This isn’t a winter project, but I was mapping my new year’s garden plans then, and mastering this powerful tool for better organic weed control was key on my wishlist–and maybe is on yours.

Dr. Sonja Birthisel completed her PhD at the University of Maine in late 2018, where she was a postdoctoral research associate focused on helping farmers by studying practical solutions for issues posed by climate change, weed management and more. That included the subject of soil solarization that many of us gardeners use, too, in the name of weed suppression. I was excited to hear what she learned that we can all benefit from, including the subject of the effects of clear versus black plastic.

Read along as you listen to the January 7, 2019 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Spotify or Stitcher (and browse my archive of podcasts here).

soil solarization with Sonja Birthiselsoil solarization for weed control, with sonja birthisel



Q. So, are congratulations in order then?

A. They are, yes. I’m now as of three weeks ago Dr. Sonja Birthisel. Very exciting for me personally.

Q. Good for you. What a journey, huh?

A. Indeed, yes.

Q. Are you staying in Maine or are you moving elsewhere?

A. Yes, I’ll be here for the next year and a half, working on a new project as a postdoc and also teaching at U Maine in the spring.

Q. Outstanding. Your thesis explored how climate change might affect weed-management strategies. Of course, you are located in Maine so that’s where you were testing these ideas and these practices and so forth. Tell us a little bit about what you were doing in your recently accomplished doctoral work.

A. Sure. I came into graduate school really interested in this issue of climate change and increasing weather variability, and what that might mean for our farmers in the Northeast. I was interested both in … I study weed management, so I’m interested in how the weed communities that we deal with might be changing and interested in how the tools and tactics that we use to manage them might be changing as well.

Q. Yes.

A. That’s a big topic. It’s bigger than I could address in one doctoral thesis, but based on getting a couple of grants, I’ve started eating away little bits of the puzzle. This work on soil solarization that caught your attention became part of this because it was a question that a farmer asked me about, and that I didn’t know very much about. I love doing work that’s practical and that is addressing people’s questions.

It’s a tactic that has not been used extensively in Maine in the past, but certainly holds some promise and may become increasingly effective as our climate in Maine continues to get a little bit warmer.

Q. That sort of speaks to—let’s backtrack—what is solarization and what has it been used for, and where, historically?

A. Soil solarization is the practice of covering moistened soil with clear plastic for a period of weeks, and this creates a local greenhouse effect. Solar energy heats up the water molecules in the soil. That heat stays trapped under the plastic. If conditions are suitable, you get temperatures hot enough to kill pests, including plant pathogens and weeds. This has been used most extensively in parts of the world that are much hotter and sunnier than Maine. I’m thinking of Israel and Southern California. There’s been work on this done in Georgia, but not in Maine as much.

soil solarization tips for all natural weed controlQ. O.K. You said a couple of things in that short description. “Moistened soils,” you said, and you said “clear plastic.” Let’s dig into those a little bit. Moistened soil because the sun will heat up the water molecules, so this isn’t so good on dry soil?

A. Yes. The moisture does a couple of things. Moisture helps conduct the heat in the soil profile. If your soil is moist, so around field capacity not super water-logged, that will allow that solar energy to travel to deeper layers of the soil so you’ll get more benefit from the practice at more depth.

Q. O.K.

A. That’s kind of the biggest physics reasons. Then also, if we’re thinking about this for weed management, it can help putting seeds into a more active life phase, maybe encouraging them to germinate. Then if you germinate under that plastic or start to germinate and it’s unsuitably hot actually, well, then that seedling is toast. It can improve weed management in that scenario.

Q. That seedling we’re talking is in the seedbed of the underlying soil, unwanted things, not the precious seed we just planted as farmers or gardeners?

A. Correct. Hopefully our garden weeds, yes.

Q. Right. The other thing you said a little bit earlier is you said “clear plastic.” I was especially fascinated because I’ve been gardening a very long time, writing about it a very long time, and I thought I was an advanced intermediate. Not a super expert, but an advanced intermediate. I’ve interviewed a lot of super experts over the years, and no one had ever really said to me that using black plastic or a blackout kind of a cloth or fabric is not technically solarization, that it’s called something else entirely. Tell us about clear plastic versus some other substance.

soil solarization and tarping for weed controlA. Clear plastic does actually get hotter underneath than black plastic. Both of these practices back in the ’70s and ’80s were considered solarization, but since then, the terminology has changed. Now we call solarization clear plastic or we call clear plastic solarization, and then using black plastic for similar purposes is called either tarping or occultation. Those are the two words I hear most often.

Q. Occult like the occult?   

A. Exactly, yes. Our word for the occult comes from the word for black. Occultation-

Q.  I love it. [Laughter.]

A. … is using black material, yes.

Q. Oh, I’m going to feel so sophisticated now knowing that. Thank you.

A. You are already very sophisticated, but here’s just one more tool for your toolbox there.

soil solarization and tarping for natural weed control Q. Yes, I love that. When you say hotter under the clear, how much hotter? I mean I know you can’t give me an exact reading probably, but is there sort of a rough range?

A. Well, I probably have some exact numbers somewhere near at hand, but at least 5 degrees Fahrenheit hotter is what I was measuring in my experiment here in Maine, and that temperature difference can depend on where you are in the world for sure.

Q. The other thing that a lot of people who wanted to use it, who are organic gardeners especially and very interested in composting and building soil life and so forth and the sort of microbiome of the soil, would say: “Well, but I don’t want to kill all the soil life.” What happens under either the occultation or the solarization materials? What happens? Does it kill off soil life?

A. This is a great question and it’s one of the most common things that farmers and gardeners ask me about. We’re definitely changing the soil atmosphere when we’re applying either solarization or tarping/occultation. Things are getting warmer. It depends on how hot it gets, but yes. My own experiments and past experiments show that especially with the clear plastic solarization, we do end up eventually killing quite a few soil microbes as part of this process, including plant pathogens. That’s where the pathogen control can come from if you’re getting hot enough soil temperatures, but the good news is that typically those populations bounce back afterward.

There will be enough good microbes surviving at deeper layers of the soil profile that even if you’re really zapping it, there will be recolonization afterward.

Q. O.K.

A. That’s what I’ve got to say about microbes, but there’s more to soil life than just microbes. There are earthworms and beneficial beetles and ground-dwelling pollinators. Unfortunately, there’s been surprisingly little research on how these practices impact those soil organisms. That’s something that I’d love to look into next.

Q. Uh-oh. [Laughter.] Curiosity is going to cause some more explorations out there in your fields.

A. Watch out world, yes.

Q. Yes, there she goes; she’s off again.

How hot does it get? Well, maybe we should go to the sort of how do we do this. Sort of the 101 of when to do this, how do we do this, what’s the basic practice for solarization and then you can tell me any variations if we’re going to use the black material. We can talk about what happens afterwards, how hot it really gets.

A. Yes, absolutely. Solarization in particular is going to be most effective if you’re doing it during the parts of the year when you’re getting the most sunshine, so around the summer solstice, but it can also be effective in the springtime. Here in Maine, people typically start preparing beds in May and planting some of their early season things then. For my experiments, I started solarizing in the beginning of May. I solarized for two weeks and then pulled up the plastic and would be directly planting into the prepared steel seedbed that I had made with my solarization. I was finding that two weeks in the springtime starting in May was effective for pretty substantive weed control afterward.

Q. This was from a smooth seedbed? A seedbed that was already cleared of growing material? I mean it was soil.

A. Exactly, yup. I would prepare my seedbed first, which in the case of what I was doing was rototilling, but you could also do this in more of a no-till setting. But clearing the soil, loosening it up if you’re using a broad fork or something like that, getting it all ready to plant and then applying your plastic for a couple of weeks. One step that I neglected to mention, but will throw in there is before I put on my plastic, I’d make sure the soil was wet enough.

Q. Right. Right. We’d irrigate.

A. It could happen right after a rain if you want to be utilizing nature in that way, or I’d go out with a garden hose and just make sure everything is nicely moistened, then I’d apply my plastic. For solarization with the clear plastic, burying those edges or putting metal pipes on them or making sure that that edge is really tightly secured ends up being important to creating a good seal around it. For black plastic, that edge securement is less essential.

Q. In May, it’s what, in the 70s where you were doing this?

A. Yes.

Q. If it’s in the 70s …

A. It might get that hot during the daytime and then it’ll dip down to the 40s or 50s overnight often.

Q. O.K. So then, what happens under this plastic? Does it get 10 degrees warmer or 20 degrees warmer? What happens underneath?

A. Our maximum soil temperatures were typically around 100 degrees Fahrenheit in an open field or sometimes as hot as a 118. I think 118 was the hottest I measured during that May time period. It can get pretty hot.

I should say that in our control, so just regular bare soil, it’s also sometimes getting hotter than the air temperature because the solar energy is heating our soil in the springtime, too. But it is typically at least 10 degrees hotter under the solarization than it is in your nicely warming soil.

Q. It gets quite a bit hotter under there. You did this in the spring to take advantage of … You’re moving toward the longer days, but to kind of get a headstart because that is equivalent to when farmers or gardeners would be doing it. I mean they can’t wait until the solstice, they can’t wait until the longest day of the year to begin prepping their beds, so that’s why you did that.

Say we have an area that’s not in use, that’s fallow or whatever, or a new area that we want to prepare and kind of clean up and reduce the weed population, the seedbed, what’s the other end of our window? How late in the year is this something that’s effective?

A. I’ve had luck with this practice all the way into late July certainly, and then more mixed success in August.

Q. That’s in Maine, of course, for people who are listening-

A. That’s in Maine, yes.

Q. … from all over. If you’re in Atlanta, you’re going to have a longer window probably. Yes?

A. Exactly.

Q. O.K.

A. Yup.

Q. O.K. You said you take the plastic off after your couple of weeks of heating things up with it well secured and so forth, but then you take it off. Do you do any other preparation besides actually sowing your seeds or putting in your transplants or whatever? Are there any steps afterward?

A. Nope. Just transplant directly into it, or sow your seeds directly into that nicely prepared seedbed. The less soil disturbance you can do after this practice, the better from a weed-control perspective because you want to avoid bringing buried weed seeds up to the soil surface.

Q. O.K., because your most effective killing off of weed seeds is going to be in that top layer.

A. That’s exactly right.

Q. Right. What if I want to add compost or if I’m doing any kind of soil improvement, would that come before I prepared the seedbed and put the plastic on?

A. I would recommend yes. You might get different answers from different people, but one of the reasons I’d say, “Yes, go for it,” is that incorporating organic amendments like compost into the soil prior to solarization can actually make the practice more effective. It can lower the temperature threshold at which the practice starts to be fatal to weed seeds and pathogens. It can also help warm the soil even a couple degrees more. The thinking is that the release of some of the volatile compounds that are in compost or fresh green material can kind of further elevate the properties of solarization that are killing pests.

Q. Because I think I read in eOrganic? Is that what I mean to say? The Extension website.

A. Yes.

Q. Or it was probably on Extension.org, but whatever, where your results were reported. Did you call it bio-solarization? Do I recall that?

A. Yes, you’re recalling correctly. Bio-solarization is our fancy word for this exact practice of integrating solarization with applying organic amendments prior to solarization.

Q. O.K. Good. Does this ever backfire? Are there some weeds that say, “You’re not going to kill me. The heck with 100 degrees.”

A. There are, there are. This practice does end up working more effectively for annual species than perennials, typically. I was having really good success with two weeks of solarization, controlling a lot of our annual weeds that are a problem in Maine. But if I wanted to get rid of quackgrass or control another perennial, it’s going to take a lot longer solarization period than two weeks in order to get really good control results, probably. I was seeing some reduction in quackgrass for example with two weeks of solarization, but by no means eliminating my problem there.

Q. In that case with the perennial weed situation, were you first removing the living plant material? Were you getting all the stuff out of there, like cleaning out the beds so to speak? Were you just putting the plastic or the tarp over existing vegetation?

A. I rototilled first and then put the tarp over-

Q. O.K., so you still tilled.

A. … existing tilled in vegetation like if you’ve got a lot of rhizomes in there. The rhizomes are still going to be there.

Q. I know this very well from firsthand experience. [Laughter.] There are certain things, like I have one plant that I bought as an ornamental many, many years ago. It was the hot plant of the moment. It’s Houttuynia, the chameleon plant; cordata is the species name. It has a variegated leaf that included red, yellow and green markings. It was this thing and everybody wanted it, and then guess what? Everybody who got it will never get rid of it in their entire lives, especially if they’re an organic gardener. It’s unbelievable this plant.

A. Pretty pernicious. O.K., well, remind me not to plant it.

Q. Don’t plant that, but I still see it. But I’ve tried the tarping, which I thought was solarization, but I’ve tried a lot of these things. What I did first was I removed as much of the vegetation as I could because it is rhizomatous as you’re pointing out.

A. Yup.

Q. If, for instance, I wanted to change an area that might be in lawn right now into an active bed for something, to vegetable beds or whatever, could I use solarization as part of the sort of prep of that?

A. Absolutely. If you wanted to take a piece of lawn and turn that into a garden, solarization would be a great first step, either solarization or tarping with black plastic. If you lay that down for several weeks, it’s going to do quite a number on your grass and make it much easier and quicker to sort of transitioning to a cropping system after that.

Q. Would you still till first and break up all the clods of grass or vegetation, or would you just sort of burn it up under the solarization under the plastic?

A. I don’t know if there’s a strong pro or con, but I guess I would solarize or tarp first.

Q. Yes, weaken it.

A. What’s under there, weaken the root system a little bit and then it’ll be easier to get your tiller in there. That’s my gut feel on that.

Q. It’s my instinct, too, but I don’t know why and clearly I haven’t been out in control plots and I haven’t been behaving as scientifically as you have been, but that would be my guess from past experiences.

I just was curious: What do you still want to know? What do you wish you knew more about from what you’ve learned? You mentioned you’d like to know maybe more about not just the microbial soil life, but other creatures, ground beetles and earthworms and so forth. What other things do you want to learn more about?

A. Other than beneficial soil life, which is very high on my list of proprieties, I’m really interested in more of the trade-offs between solarization with clear plastic versus the tarping with black plastic. I’ve done a couple of experiments comparing these things and found really mixed results. Some years in some situations I was seeing better results with the tarping and other years it would be the other way around. I think that what species you’re trying to control and exactly how hot it’s getting and other aspects of what you want to use these practices for really factor into that trade-off. I’d like to dig deeper there.

Q. Just as a totally practical point, we all hate wasting plastic, one-use kind of plastic of any kind. If I’m going to get the tools to be doing this as a regular part of my weed control each year, what am I looking for? What mil thickness? Is there any hint of how I should go about getting the right material?

A. Sure, yes. If you’re a backyard gardener, so if you’re not going to grow 10 acres of mixed veggies, I would definitely suggest going with a thicker plastic. One thing you could look to do, either buy a silage tarp if you want that black plastic; those are pretty heavy-duty, thick and reusable. You can roll them up and use them year after year. [Note from Margaret: Though some products labeled “silage tarps” are just 3 mil thick, I suggest a minimum mil thickness of 5.5 for good reusability.]

If you want to try this clear plastic solarization, I’d suggest using greenhouse plastic. If you can find a farm that has old plastic in their hoop house that they’re not using anymore because they replaced the hoop house cover, I use that in some of my experiments and found that that actually worked really well as a solarization material. You may be able to salvage and reuse it and be kind to the environment that way.

Q. Fantastic tip; that’s a great tip. I happen to be neighbors to many young organic farmers who have hoop houses. I’m on my way down there. [Laughter.] [Note from Margaret: Greenhouse film with a 4-year durability rating and 6 mil thickness like this will give years of service for solarizing.]

A. Excellent.

Q. Well, Sonja Birthisel, I’m so happy to speak to you. Thank you so much for this work, because it’s so important and I think there’s so much misunderstanding about it. I can’t wait find out more about those organisms that live in the soil besides the microbes when you explore that in the next year or so.

A. Thank you. It’s been a pleasure talking with you and your listeners today. Anyone listening in, don’t hesitate to email me at sonja.birthisel [at] maine dot edu if you have more questions about this practice or want to know more.

how soil solarization kills weedsmore about solarization, weeds, and prevention

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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 10th year in March 2019. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the January 7, 2019 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).

  1. Ellen Kirby says:

    I appreciate the ideas for solarization. Your dialogue is most helpful as well. At the Food Bank Garden in Winston-Salem, we use biodegradable paper (sold at Lowe’s) underneath a thick layer of weed-free straw in our watermelon fields. This is a form of solarization that is also an improvement to the soil when it is tilled under. It is also great because the watermelon can sprawl into the paths between the rows.

    We also use the thick plastic (available in biodegradable) to cover hr planting beds. See photos on Facebook: foodbankgarden to get an idea of how we do the watermelon field. Cover cropping is another great practice we use for weed control. Even said, we still have lots of weeds to manage so all ideas are needed.

    I’ve written a paper on this year’s crop yield. Due to two hurricanes, we had a lower yield than in the past. I’ve posted a paper about that as well.

  2. gloria says:

    Science will out-all of these years we just assumed that the black plastic was the best for solarization.
    Thank you for this illuminating article.

  3. Janet says:

    Good information for the future. I have a tree line in the back of my property that japanese knotweed has taken over. I chopped it all back and used our old swimming pool covers that were no longer useful for the pool. Its been 2 years now and I think, possibly, this upcoming season may be the one that eliminates it. Time will tell.

  4. Bill says:

    Some 50 years ago we moved into our new home in the fall.We wanted lawn only for a narrow area. I sowed grass seed, bought several rolls of clear plastic and used it to cover the newly seeded area. It worked great. It kept it moist and warm. After a few weeks when the seed had sprouted, we had a lawn. I would recommend this to anyone.

  5. Mariele says:

    I wonder if this could work on containers. I grow my vegetables in large boxes made out of old doors and some in corrugated metal. Could this also help with some of the insects that are waiting there to eat my produce in the summer? I am particularly talking abut cabbage caterpillars. Do the white butterflies hatch somewhere else or are they in my containers?

  6. Ferne says:

    Thanks Margaret to you and your guest for the latest in scientific info on this subject. Very useful in my sunny B.C. Okanagan climate. And for anyone else who are desperate to get rid of some of the worst of particularly stubborn weeds. Works so well with so much less effort. It was a fascinating talk.

  7. Louise says:

    I wonder how I could use this between my garden fence and a neighbor’s fence where thorny vines grow. Would I need to weed them out, to put the plastic down? Or could I mow it?

    On the wet side I can’t easily walk for much of the summer. Many weeds thrive there.

    I hope the research continues and I see some helpful ideas.

    1. Ann says:

      I’m doing this right now to get rid of bishops weed. I’m using clear plastic that came with some ikea packaging. Pulled up the plant and most of the roots that I could. It took about 2 days. Then covered with plastic and bricks. I hope this helps. I know that bishops weed is persistent.

  8. Tibs says:

    I didn’t mulch a large perennial bed Las year. Kept the summer weeds under control but now I have loads of a type of bitter crest. If I do the solar clear plastic thing will it kill the spring bulbs and peonies day Lillie’s phlox?

  9. Sarah Young says:

    Thank you for this information. I have tried solarization with clear plastic as well as tarping with black plastic. I did not know about adding water first but it probably would have helped. My problem with the plastic is that it degraded after a few months and turn into tiny bits of plastic which I could not remove from the soil. It will always be there. I recommend keeping an eye on the state of the plastic and removing before it falls apart. We have a terrific problem with bindweed here in Colorado and that was the reason for the long solarization.

  10. Joe says:

    “Unfortunately, there’s been surprisingly little research on how these practices impact those [beneficial] soil organisms. That’s something that I’d love to look into next.”

    The possible negative impact on beneficials is definitely my biggest concern, and I’m disappointed this hasn’t been looked into yet. That being said, I’ve got some leftover thick clear plastic from building a little cold frame, so I’m going to give solarization a try this year!

    Incidentally, I accidentally “solarized” part of my lawn last year. I was replacing our sliding patio door, and after removing the old door I left it laying on the lawn for a few hours and finished installation of the new door before cleaning up. It was a pretty short solarization period, yet still zapped a little square of the lawn pretty good. Consistent with what Dr. Birthisel said about perennials, it bounced back within a few weeks without issue.

  11. Heidi Mass says:

    I live in NJ and never had good luck with solarization. My plastice always turned to pieces like a previous comment mentioned. I think it was because I was using it for a long term. I also didn’t roto till first. I switched to newspaper and it works much better for me.
    Great tips, thanks!

  12. James says:

    I would like this…but I have a porch garden, so pretty much I am working out of two big pots with maybe a cubic meter of soil at the most. To recondition the soil I’m not covering it, instead we’re composting, adding a bit of fertilizer and using a ton of earthworms. J

  13. Carmen says:

    I live in NE Pennsylvania and I tried solarization on my raised beds this Spring. I thought the soil was damp enough. I thought the plastic was pinned down fairly well. What I got was an explosion of weeds! I think I’ll try tarping next time.

  14. Jean Gavy says:

    I am dealing with a fungus which for two years caused my tomato plants to lose leaves (turn brown with spots and drop off).
    Would black or clear plastic get rid of the spores?


    1. margaret says:

      Hi, Jean. Needs diagnosing first before you can figure out what will help. I have summaries of various “tomato troubles” on this page (halfway down there is a paragraph or two that starts “Tomato foliage…“). Follow some of those links if you are not sure (like to Cornell’s diagnostic site for tomatoes). So the answer is maybe yes — and here’s more on “tomato hygiene” aimed at prevention or reduction of disease that is present in soils and so forth.

  15. Faye says:

    My garden bed where I had Vinca growing as groundcover became invaded with wind violets and clover last fall. We dug up and pull as many as we could. Took us 2 weeks. This spring they started to come up and take over the bed again, so my husband covered the bed with black tar paper. The paper has been on for 30 days. My worry is that there is a Magnolia tree planted in the bed along with Japanese rose plants. I’m afraid the roots of the plants are going to get too hot and die. The tar paper is about a foot from their trunks. I am watering around the trunks. Any advice at all would be greatly appricated! I have been fighting wind violets for 3 yrs. and they are winning..

  16. Rose Unes says:

    Late last summer I covered a newly cleared site, about 200 sq. ft., with clear plastic and followed all of the above directions. As the months progressed it acted more as a greenhouse rather than killing the weeds. This is January and I just went out and saw that under the clear plastic was green growth. I did attach the edges of the plastic as securely as possible when installed.
    My hope is that by next fall I can plant a wildflower/pollinator garden. I will leave the plastic on until then. What am I doing wrong or do you have any suggestions?
    I live in Rosendale, same zone as you.

  17. Ellen Hochberger says:

    I am re-reading this article on solarizing. I followed your instructions 2 years ago to get rid of, your old favorite, Houttunia! It was a success after leaving the tarp on an 8’x10′ section for the entire season. when I removed the tarp I planted lambs ears as a ground cover for now.
    Welcome spring 2021 and it is still clean! However 3 ft over, there they are again but this time surrounding the base of a 3 year old beautiful Katsura tree that I planted from a propogation.
    Question is, if I try to solarize this area, will I hurt the Katsura roots?
    Also, is this moot because apparently those devilish little roots were just hiding and moved over to the next area? (in which case, time to move).
    Would so appreciate your opinion on this. Just the sight of houttunia makes my blood boil!
    Thank you!
    Ellen Hochberger
    Summit, NJ

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