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Garden solarization

Goat eating oats

Our next garden-experiment-that-I-may-live-to-regret is solarization. I'm trying all of these experiments for my upcoming soil book, but this one was also spurred on by my fall oats cover crop not dying as expected. I suspect the uncharacteristic overwintering ability of the oats came about because I grazed it repeatedly in the fall, which kept the plants at a vegetative state rather than ever getting close to flowering. No matter why the oats survived, I was left with a conundrum --- how to turn that area back into plantable ground without tilling up the oats or lots of hand weeding?

Garden solarization

Solarization might be the answer. The idea is that you prepare your beds (in my case by letting Abigail eat the oats as low as she could and then begging Mark come in with the weedwhacker to finish off the job), then you stretch a piece of clear plastic tight over the ground to bake what's left behind. Solarization only works during the sunny part of the year and can take anywhere from one to three months to kill weeds and pests in the earth. Of course, the biologist in me says --- what's to prevent solarization from killing all of the beneficial soil microorganisms too? And, since the plastic dropcloths often used for solarization aren't UV-stabilized, will we end up having to pick plastic out of our soil when the greenhouse layer disintegrates in the garden?

Weighing down garden plastic

Mark always rolls his eyes when I poke holes in techniques I haven't even tried, so I shrugged and decided to give solarization a whirl. Worst-case scenario, we'll have a biologically dead bed that I can perk back up with some well-behaved cover crops and compost. Best-case scenario, we'll have a bed ready to plant into in June with very little work on my part. Stay tuned for more details as the experiment progresses!

Update: It works! Check out my ebook Small-Scale No-Till Gardening Basics for more information.

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Although I haven't put clear plastic down for solarization I was always under the assumption that use of black plastic was better. I have used black plastic to attempt to kill off weeds, grass and especially that bane of my yard: Johnson grass. It does kill off the grass and weeds (Johnson grass not so much), and I found that the plastic lasts a few years (3 at the most) before it disintegrates enough to have to pick plastic pieces everywhere. Does it kill off beneficial bacteria? Well, this site says no:
And apparently clear plastic is better than black plastic. Hope this helps!

Comment by NaYan Fri Apr 24 09:15:33 2015

Yes Anna, I hate to say it but you really need to use black. Despite the heat, things will grow well under the clear plastic. Even worse, weeds might thrive more than the cover crop.

It's also true that you don't want to use the plastic for more than a couple of years. I have a friend with a yard full of crumbles. But you can watch it & catch it before it gets that bad.

Comment by Terry Fri Apr 24 11:47:05 2015
My barley doesn't look like it is close to blooming either. Deer have nibbled some, but I don't think they grazed quite as diligently as your goats. I will wait another week, when we reach the 14 hour day mark, but will install a similar trial here. I will still try to mow kill half of it. I can do that in the part of the garden I plan for the fall garden . . . that way I can leave it in place until July 1st - ish.
Comment by Charity Fri Apr 24 11:57:47 2015
I probably should have been more clear. Black plastic mulch and solarization are quite different technologies. The former works like a kill mulch, and is most appropriate either in weed-free ground or around perennials --- you wouldn't want to lay down a cardboard kill mulch or black plastic mulch and plan to plant vegetables into it into the near future. Solarization is a speedier endeavor that works by heating the soil, rather than by blocking the sun. Basically, you're cooking all the weeds (and microorganisms), and then you remove the layer and plant into the bare ground left behind. The extension service sites I researched the idea on recommend against black plastic for solarization because it actually doesn't get the ground as hot beneath as the mini-greenhouse created by the clear plastic gets.
Comment by anna Fri Apr 24 12:12:15 2015
It may be that you planted fall oats...they will not winterkill as they are supposed to survive the winter and put on a grain crop in spring. For winterkill oat covercrop plant spring oats, usually known as white oats, fall oats are usually known as gray oats.
Comment by Nita Fri Apr 24 12:58:32 2015
Nita --- I was wondering if it could be the variety! Probably proves Mark right, who tells me that it's not worth the savings to buy cover crop seeds from the feed store where varieties are unlabeled. But it's so much cheaper than online.... :-)
Comment by anna Fri Apr 24 16:22:40 2015
I don't understand why you would think black plastic would be inappropriate in the veg garden. You want to kill weeds and cover crops, right? Nothing does that better than excluding all light. Whatever tries to grow is killed by absence of light, helping diminish the weed seed bank and leaving excellent tilth. I use a double layer of woven landscape fabric with a 20+ year, uv-stable life. Huge labour saver, and no-till soil saver. Jean-Martin Fourtier of "The Market Gardener" fame has been using 6 mm black silage tarps for almost a decade and credits it as a major reason for the success of his micro-farm operation. Apparently the technique, called "occultation", is widely used by organic growers in Europe.
Comment by Jackie Fri Apr 24 19:02:36 2015
Jackie --- It seems that I must be being very unclear today.... I wasn't saying that black plastic mulch is inappropriate in the vegetable garden (although I do have reservations about its use from a sustainability perspective). What I was saying is that you wouldn't want to take weedy, problematic ground like I'm working with in this area, put down a black plastic mulch, then plant into it in the near future. Like a kill mulch, black plastic will prevent future weeds from growing and will eventually kill existing weeds, but it's not a quick fix for an area you want to plant into in the near future if that soil's not already in good shape. That's the potential niche of solarization --- you can reclaim weedy ground quickly without tilling if the technique works as advertised.
Comment by anna Fri Apr 24 19:31:26 2015

I love how "lively" the conversation is on this post and how diplomatic Anna is with her responses. We love you, Anna! It's interesting to see how passionate people are about their black plastic for killing or preventing weeds. To the black-plastic passionate supporters: yes, black plastic does absorb more heat than does clear, but did you know that dark colors actually release heat more quickly when the sun goes down than do lighter colors? (thank you physics class) Solarization is a totally sound, documented technique that Anna is using in just the right circumstance. Black plastic is useful for other purposes. And so is cardboard. And deep wood mulch. And so is chocolate, but i digress...

I recently participated in a volunteer activity (again through my college bio/ecology class) with Save the Bay in Marin County (SF area). We were helping the coordinator experiment with killing non-native invasive Harding grass with solarization very similar to what you (Anna and Mark) are doing. In the case with Save the Bay, Harding grass is wicked-difficult to kill/remove, so they were solarizing after trying hoe and to whack down as much as we could of the vegetation, then covering with hay/straw, then with the plastic. The Harding grass is very pokey, so they are hoping the hay will cushion the pokes to the plastic. They've had a hard time with other experiments with various kinds of clear plastic that do break down in UV light. This round of experimentation was with UV-protected plastic. They also started a side experiment with a piece of sort-of thin acrylic (poke-proof and hardy to UV) held down by weights. It takes at least 7 weeks to kill the vegetation. Then they plant native species.

Comment by jen g Fri Apr 24 21:16:16 2015

jen g --- Thanks for posting! That kind of fascinating, firsthand information is what keeps me blogging. :-) I was actually wondering about using those clear-plastic roofing panels, or maybe the double-plastic-with-an-air-gap greenhouse panels --- both seemed like more of a long-term choice for solarization if I like what it does with the ground. Sounds like I might be on the right track!

In the meantime, any chance you'd like to email me a photo or two Save the Bay's experiment? I'd love to add it to the book (and am always willing to plug a nice non-profit. :-) ) My email address is if you're interested, but don't feel obliged!

Comment by anna Sat Apr 25 07:38:36 2015
Maybe it's just the feed store employee knowledge...I buy my cover crops at the feed store, and it's in town actually. Cayuse is a dependable white oat and should be widely available. Just make sure you ask for seed oats not feed oats, because seed oats are much cleaner. There's always next year!
Comment by Nita Sat Apr 25 09:27:56 2015

Nita -- Cover crops, unfortunately, haven't really made it to our region. So I'm pretty sure all the seeds I've been getting are meant for feed. (The store certainly doesn't stock any cover crop seeds that aren't also used for animal fodder.)

We do get the occasional weed seed in the oats, but it's some kind of forage turnip that doesn't cause any problems. I consider it a semi-intentional polyculture. :-)

Great to get your oat variety recommendation! I'll try to track that one down.

Comment by anna Sat Apr 25 10:39:31 2015
I didn't make it clear, but the woven black plastic, which is permeable to air and water, is removed after the weeds and cover crops are killed. It simply takes the place of tilling it all in and waiting for it to decompose. The soil life does a nice job of incorporation and you've got a beautiful ready-to-plant no-till bed. True, the plastic is a petroleum product, but a one-shot purchase, and Fortier and I agree that hand tools are the most appropriate technology on a scale of less than 2 acres.
Comment by Jackie Sat Apr 25 10:53:06 2015

In summary . . .

black works via light exclusion, similar to a kill mulch of cardboard or deep mulch.

clear works via high heat. It 'bakes' the top 6 inches of the soil. Interesting to hear about the Save the Bay use of hay/straw. I saw that mentioned somewhere else. The way it read (ensuring the hay/straw as wet), it sounded like the heat was ramped up with the process of composting. I'm wondering if a high nitrogen moisture under the clear would also help with temperature levels . . . i.e. hay/straw layer, possibly shredded to enable the plastic to be tight against it, soaked in a diluted urine solution. Just curious.

Comment by Charity Sat Apr 25 13:23:33 2015
I'm sure you have already thought of this, but I would think that your Southern States would be able to order you 50lb bags of actual cover crop seeds from their main suppliers. And for the most part they can be ordered treated or untreated, so that you can feed excess/unused to your critters, if you wish. For a once or twice a year shopping adventure, if your local one doesn't, they might be able to track one down which would in a reasonable distance.
Comment by Charity Sat Apr 25 14:22:34 2015

The winter kill on oats definitely has to do with their phase of growth, and I've had it go both ways. If I plant oats (whatever the feed store has) in September (zone 6b) they will get up to a foot tall or more quickly and usually winter kill in January. If I'm late getting them in until October sometime they will hold over the winter as small plants and continue growth in spring unless the winter has been particularly hard.

As for the black versus clear plastic debate, I use black plastic (generally the tarps of of lumberyard bundles which last many years) extensively as it's the only thing I've found that slows down bermuda grass. The times I tried clear plastic it acted much more like a greenhouse and took longer than the black to kill what was underneath. I'm sure this depends on the weather, so clear may work in August but not as well in April. I do not use the black plastic as a "mulch" but as a temporary covering. Right now I have it over the area where corn, sweet potatoes, and watermelons will go in this week. The oats in that area were thin and the black covering is a stop-gap to retard weed growth over the past month while waiting for summer plantings. I'll remove the plastic completely and plant.

As for your concern over soil life, I had the same. Portions of my sunniest area are infested with root knot nematodes and solarization is one of the prescribed methods of management. I was worried about the soil life, but our nematode specialist at the University of Arkansas is of the opinion that bacterial, etc., populations will rebound quickly (more quickly than the nematodes) as the effect of solarization is fairly shallow. I've decided, however, to just go with plants that don't host RKN--- sesame plants are beautiful!

Comment by sweetgum Sun Apr 26 23:43:59 2015