soil solarization

kqcrna(z6 SW Oh)April 9, 2007

I am considering solarization of my (small) tomato beds. I am battling cilantro and my neighbor's purple deadnettle, and I do mean battling. I wish my basil would return like the cilantro. And I know that the deadnettle will creep back in later, but I'd like to at least get rid of what's already there.

I realize that the purpose of solarization is to kill pathogens, weeds, and seeds, but does it also kill off beneficial organisms? What about beneficial insects and worms?

Any of your always expert comments appreciated,


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pablo_nh(z4/5 NH)

"I realize that the purpose of solarization is to kill pathogens, weeds, and seeds, but does it also kill off beneficial organisms? "

That's what I hear. I dunno about deadnettle or cilantro, but heavy mulch early in the season is normally the best way to deal with most weeds (I have crab grass, dandelions, and a normal variety here). If you add lots of OM and DON'T STEP ON THE BEDS!!!, then pulling weeds is a snap.

    Bookmark   April 9, 2007 at 10:54AM
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You're battling cilantro Karen! I try cilantro every year and end up with a seedy looking weed. I'd love to be able to grow cilantro like you get in the stores but I can never seem to keep it from going straight to seed. I've never even had any real cilantro leaves all I get is some kind of dill looking thing. What kind of soil do you have it planted in? Maybe the soil that IÂm putting mine in is too good. IÂd love to know what youÂre doing different than me.

Thank you,

    Bookmark   April 9, 2007 at 11:27AM
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kqcrna(z6 SW Oh)

I just find that early, think mulch also keeps the soil colder, longer. I want to plant tomatoes there. Our growing season is fairly short for tomatoes so I'd rather have the soil warmer, earlier, if possible. For that reason I don't usually mulch until the weather is consistently warm and the soil is warm, too. But by then these undesirable weeds tend to be out of control.

Am I stuck between a rock and a hard place?


    Bookmark   April 9, 2007 at 11:29AM
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pablo_nh(z4/5 NH)

Mulch when you plant. If you don't- you'll have to water more and have more issues with watering (and possible BER), you'll have more weed issues as well. A good mulch composts in place and builds soil, as well.

Once solarized- the weeds don't stop. Seeds blow in from wherever and boom- weeds. Whether you solarize or not- if you don't mulch, then you're really doing more work than necessary.

I live in NH zone 5, and I mulch every year. I do, however, have raised beds which warm up faster (I pull back the mulch for a couple weeks before I plant)

    Bookmark   April 9, 2007 at 11:44AM
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rhizo_1 (North AL) zone 7

Karen, studies have indicated that although the populations of 'some' essential microorganisms are damaged by solarization, they restablish themselves very quickly. Greater numbers of a wide variety of beneficial bacteria and fungal species have been found a few weeks after the process than were recovered before.

The best guess about earthworms is that they leave the location during the solarization process; either escaping to deeper soils or evacuating via horizontal channels. Anything that is reasonably mobile will leave the steaming soils until things get back to normal!

    Bookmark   April 9, 2007 at 12:55PM
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kqcrna(z6 SW Oh)

Thanks for the replies. I typed a reply and somehow GW lost it, asked me to sign in,...

Strouper, I just sprinkled cilantro seeds into the bed a few years and have had it come back ever since. You can prolong the life of the plant by pinching off the blossoms immediately.

Re: mulch. I do mulch with a couple of inches of leaves in fall, by spring they mostly either blow away or are digested. I rake aside as the temp warms to promote soil warming. We plant tomatoes around Mothers Day and mulch with shredded mulch. Weeds keep coming.

At the fence line where the purple deadnettle is spreadling from the neighbor's hillside to my yard, I mulched with 6-12 inches of leaves in the fall. That crap is coming up right smack dab in the middle, thru all those leaves. I've read that it spreads by both roots and seeds. It's blooming now.

    Bookmark   April 9, 2007 at 2:34PM
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I'm definitely with Pablo on using lots of mulch. I've started planting exclusively in raised beds and lasagna beds. Even when weeds pop thru they pull up easily, or I just throw more mulch over them. Since November I have had hundreds of pumpkin plants come up in my new beds (chopped up pumpkins have been great for my lasagna gardening,) and they all turn into greens for my compost pile. I haven't had experience with deadnettle, but every time I lay down cardboard for a new bed, nothing seems to break through. Might be worth a try for you....
good gardening and good luck!

    Bookmark   April 9, 2007 at 2:52PM
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kqcrna(z6 SW Oh)

I've thought about the raised beds- would love to have some. Time and materials are a problem though. I do have 60 wintersown containers to get planted out into beds, too, and still more seeds to sow.
I also have some physical limitations, then there's that little matter of the job that interferes with my gardening time. And new carpet for the first floor arriving late this week or next. Have to pack up all the china, crystal, sculptures, computer, tv electronics. Would have replaced this worn out carpeting long ago if it weren't so much work. Blah!

    Bookmark   April 9, 2007 at 3:44PM
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bejay9_10(zone 9/10)

Most of my veggies are grown in raised bed/boxes. The only soil I put in them is from compost that has been heated to kill off the weeds and any pathogens. If I just add fresh shredded leaves/grasses/clippings, then I will plant weeds in the beds. Best to compost first.

If planting out in open ground, I first cover the area with cardboard, in order to kill weeds/bermuda, etc., then add compost, plant and mulch when stuff begins to grow. Unless my ground up regular stuff is piled 5 or 6 inches or so, (which is usually put around trees), the weeds will crop out of it.

For veggie boxes, I use straw - works OK, but also invites sowbugs, especially in strawberry beds. Would like to find something that didn't have a lot of fine particles - but at least straw has fewer weeds.

Just my 2 c's.


    Bookmark   April 9, 2007 at 4:46PM
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Karen - guess I am just lucky to have plenty of stuff available for lasagna beds. Three weeks ago I decided to plant some sweet corn. In about 1 1/2 hours I was ready to go. Usually I wait awhile before planting, but here's my recipe for a quick bed:

1. flattened and soaked cardboard boxes(overlapping) about 4 ft by 10 ft.
2. a couple buckets of almost finished compost.
3. kitchen scraps, refrigerator waste, and fruit and veggie pulp( free from my local juice bar)
4. lots of dry leaves and grass
5. lots of coffee grounds, compliments of starbucks and my local coffee shop.
6. old straw and hay
7. more coffee grounds
8. shredded leaves
9. compost for planting holes
Plenty of water on each layer of "browns".
Succession planting - one row of corn is now 8" tall, one is 3", the third row should be sprouting soon.

There is plenty of good info about different materials and methods available on this site - I just use whatever I can scrounge for free. A few beds of mine have sides (logs, scrap firewood, etc.) I'm pretty new at this type of gardening, but it definitely has me hooked. No more backbreaking pickaxe digging in terrible rocky soil for me!

good carpeting to you!

    Bookmark   April 9, 2007 at 9:56PM
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kqcrna(z6 SW Oh)

melonhead: You are describing lasagna gardening, not solarization. I have built quite a few lasagna beds in the past, the last 2 completed about 2 weeks ago.

It's not easy to get my husband to bag grass clippings, he wants them mulched into the lawn. Neighbors do the same. I have used 1/3 of my hoarded leaves from the fall to build the current beds, and used up the compost too. I save the leaves to produce compost thru summer. Aside from no Starbucks near me, I have gone to 3 or 4 in the city who don't, and won't, recycle grounds. No straw or manure easily available here in suburbia, and even if I found them, would you suggest I haul it in my Honda Civic or my husband's new Cadillac?

My point- it's not always easy to get lasagna ingredients in suburbia, and I just finished 2 beds.


    Bookmark   April 10, 2007 at 6:44AM
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Stop and think about this. If soil solarization kills pathogens, disease causing bacteria, what would it do to the beneficial bacteria? Since pathogens are meaner and tougher than the good bacteria, and if the good bacteria fairly rapidly restablish themselves after solarization what happens with the pathogens which are hardier and better able to adapt to a new environment.
Soil solarization may work to control your "weeds" but it will need to be there for many weeks, most of the stuff I have been taught about it is at least 12 weeks where the sun shines most days. Less than that and it will not be effective. While most of what I see does say the plastic needs to be in place 4 to 6 weeks UCDAvis says the longer the better and all summer is not too long. This solarization only cooks those thingys that will be in the top 2 to 3 inches also, so if the beneficial bacteria will move lower to escape the heat does it not stand to reason that the pathogens would also?

    Bookmark   April 10, 2007 at 7:08AM
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kqcrna(z6 SW Oh)

Kimmsr: yes, what you say about the bacteria does sound logical. I guess I always hope when I add organic mulch or lasagna ingredients that the good guys will overcome, but to my knowledge I've never had a bad problem with pathogens in the soil. My nemesis now is just those !@#$% weeds and seeds.

I guess I'm kind of late for the solarization, yet again. I should have done it prior to that unusual hot weather we had prior to this cold snap, but who could have guessed that heat could last so long in Mar and April. I wanted to give it a shot last fall but my health issues and additional major surgery Aug. 29 shot that idea. One of these days, I'll get it conquered, too, I guess. It will remain on my list of projects to try.

As always, thanks for the suggestions and comments.


    Bookmark   April 10, 2007 at 10:31AM
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rhizo_1 (North AL) zone 7

Though Rhizobia bacteria are affected by solarization, most other essential and beneficial bacteria and fungi are not. As a matter of fact, it's been shown that once the pathogens are killed, the beneficial soil microbes repopulate rapidly with little assistance. Some are actually very little affected by the heat. This has been observed in many practical studies.

In other words, IF you are having a problem with soil borne diseases, weeds, etc., the benefits of proper solarization FAR out weigh any temporary decline of beneficial populations. The plants installed into recently solarized locations respond immediately to the improved environment. This elegantly simple process has saved (literally) agricultural soils that poor farmers thought lost to them forever.

Since very high temperatures have to be reached for 'sterilization' to occur, the best time to achieve the best results is in the hottest and sunniest time of year....summer. Soil temps under the clear plastic depends upon outside temperature, surrounding soil temps., amount of sunlight, etc. It can also take several weeks, not just a couple. If it's not done correctly, then you end up creating a greenhouse, rather than a killing steam. Yikes!

    Bookmark   April 10, 2007 at 1:23PM
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