Large brown tarps work wonderfully to kill weeds underneath; perennial bindweed roots come back, but that goes without saying. Other tough perennials are greatly reduced or killed, and annuals are killed. I'm thinking this would be a good no till way to kill off rye cover crops, leaving a dead mulch on the surface.

But, as a routine event (once a year for a few weeks), would this do a lot of damage to the soil? And, if so, would there be any way to mitigate this?

I do remember reading a study showing that growing in plastic row mulch developed more plant pathogens over time, however if you're only using tarps to kill off weeds and they're only on a few weeks, I wouldn't worry too much. I'd probably just wait a few days after uncovering and before planting in so as to let the soil aerate. Plastic tends to increase soil moisture by up to 50% and it can help speed up plant residue digestion, you just have to be careful the soil doesn't become waterlogged and go anaerobic.

Covering the soil will kill off a lot if not all the microbes in the soil under the tarps.

While you can reintroduce microbes and they will return after the cover is removed, it does effect the soil health.

That said, covering to kill off unwanted plants is still a highly recommended and useful technique. Just be aware that you will be killing the microbial life as well and know to do something to restart that life and you will be fine.

How deep do you suppose the microbe killing would go? If the tarp was tented up a bit, would that help things stay alive?

Does cardboard have a similar effect on soil life?

Mycorrhizal fungi need living plants to survive themselves. They have evolved into a pure symbiotic organism. I imagine you would lose these if they were present in the soil. I also imagine the fungi that live by decomposing would be just fine. Bacteria would shift to different types I am guessing. Not a huge deal compost fixes that. Life has taught me that your insects will be fine.  All in all better than tilling IMO.

Can mycorrhizal fungi survive as spores? Also, in a vegetable garden, where a different kind of (usually) annual plant is grown in a bed each year, would there be many species specific mycorrhizal fungi around in any case?

I guess to protect the living soil web, I should intersperse patches of untilled, untarped area where living symbionts can survive; this should be a permaculture practice anyway.

As I keep thinking about this, since this mid scale garden is mostly annual, the soil disturbance may not actually be a bad thing; an annual's niche is disturbed soil. This seems like a better way of disturbing things then burning a lot of fossil fuel to mash the soil up.

Gilbert Fritz wrote:Can mycorrhizal fungi survive as spores? Also, in a vegetable garden, where a different kind of (usually) annual plant is grown in a bed each year, would there be many species specific mycorrhizal fungi around in any case?

It depends. In my annual garden there is, but I take extra steps to make sure of that. That being said annual gardens are primarily bacterial to my understanding.

Gilbert Fritz wrote:I guess to protect the living soil web, I should intersperse patches of untilled, untarped area where living symbionts can survive; this should be a permaculture practice anyway.

If you have any perennials that you don't mind, these could act as fungi refuges if you leave them and a small space around them. Again this only works If the fungi is there.

Gilbert Fritz wrote:As I keep thinking about this, since this mid scale garden is mostly annual, the soil disturbance may not actually be a bad thing; an annual's niche is disturbed soil. This seems like a better way of disturbing things then burning a lot of fossil fuel to mash the soil up.

This is true. It is way better than fossil fuels. I personally use no till in my annual gardens with the exception of preparing new beds, but a lot of other people seem to have trouble with it.

I've had trouble with no till as well, but I think the key is cover crops that grow enough root to, in effect, till the soil. (At least with my heavy compact clay.) Then the question becomes, how to kill off said crop, which is where the tarp comes in.

For more understanding of what I am trying to do, I have a quarter acre mostly annual garden.

Fallow studies show you shouldn't lose mycorrhizal fungi colonization if roots aren't disturbed for 30 days. However for every further 30 days you do reduce the mycorrhizal root colonization by ~12%. If you do till those root residues then that reduction starts from day 1 and not 30 and also significantly reduces the ability of that tilled mycorrhizae to recolonise new plants.

Also, while a month under the tarp tends to reduce the overall microbial abundance, changes in community and diversity are mostly because of temperature or moisture extremes. For example solarization of the soil for a month with clear plastic changes the soil community towards more thermophilic microbes and will kill off some mesophiles that like temperatures under 45C (113F).

If I were to tarp, I'd only go a month at most, and choose one that doesn't heat the soil excessively and maybe poke a few holes in there if it doesn't breath or if it covers a large surface area.

Gilbert Fritz wrote:How deep do you suppose the microbe killing would go? If the tarp was tented up a bit, would that help things stay alive?

From my understanding, part of the killing comes from "cooking" of the soil under the tarp trapping the heat under the tarp. A tarp or cardboard or sheets of plywood or any other covering heats up in the sun and transfers that heat into the soil along with cutting of air and light. This creates a rather inhospitable area for most life.

I don't think tenting the tarps up would really help, rather it would likely just make the whole process less effective. Making you have to keep the tarps covering longer to kill off the undesirable plants which would likely mean sterilizing the soil more effectively.

I am not an expert in the microbial life and haven't followed studies on how sterilized the soil gets etc, but I have used the covering technique many times, as well as know a lot of people who do it too. The microbial life will return naturally once the cover is removed. Like me and many others you can jump start the process by adding compost, compost tea, a bit of fresh earth with microbial life, etc into the area after you remove the tarps. the microbes will recolonize the area quickly.

Since your plan is to do this to the same area only once a year, I would not think it would overly harm the soil as long as you reintroduce the microbes after the event. If you were doing it several times a year then it might be worthy of concern.

Something you might consider if your worried about stripping the microbes from too large of an area would be to section the area into smaller areas to cover with neighboring areas that you can leave uncovered. This could allow for microbes to have refugee to stay healthy to reintroduce themselves after the covering is removed. I don't know how your garden is laid out and if this is possible. Another option might be alternating covering, so that there is always an area next to the covered one that still has microbes in it. Both options you would still want to reintroduce microbial life to give the soil a jump start back into healthy micro organisms.

Good ideas!

Interestingly, I used two kinds of tarp; one a standard light blue one, and the other one which was silver on one side, brown on the other, with the brown up. This was during the heat of the summer. I also used some sheets of cardboard.

Under the blue tarp, the weeds went to town. They slowly but surely lifted the tarp into great billows, until I had to remove it. (All weeds had been whacked to the ground first.) Thistles and bindweed did particularly well, though they were a little more pale then usual.

The brown tarps quickly and completely killed the top growth of all weeds, probably as you suggest due to over heating. Bindweed came back when uncovered, but there weeds were completely dead.

Cardboard killed the weeds more slowly, and bindweed found cracks to grow through.

The weeds stubs from heavy plants were very hard on the tarps.

Bindweed will take more than a few weeks.  I have some plastic on a section now and imagine it will be dead dirt soon. Has anyone tried using gall mites for killing bindweed? This is used by farmers in field crops and supposedly does not harm other plants (cousin-morning glory excluded)
I really like to ask should we rather than could we due to unintended outcomes. Introducing mites to my property that are tough enough to kill bindweed is a little daunting. Any one have information?
And by the way, that tarp can be inviting to gophers. I lost a few plants that way.

I looked up the mites a few years back. Basically, you need to leave lots of bindweed around to build up a population, and for a small area it didn't sound like it made sense.

I don't mind that the tarps don't kill bindweed; I mostly want them to kill the tops, other weeds, and cover crops, all of which they do well. Nothing kills the bindweed, but established stands of squash and other cucurbits really suppress them in my experience, as do stands of sunflowers.

Hello to all,Just moved here from acreage outside Phoenix now living in urban Dayton Ohio area.  Turning backyard lawn into veg garden.  I  have access to cardboard, tree trimmings, composted manure, and composted forest material.  I have made many raised beds and lasagna gardens with great success back in the desert.  Permaculture techniques  worked wonders but what a difference in beautiful  GREEN WET OHIO!!!     Oh and my very first dig into lawn produced a big fat worm.  Heavenly.   It is almost September and staked out about 30 by 50 slightly sloped area and want to create soil. Planned to layer manure cardboard tree trimmings straw but was wondering is I should cover with black plastic so to cook over winter.  Anyone with better ideas?  Thanks to all for your responses...Ellen

Gilbert Fritz wrote:I used two kinds of tarp; one a standard light blue one, and the other one which was silver on one side, brown on the other, with the brown up.

Under the blue tarp, the weeds went to town.

The brown tarps quickly and completely killed the top growth of all weeds, probably as you suggest due to over heating. 

I used lumber protection tarps from my local home improvement store (lumber is shipped to the store wrapped in them and then recycled by the store).  They did a great job of killing the weeds in a month.  I believe the reason the brown/silver tarp worked and the blue one didn't is because it blocked all the light. 

Lumber protection tarps; great idea!

You bet!  I got a bunch from one home improvement place and then they started wanting to charge me for them.  Then I went to another place that specializes in homes and depots and they said I could have them for free.  They are all I use for simple tarp applications.  Covering garden beds to kill weed seedlings, tarp to put mulch on temporarily, drop cloth, etc...