Jenny

You could solarize the soil, but it requires coverage during the hottest part of summer, so you probably wouldn't be able to sow any seeds until next year.

While I don't know if it would work, I've thought of solarizing parts of my garden each year and setting plants out in containers on top of the plastic so the growing space isn't entirely wasted for a season.

Do you have other hard to control, ugly weeds in the yard? I ask because your dandelions and clover are pretty wildflowers, and clover will almost certainly be a component of your wildflower seed mix, I bet. I have an unmown yard and the dandelions and especially the clover (mine is a tallish, huge-flowered purple and pink and cream variety), and one type of buttercup are really pretty in the spring.

My soil is terrible after my virgin soil got robbed during construction ten years ago, the deep dark soil dug out and replaced with gravel/soil. A lot of ugly stuff came in with that replacement dirt. A blackberry, something with burs, and the worst of all, a pernicious (I've wanted to use that word this morning somewhere. (It doesn't quite mean what I thought it did, but what the hey.)) buttercup.
This little, evil, nondescript-flowered, buttercup is taking over, so I'm trying to smother it area x area. I covered a huge area last fall with a 3' layer of compacted leaves. I was astonished to see the first green of the spring this year: those buttercups poking their heads out of that deep mulch. I am kicking myself that I didn't put a newspaper layer down first. They're slightly weakened, so I'm going to cover them with newspaper, seaweed and hay as soon as I can get to the beach, and hope that I can finish them off and quickly get to another area they're taking over.

In other areas of my yard where the evil buttercup hasn't arrived yet, I am growing or buying or digging up wildflowers that are more attractive than my weeds and plugging them into those areas.

My neighbors are really rude about my yard (I live on a dirt camp lake road, where originally no one mowed, so no one can really get annoyed with me.), and when they are in my yard, they think nothing of stepping off my indian paths and flattening a clover or a particularly handsome buttercup. When my neighbor Phil stepped on a nice, fat purple clover one day, offpath, I said, "You're stepping on a flower," he said, "What? This mess?" I took him by the arm and made him stand still and I pointed to the clover, the buttercups and the daisies in bloom under his feet and said, "You cannot tell me that that is not pretty." Then I took him to a less robust part of the yard and showed him my beautiful patch of fine grasses and yellow, red, and orange indian paintbrushes.

"Ta-Dum!!" I said triumphantly.

He still couldn't see it.

Anyway....I would try to overwhelm your own wildflowers---your dandelions, your clover---with plugs you put in among them or I would smother the area with newspaper and a nourishing mulch this year and plant the wildflowers next year. As an organic gardener, the herbicide route is not an option for me. I've made a commitment to organic gardening that I keep even when the going gets tough. It'd be nice if I weren't an organic gardener, though, because dealing with my little buttercup is a difficult battle I may lose.

Good luck.

I'll have the huge amount of dormant seeds competing with my native seeds, right?

Yes. This is your issue.

Dan

Re: worry about dormant seeds ... why not go ahead and sow the wild flower mix and see what happens this year. If the unatractive weeds overwhelm the pretty wild flowers, then go the smother/layer route.

Laying down newspaper and then covering that with a mulch will aid in killing the "weeds" that are growing there and will help add organic matter to the soil. The newspaper and mulch does not "smother" the plants growing there now it deprives then of access to the sunlight they need to grow and without which they will die. You could seed right into the material you mulched with and as these native plant species grow, and the soil bacteria digest the newspaper, they will root into the soil under that mulch.
Solarizing soil requires several weeks of good sun, about all of your summer, and will also kill off a good portion of the Soil Food Web which will then need several years to recover. Covering any soil with any plastic, because that so limits the exchange of air a good, healthy soil needs, is not a good organic practice.

Kimmsr

About this: Solarizing soil....will also kill off a good portion of the Soil Food Web which will then need several years to recover.

Your statement above ignores a great deal of research, such as this information from the University of California. The reason for some of its beneficial effects have not yet been identified. Increased plant growth response. Plants often grow faster and produce yields of increased quantity and quality (size and appearance) when grown in solarized compared to nontreated soil. This phenomenon can be attributed, in part, to pathogen and weed control, but it is largely unexplained. For example, when soil apparently free of pests is solarized, increases in plant growth are still observed. A partial explanation may be found in a combination of mechanisms. First, because major pathogens and pests are controlled, it is likely that minor or unknown pathogens and pests are also controlled. Second, some soluble nutrients such as nitrogen (NO3-, NH4+), calcium (Ca++), and magnesium (Mg++) may be increased and made more available to plants in solarized soil. Third, beneficial microorganisms, such as mycorrhizal fungi, Trichoderma sp., actinomycetes, and some beneficial bacteria survive the solarization process or recolonize the soil rapidly. These in turn may contribute to a biological control of pathogens and pests and/or stimulate plant growth.

Special Uses for Soil Solarization. Soil solarization has been used successfully on a large scale to control Verticillium wilt in pistachio orchards in California. Using both hand labor and plastic-laying machinery, the orchard floor was completely covered in plastic. Plastic strips were hand applied around tree bases and connected to a strip of machine-applied plastic between the tree rows with heat-resistant glue or by using narrow bands of earth to hold the strips down. The 5 to 10-year-old trees partially shaded the soil and plastic but not enough to prevent soil heating, which the trees survived with no visible detrimental effects. Although expensive, success of this method in existing orchards indicates the high potential of solarization to control some soilborne problems.

Soil solarization has also been used successfully in Israel to control soilborne pests in greenhouses.

Special Considerations of Soil Solarization. Soil solarization requires that soil be out of production for at least 2 months during the summer. To avoid losing a growing season, special attention should be given to crop rotations and sequences that allow solarization but also take advantage of the land before and after treatment. In the Imperial and Coachella valleys, where summer temperatures are too hot for most crops, soil can be solarized during summer and planted during fall or winter.

Sensitivity to heat of plant pathogens, weeds, and other soilborne organisms differ. Some organisms will not be controlled by solarization and will require other control measures. This appears to be the case with sweet clover (Melilotus sp.) and some high temperature fungi in the general Macrophomina, Synchitrium, and Pythium. Conversely, some organisms difficult to control with soil fumigation, such as seeds of cheeseweed (Malv parviflora) and field bindweed (Convolvulus arvensis), are easily controlled by soil solarization.

From the conclusions in the UC link above:
Does soil solarization kill beneficial soil organisms? Populations of some beneficial organisms, such as Trichoderma spp. or actinomycetes, may be increased by solarization. Other important soilborne organisms, such as mycorrhizal fungi, may be decreased in the upper soil profile but not enough to lessen their beneficial action. Populations of some microorganisms, such as beneficial bacteria (Bacillus and Pseudomonas spp.) are partially decreased during solarization but afterwards recolonize the soil rapidly. However, populations of Rhizobium spp. of bacteria, which fix nitrogen in root nodules, are killed and must be reintroduced with seeds of legume crops. Survival and activation of beneficial organisms appear to play an important role in the increased plant growth commonly observed in solarized soils.

It appears that of the important soil bacteria, only Rhizobium spp. is destroyed by solarization to the extent that they need to be replenished by planting legumes, probably with a nitrogen inoculator to most rapidly bring the soil back to its optimum bacterial balance.

I started a similar project about 8 years ago here, using almost but not completely organic methods. My two biggest regrets are not eliminating more of the bermuda grass right away, no matter what it took, and engineering my yard to provide the best possible drainage and tilth.

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My suggestion is that if you can, get a giant compost pile going simultaneously during this time using mainly deciduous leaf mold and inoculated with shovelfuls of your native top soil. Ideally, you would kill the weed seeds with a hot compost technique, yet maintain a large store of soil flora and organic matter for mixing back in the yard after whatever weed killing methods you use.

Anney, If I read that right solarizing soil will kill of disease pathogens but not beneficial soil organisms, right? That is not going to happen no matter who wrote the article. Disease pathogens, Bacteria that cause disease, are generally tougher, hardier, then any of the beneficial bacteria.
An article from the University of Georgia I saw some time back indicated the soil needed to be out of production for 8 to 12 weeks, about the whole growing season in Minnesota. Soil solarization may be of some benefit in Georgia and other southern states with a long growing season, but few of us up north will find enough sunny days to try that.

kimmsr

Yes, we have a longer hot growing season here than folks in Minnesota do, but even so, the solarization process in the summer takes away a lot of the growing time for plants that need hot weather.

The link does not claim that solarizing the soil will spare beneficial organisms but that in the field tested, they quickly recolonize with the exception of nitrogen-fixing organisms. What I suspect is that probably most organisms are killed and some DO repopulate the soil quickly if the soil is otherwise healthy. Conditions need to be right for the repopulation of beneficials to occur, of course.

If the soil is otherwise healthy is the key. If you already have a good, healthy soil then solarizing it would be of no benefit. Solarizing is only needed on soils that are unhealthy.

Well, no, kimmsr, healthy soil can have invasive plants that are impossible to eradicate with other methods. I know this is your mantra, but it just isn't true. It isn't true about insect pest damage either. I think it's like anything in nature. Even with healthy soil we'll need to be able to accept some pests and some weeds and some less-than photogenic plants and not try to make a garden be perfect as many photographs would have us believe we can accomplish.

If the soil is otherwise healthy is the key. If you already have a good, healthy soil then solarizing it would be of no benefit. Solarizing is only needed on soils that are unhealthy.

Puh-leez.

Dan

If the aim is heat for sterilization, why not skip the 6 weeks of inactivity and get a garden torch (a lawn dragon or whatever) and fry the whole mess crispy? You can rent them at some places or a flame weeder plus the propane is still under $100 to buy.

dicot

I don't think a flame weeder would have the same effect. It would certainly kill off some surface weeds and maybe some surface microorganisms, but solarizing the soil kills everything several inches deep, the roots, insect pupas, and most of the bacteria and funguses that are causing problems. I recall reading somewhere that earthworms will burrow quite deeply into the soil to escape both heat and freezing, so they are not destroyed.

"but solarizing the soil kills everything several inches deep"
anney, that sytatement contradicts what you have been trying to say elsewhere and fully supports what I have been saying about solarizing soil. There simply is no way that solarizing is going to distinguish between the beneficial soil dwellers and the bad guys and kill only the bad guys, especially since most of the bad guys are meaner and tougher then most of the good guys.
I am aware that there are some people here that simply cannot believe a basic premise of organic gardening that a good, healthy soil will go a long way toward curing the problems many people have in their gardens, because they have never gotten there.

kimmsr

RE This: anney, that sytatement contradicts what you have been trying to say elsewhere and fully supports what I have been saying about solarizing soil. There simply is no way that solarizing is going to distinguish between the beneficial soil dwellers and the bad guys and kill only the bad guys, especially since most of the bad guys are meaner and tougher then most of the good guys.

Do you have problems with comprehension? Or do you just not read posts? I said this: The link does not claim that solarizing the soil will spare beneficial organisms but that in the field tested, they quickly recolonize with the exception of nitrogen-fixing organisms. What I suspect is that probably most organisms are killed and some DO repopulate the soil quickly if the soil is otherwise healthy. Conditions need to be right for the repopulation of beneficials to occur, of course.

Some people are just fanatics with closed minds.

Since both bad and good soil organisms will repopulate after soil sterilization measures, what is the purpose? Weed seeds that might be killed can blow in from neighboring lands, and it seems that controlling weeds can be done in far less intrusive ways. The OP is in MN, so perhaps the subject of soil sterilization is moot. I think that my advice to the OP is on the laissez fair [lazy/hope for the best] order. Sow the flower seeds this year. If there is a problem with unwanted/noxious plants, then take more stringent measure. But chances are, that even in that event, there would only be 'islands' where more labor/time intense measures would be required. BTW - dicot's pictures are lovely. That's the look I'm hoping to achieve in a side yard with no water.

I think with all the combative language, we have missed an important part of this site: there are TREES there. So the effects of solarization will be substantially reduced. Solarization is mostly used to control soil diseases and nematodes that run rampant in parts of California, studies have shown it only effects about 2" of the soil in terms of sterlization of dormant seeds - it is not really what you need.
Just scrape off whatever you can, get some black filter cloth - it blocks light but not water - and put it down, then starting building up some bew compost with whatever you can get your hands on - layer it onto the black cloth and try to get it about 12" deep. You can cut the cloth open in some areas to put in a larger addition, but be sure to remove the soil from the hole and dispose of it rather than mixing it into the compost.
Your area will not be hot enough long enough to use solarization for weed removal and the lack of a heat containment below soil during solarization makes its efficiency just terrible related to the work and time required to do it.