Mulching to control weeds; mulch thickness?

Greetings from Zone 5 in Iowa,
Am conducting a bit of an experiment wrt mulching and would like to know if I'm going about it wrong or what..
I am using grass clippings, layered on in about 2-3 inches at a time, and am now on the third layer. Here's the problem: weeds are still coming up through the mulch.
Am pretty sure - not 100% - that these weeds are NOT part of the grass mulch, since there aren't weeds going to seed in my yard. And the soil environment under the mulch is, of course, ideal for germinating plants of any kind. So how much mulch is needed to prevent weeds from coming up..?
I think the layers have compressed naturally to maybe 2 inches. No odors evident, the previous layers are a golden straw color. So far if the weed is big enough I just pull it, otherwise am putting more mulch over the weeds.
I haven't read the Ruth Stout books, but have heard somewhere she used TONS of mulch material to conquer (sp?) her weeds. That's a LOT of mulch, and sounds like more work than just regular weeding. And of course hoeing and mulching are mutually exclusive activities.
So anyone mulching have a recommendation of how much mulch is needed to actually control weeds? I've also heard that close planting will reduce weeds, but my cold frame is planted pretty tightly and I still have weeds coming through the crop "canopy". Starting to seem like any technique that promises weed control without hoeing is a bit of snake oil to me.. Still digging through weed control posts in this newsgroup, would like to hear from folks that are making it work for them.
Thanks!
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Personally, I think it's the type rather than the thickness of the mulch...
This is a bad time of year, most plants are boosted by soil and weather conditions.
No amount of organic mulch is going to stop the bindweeds. I just have to get off my lazy early morning tailfeathers and pull up the damn weeds and make my chickens happy by tossing the weeds into the henyard.
Either that or use landscaping fabric which has density and color to it's advantages.
There is a time and a place for organic control. Mid to late spring is not it. ;-)
Just my 2 cents!
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K.

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Sounds like it hasn't matted down much or started decomposing. Was it green when you put it down or already dry and fluffy?

It takes only two to three inches to prevent germination if you put them down over clean ground. But already growing weeds will easy push thru a LOT of mulch. Weight matters, as does matting down. If your clippings are fresh, less are needed.
To kill established grass/weeds, close mow, put down a a few sheets of newspaper or paperboard or cardboard, put the grass clippings on top, water well to matt things down. If you think the newspaper will blow in less than a gale, you don't have enough clippings.
If you don't want the paper, you will need huge piles of mulch and even then some will keep poking thru.

Works for me, and for my parents before me for many years. Now my wife is converted, but not yet her parents.
sdb
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Good to hear from you, thanks for the info. I have started using the newspaper layer technique in flower beds, and it does seem to do the trick. Will give it a shot in the edible garden as well then.
Shouldn't be a big deal to lay down between rows, etc.
FWIW the grass has been put down both green and semi-dried, and it has dried out on the beds as I mentioned. No probs with odor, just those pesky weeds :) I laid the mulch down over ungerminated (non-germinated?) soil, but of course it must be alive with weeds, and so they are pushing right on up. Will replace with that semi-impermable layer and see how it goes.
Thanks again!
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I use newspaper to mulch with..... lay down a thick layer of newspaper, then top with grass clippings, leaves, whatever. Nutsedge doesn't even grow thru a good layer. Eventually all will decompose and can be mixed into the soil. Nicole in sunny CA

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