Don't use it til it turns more brown than green if plants are already in the ground. You need browns to mix in if you start a pile/composter or it will stink and probably shrink down to nothing quickly. Fall leaves and paper with safe ink like newspaper shredded, any high carbon decayable organic matter. You need lots of browns and smaller amount of greens like grass clippings. Fall leaves are the best in my opinion, shred them first if you can.

Simply because you see "weeds" growing in your lawn does not mean that you will be transfering them to your garden if you pick up the clippings and use them for mulch. If those "weeds" have produce viable seeds you will be doing that but not even "weeds" are producing seeds every day so most of the time you can pick up those clippings, with the parts of the "weeds", with no problem.

The solution to mulches sprouting weeds is to apply more mulch. I use all kinds of "weedy" mulches---hay, horse manure, and grass particularly.

I love mulching and when I see a weed, I toss something on it.

To be nit-picky, you won't really be amending your soil, initially, with a mulch, unless you incorporate it into the soil at which point it isn't a mulch---it's a soil amendment.

Yeah I didn't make that real clear... It's a new garden are that already has grass in it, so in addition to putting compost, etc. in the soil when we till it we will be mulching to keep the weeds and grass down. Just wasn't sure if using grass as a mulch would be counterproductive.

Grass as mulch works great. Also, most people mow their lawns well before the grass goes to seed. So grass clippings can be pretty weed (seed) free.

If you do have weed seeds in your clippings, hot composting will kill a lot of them. You do need an optimal mix of grass clippings with browns, and turn the pile while it's hot to move the cooler outer layer to the inside so everything is cooked.

A lot of us use grass clippings as mulch though. As long as they aren't sprayed with herbicide, they work fine.

I use grass clipping mulches extensively. I have a mixed clover/grass lawn and the clippings serve as the main nitrogen source for the plants. It breaks down over the course of the season, the worms love it.

As others have said, if you put it on thick enough there are very few weeds. I try to put it on 4 - 6 inches thick. That will last the entire growing season.

The house I purchased has an evil bermuda grass lawn. I do not put grass in my compost nor use it as mulch for this reason, as a single rhizome (is that the right term? A "nubby" part... whatever it's called) can sprout a whole new bermuda plant in my garden. And I get enough bermuda intrusion as it is. But if you have a nice fescue lawn, then yea... what they said. ;)

I feel your pain JonCraig. Yes it is 'rhizome' and they are awful. My Bermuda patch started near the compost pile in the first place, so I think it actually came in with a bag of someone else's clippings. If I had known what it was I would have had a chance of nipping it in the bud, but it's too late now.