did you actually compost the horse poo or just let it dry out and spread it? Uncomposted or dried poo from farm animals tends to bring in tons of weed seeds, sheep tends to be one of the most loaded that I've ran across! The poo really helps make wonderful compost, but if not hot composted, watch the weeds take over. Don't look at this as you making a mistake, look at the endever as learning by trial and error! I doubt if spreading grass clipping has anything to do with this--that should infact help surpress some of the weed seedlings comming. I mulch with grass clippings yearly and my major "weed" issue is maple seedlings popping up everywhere, but they just end up as greens in the compost.

When you cut the grass were there seeds growing on that grass? Could those other "weed" seeds have gotten in on the wind, by birds, attached to wild animal fur? Was that manure fully composted, ie was the ratio of manure to vegetative waste 1:3?

champagne, I mulch with grass all the time and have never had any sprout - but - I bet you have a different kind of grass than I do!! There are, of course, lots of kinds of lawn grasses. I think I've read (here on this forum, probably, some years back) about kinds of grass that are able to root from cuttings. I don't know which those are ... but ... if grass is sprouting everywhere, it sounds like maybe you have a sort that can do that.

It sounds very frustrating, going from nearly-weed-free to this. I think once you pull all the grass you'll be done with it (if indeed it was sprouting from cuttings). The field weeds from the seeds in the horse manure will be more challenging - though, since I imagine they're near the top of the bed, it's likely that they'll germinate fairly quickly (giving you the most weeds this year, and fewer next, and so on).

Champagne - What you did was spread 'sprigs" of that grass. Was it Bermuda?

Mulching with a grass that sends out runners that root has one extra step: spread the grass out on the patio or dirt and let it get dry ... really dry.

Or compost it with layers of dry leaves and shredded paper and use it next year.

Pull those sprigs and weed when you see them and the problem will end. It's a good use of a "hula hoe" (those D-shaped hoes for scraping out weeds.

Thanks for the input everyone. Seems I did do quite a bit wrong -- not fully composting the manure, using "green" clippings, etc. I think after this experience (I tend to learn best at the school of hard knocks) I will stick to mulching just with totally finished compost from the bin and/or supplementing with purchased compost. And in the meantime, pull and hoe the multitude of sprouts for a while!

Yeah if you have Bermuda grass I'd be leery as using it's clippings without letting them dry out first. If you have Bermuda grass it should be pretty obvious though; because it invades everything it's near and is hard to kill short of liberal use of roundup or thoroughly solarizing in the middle of summer. I mulch with Zoysia clipping and have never had a problem. It is similar to, but not quite as aggressive as bermuda. Honestly your weeds probably came from the horse manure. A horse stomach isn't as efficient as a cows and most of the seeds just pass on through. If the manure isn't thoroughly hot composted and aged weeds will be a problem. My solution to weeds popping up through the mulch is to just smother them with more grass clippings.

Unless grass is cut really low and bits of roots, stolens, rhizomes, etc. are also picked up with the grass clippings the only way you would get grass seeded in a mulched bed is if there were seeds in those clippings. While there are many plants that will stem root I'v not heard of any grass species that will, yet.

kimmsr ... you are in MICHIGAN. You have never encountered Bermuda grass. It has no trouble growing from the bits of stolons that end up in the clippings even during normal mowing.

Bermuda grass is not the only type of grass that can create these sorts of "weed" problems, although it does have a reputation for being one of the worst. A great many other common turf grass species can display this trait. Turf grasses come in three different growth habits -- clumpers, rhizomatous and stoloniferous -- and most common lawn blends, both warm and cool season grasses, combine a variety of both. Bentgrass, Kentucky blue and creeping ryegrass -- all of which are common to various cool season grass mixtures -- are either rhizomatous or stoloniferous and can create rooting, weedy problems when applied as a mulch to a planting bed. There are a number of other various grass species as well that will create similar issues.

Bermuda and Zoisya grass are well known for this problem as well and they tend to be used most often in warmer climates (warm season grasses) and often as a single species rather than as part of a blend of various turf grasses.

Canary reed grass also roots from grass clippings that happen to have stem segments.
But I'm betting the case above was from the turf being in seed. Don't give up on mulching. Your garden is better off than before and well worth learning these lessons ealier than later.

Bermuda well deserves being called DEVIL GRASS! But for other mulching materials, hope you will consider tree trimmings/chipping from an arborist/trimmer. I swear by them, and find them very attractive in a mulched flower bed.