I have used pre-emergent without problem. Wind brings a new crop of seeds regularly so back to my knees.

The mulch problem is more serious. If it's covering any rhizomes when rain arrives ~~~ hello rot.

Rhizomes like their backs to the air.

I multched mine last winter and lost about half of them to rot. I was able to kill the rot off the other half with bleach.If your going to multch make sure you get it off really early. I waited unti I saw sighns of growth and that was too late.

Orchard Supply Hardware (OSH) has a bag of pre-emergant called Amaze that works wonders, there is also a spray called Surflan that you can get at a western farm supply but you might need a permit, it to works great at keeping the weeds out. Each needs to be watered in and will work for 3 to 6 months, then reapply.

Thanks, all. I figured mulch wasn't a good idea. I've never mulched my iris beds and am careful to keep the rhizomes' backs to the sun. I killed all the Creeping Jenny today (I hope) by spraying the whole bed with RoundUp. (No iris have been returned to the beds yet and for this zone, I hope I have time before it freezes - the RoundUp jug said it takes a week to two weeks to completely kill the weeds). I will look for Amaze. Can't find OSH stores in Illinois so have to either buy on the web or look elsewhere. Thanks again to everyone.

Westy

Last year when I planted my new iris bed, I put the pre-emergent Preen on it after planting the iris.

Now I am a big believer in weeding as much as possible before you plant. The combination of very careful weeding, and the Preen along with very. very occasional spot pulling up any weed or grass sprig (very rare) that cropped up as soon as it cropped up...kept the bed weed free almost totally, for a year! (No mulch at all, of course)

I have now put Preen down on that bed again.
kay

Pre-emergents work on seeds. Existing weeds will not be affected. I have heard of success with Preen, and I believe the active ingredient in that is corn gluten meal, which I'm thinking that we should be able to get at grain elevators/feed stores pretty readily here in farm country... (I just haven't gotten around to it yet.)

I have also found that a nice, thick stand of Canada thistle will suppress most weeds. ;-) (Bane of my existence, that stuff! And field bindweed...)

I do mulch some, but not over the rhizomes - just around the clumps, preferably with a light mulch like wood chips or pine needles. I've used a thin layer of grass clippings successfully, too. I'd hesitate to mulch with leaves, which will mat down and hold a ton of moisture, and unless you shred them, blow around and end up collecting in the center of iris clumps.

I dig the weeds, use weed killer on any you can't get to pull, broadcast corn gluten meal, found at feed stores, natural equivalent to preen but much more affordable at $25 for 40 lb bag. I use dried pine needles for mulch around the clumps but not over the rhizomes. Key to bloom is to plant shallow, 1/3 rhizome exposed with no covering. Supplement water only in very dry season.

When you hand dig out the weeds it's a good time to scratch in some bulb booster high phosporus or alfalfa pellets