Here's an old thread with information about pre-emergent weed killer.

http://forums.gardenweb.com/discussions/2007712/preventing-weeds

Sorry I don't know of safe herbicides for existing weeds. RoundUp stirs up a lot of controversy but every few years I do paint the bindweed with it. When the hand weeding is losing the battle. If you see a picture of a distorted iris bloom RoundUp is the usual suspect. Even a light drift from a neighbor may affect iris.

Herbicides are typically divided into 3 types - broad spectrum herbicides that can kill/damage any plant (like RoundUp, vinegar, etc.); those that target broadleaf weeds (usually used for lawns) and those that target grasses and other monocots (like iris, lilies, liriope, etc). None of these is going to get rid of the weeds completely without damaging the iris. The broadleaf herbicide shouldn't harm the iris, but it will have no effect on the weedy grasses in the bed.

A fourth type are the pre-emergents. These will not harm exisiting plants but will prevent weed seed germination or seedling development. So you need to hand weed or otherwise remove all existing weeds before these become effective.

Sorry not to have an easy answer for you but you may be faced with doing at least some hand weeding. Once you get the weeds removed, do try a pre-emergent (Preen is one brand name) and then mulch as well as you can between the rhizomes. Mulch is the best "natural" weed control :-)

So Preen is safe to use around iris? I knew I would have to hand weed my bed was really looking for something to keep the weeds from returning.

Yes, most pre-emergent herbicides (with the exception of Casoron) are safe to use around any established plant.

It is probably too late this year, but I get very aggressive with Roundup in early spring. I often use it almost like a hoe. I only do it on a quiet (no wind) morning with the sprayer wand close to the ground. I can get well within 6 inches of plants with little chance of damage. Of course, that is when weed pulling is easier too.