Sorry, not a good idea for plants to cover/smother the rhizomes. I plant in front and in back. Tall bloomers behind attact the eye.

If I remember Sweet Woodruff, it is an assertive grower. And had a "moist" leaf--- as opposed to something like yarrow.

iris_gal always knows...better listen
kay

I don't let the sweet woodruff get near the irises, since it wants to smother the rhizomes. If you have the irises in clumps, you can grow things right up to the clumps. That's what I do- I keep the daisies, sweet woodruff, and baby tears about three inches away from the clumps.
Renee

I have scotch moss (it's neither) growing up to my irises. It stops dead when it reaches another plant, no smothering. In fact, it won't get any closer than 2 inches to my pumila iris.

I have sweet woodruff (galium odorata, not the native)elsewhere, and I find that, although it grows through stuff, it isn't smothering anything. It will spread far and fast, though. Milage seems to vary widely on sweet woodruff, and it doesn't help that there are two different plants sold under that name.

I've taken everyone's suggestion and not planted a ground cover around the iris. I'm keeping the iris bed weeded and mulch free.

I do wonder a bit if the notion of being so careful with iris about keeping the rhizome on top and not covering it, not planting groundcover over it, no mulch, etc. is an old-wives tale that people haven't bothered to test in a while. Since asking the question, I've watched some front garden beds more closely in my travels. There is a house down the road from me that has bearded iris growing up though Bishop's Weed. Ton's of flowers poking up through a mass of green and white leaves so I assume the iris are doing well considering the lack of air movement, lack of sun, and moist conditions under it all. Also some perennial beds are so jammed with flowers with iris sticking up through, that it seems the same conditions prevail.

Just some thoughts. Thanks for everyone's help.

I would tend to agree that the often read phrase that rhizomes need to 'ripen' in the sun is hoo-haw...planting depth is more about drainage I think... I have had Iris growing with various groundcovers and my experience is that the Iris don't compete well with others, but it's not the death knell. As long as they are not shaded out it seems to be fine, and yes I have had sweet woodruff growing around the iris.
I have seen rot problems when lots of leaves cover the rhizomes through our wet winters (zone 9 no snow), so they do need to dry out occasionally, so mulch with caution.

My best success with groundcovers is using various summer growing oxalis - particularly the purple leafed ones which contrast nicely to the iris leaves.

Chad

If there is not good ventilation around your iris (due to other plants growing up close) you will be susceptible to fungus. It likely won't kill the iris but the leaves will get ugly brown spots on them and it is INCREDIBLY hard to get rid of. The fungus gets into the ground and any rain will splash up on them.
I've been fighting this in many of my flower beds and just hoping maybe I've gotten on top of it finally (after 4 years).

The only reason to avoid groundcovers over the iris rhizomes is to prevent rot from moisture. I plant very thickly in my beds, and I have had very little rot. But I don't let dense groundcovers like baby tears or sweet woodruff cover the irises- they trap too much moisture Nasturtiums can cause problems too in my garden. No problems yet with daisies or columbines- things like that.
Renee