Thinning will help them a lot! full sun would be better. Throw away the old center rhizome and re plant the Rhizomes that were attached to it. I moved all of mine out into full sun and they rewarded me with a spectacular show last summer.
I add bone meal when replanting but no other chemicals. Let dry between watering. They rot if to much water is used. Hope this helps . I have attached a link to the Flowers I got last summer.

Here is a link that might be useful: Gallery

When did you thin them, and how did you thin? Iris really benefit by being dug up & divided, as aqua said, tossing the old mother rhizome & spreading them out. It can really be a chore when they are that overcrowded. It takes a while sometimes for the iris to recover when they have been as overgrown as yours. Iris do need plenty of sun to do their best, but I've seen some wonderful plantings in less sunny spots.
I know someone locally who fertilizes with 10-10-10, & I've heard super phosphate does well. I don't use anything on mine, & most bloom quite well. I do add some compost to the soil when I divide them. The few that don't bloom as well, I just blame on being a less vigorous variety :)

How do I tell which is the center rhizome? They seem all connected, like a web of interconnected um poos to be blunt. Basically I dug up the ones that were on top of others and separated a chunk of rhizome that had a sprout of leaves and moved those to other spots. The rhizomes are ginormous, they look like the largest hunks of ginger seen in my grocery store. I did the separating last fall, so maybe this year I'll get better blooms from those still behind. I took a pic but can't figure out how to post it...

Here are a couple of sites that show how to divide iris

http://gardening.about.com/od/perennials/ss/DividingIrid.htm

http://www.irises.org/About_Irises/Cultural%20Information/Grow_Bearded.html

Here is a link that might be useful: dividing iris

what the above replies say ... IS THAT YOU HAVE TO DIVIDE THEM ...

do NOT skip that part.. and think fert will do magic ...

also.. when a plant .. besides having no room to grow.. produces a LOT of green and sacrifices flower.. that
MIGHT be an indication of excess nitrogen ... such as is contained in lawn fert ... any chance the lawn fert'g guy is somehow apply it to your iris at the same time ????

and upon dividing .. and relocating.. go for all sun ... as much as humanly possible ...

i can not say it enough.. YOU HAVE TO DIVIDE ...denial is not a river in CT.. lol ...

ken

HAHAHA lawn fertilizer guy? The gardening is done by ne, lawn cutting by my husband, thats the extent of the lawn services here....nope no fertilizer going on anywhere on my property, we live on a lake and have well water, so no chemicals are used around here. I dug up some and divided them and moved many to another area so that they weren't on top of one another, but sounds like i have to do it more aggressively. I had planned on putting daylillies in my front yard where i get full sun, but maybe I'll make it iris and daylilly garden. Thanks everyone for your help!

I was just passing through this site and want to confirm everyone else. I had a similar, but more obvious situation to yours. Our neglected iris had been growing for 20 years. Not surprisingly, they acted like yours. I divided them in the fall, leaving space between plants and also added bone meal. All plants, half shade and full sun, went gang busters the next year.

I have never done as serious a job of adding bone meal in later divisions and the results have been less dramatic. By the way, the older center stuff that others say to throw away, doesn't have leaves, so you have done what they suggest (you said you planted sections with leaves).

And, regarding photo posting: there is an entertaining current posting on this over in the Appliance Forum.

Here is a link that might be useful: How to post photos

When is the best time to apply the bonemeal, spring(now) or should I have done it in the fall. Should iris be mulched, or should the roots be exposed? It is hard work trying to stay ahead of the grass, so I thought about mulching with something. Any suggestions will be greatly appreciated.

Well, I worked the bone meal into the soil a little, and since iris transplants usually go in in the fall, that would be the time to do it. Again, not an iris expert, but the logic seems reasonable.

I would strongly advise against mulching - particularly in hot humid areas like South Carolina, you're just asking for rot to set in. Some folks in very cold winter areas mulch in the winter, but in general it's a very bad idea.

The roots should not be exposed, but in most areas it's a good idea to have the tops of the rhizomes exposed (the only exception would be very hot dry climates where planting with the rhizomes just under the soil is OK). In my experience, rhizomes will do whatever they want after you plant them - some will stay put, others will "dive" under the soil :-)