My SIBS seem to have diminished in blooms and growth. Could it be that they are too crowded? Should they be divided every few years?

Not as often as TB's but yes, they need to be divided.

Best to do in the spring or in the fall?

Spring or fall is fine. You usually notice some dying out int the middle of the clump when it's time to divide, but for your backs sake I recommend every three years at most.

Thanks; I know its been much more than 3 years, my poor back...

yes, after 3 years they are tough to work with.

Siberian iris are usually dug in the fall in milder climates. One thing to remember if you are going to give the extras away is to keep the roots damp. If the roots dry out, the plant will die. The growers who mail them use the polymer crystals that absorb water to keep them wet while in the mail. If you are just giving them to friends you can wrap the roots in a wet paper towel and then put them in a plastic bag. Leave the leaves out of the bag. They can be put in the refrigerator for a week or two if you need the time to redig the bed. Don't trim the roots when you plant them.

thank you for the info Carl. I have gotten LA irises at our plant sale and swaps and the roots are always enclosed tightly in a little bag with water in it. I bring them home and put them in water until I can get them planted. They must like it as they bloom the next year.

The LA iris I have had shipped to me also had bags of water around the roots, then the plants were rolled in paper to keep from breaking, then put in the cardboard box.