This is my sixth summer with my garden, which I inherited from the original owner. There is a a clump of siberian iris in a sunny corner of the garden, that is the most stingy bloomer. I've never gotten more than 2 or 3 flowers from the clump. It died out in the middle, and I split the original clump into three plants. That was years ago. Now I have four nice clumps, all about the same size, in full sun for at least 6 hours a day, and they all behave the same, just a few blooms.

My soil is great (amended clay, slightly alkaline) and I use a ton of compost.

I just read they prefer alkaline. Is there anything I can do. When I see photos in the gallery of your siberian irises I realize I've missed the boat!