Zuzu......I would guess that the iris have used up the fertility in the soil that they need. Over the years I have rejuvenated several beds with good success. In two beds I dug the iris during the summer and then added lots of organic material like leaves, compost, alfalfa pellets, well aged manure and also 10-10-10 fertilizer and tilled it in. I left the beds empty til the next year. In one I planted seedlings that I had germinated during the winter. They grew like gang busters and nearly 100% bloomed the next spring. In another bed the city had to dig up a few roses to put in a new water line so I worked up the soil and planted some small leftover rhizomes. Only a couple bloomed but the rest are huge with several increase and should be top quality bloom next year. One more comment. I often see it suggested that older iris are more vigorous and grow better than the newer ones. I don't think as a general matter this is true. The older iris with the good reputations were introduced the same year as many iris that are long forgotten....survival of the fittest. I think that in 20 years some recent intros will have proved their vigor and will be widely grown and appreciated and others which didn't grow well will be forgotten.
