I was thinking the same thing that Poly said in the Iris forum; why fight nature on an issue like this? If you are doing some type of research, that's one thing, but if you just want a shorter iris, get a shorter iris.

Bonzi is not labeled for outdoor landscape use. I've used it for various greenhouse crops, but iris is not one of them. I'd never use it on installed plants.

are you producing iris for sale commercially, or you planning to treat them in the landscape? low light and crowded conditions may result in plants that are stretched, or taller than is desired. under these conditions, they might respond to growth regulators. pgr's will only affect the new growth that is put on, not any leaves that are all ready too tall.

in commercial production, b-9 is labeled to be used with cycocel; combinations of the two are often more effective than either one alone. easter lilies and many bulbous crops respond to a-rest. check these labels for labeled uses and strengths. grower product news prints research on pgr's on various crops and might be a resource. gpn.com i think.

Many growth retardants work by interfering with gibberellic acid and/or brassinosteroid synthesis or response. Arrest (ancymidol) is only a gibberellic acid antagonist. If a plant doesn't respond to externally applied gibberellic acid, it won't respond to Arrest e.g., Alocasia macrorrhizos. Bonzi (paciobutrazol) and Sumagic (Uniconazole-P) are reputed to be both gibberellic acid and brassinosteroid antagonists. I have experience with Bonzi and I never met a plant that it couldn't shorten. Sumagic is reputed to be strong enough to shorten lawn grass but I never tried it.

Anyway, these typically shorten stems and petioles much more than leaves. Don't irises already have short stems and petioles?

These are not very effective when applied as a foliar spray. They practically have to be applied as a soil drench in pots.