Iris dichotoma longevity

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I've heard that Iris dichotoma has a reputation for being short-lived.
However a plant I grew from 1998 seed is still alive. It blooms in
August, sets seed, then dies back. However, I've noticed new growth at
the base after the die-back. I'm wondering if the fact that I live in
zone 9 gives it the time it needs to prepare for winter. The new growth
does not get very big before it goes dormant. Does anyone know about this?

Ken