Depends on what your soil needs ;)

Assuming you mean bearded Iris...
recommendations include:
using gypsum if you have clay soil.
Lots of compost mixed into top 12"
Balanced fertilizer (ie 10-10-10) used in fall and early spring at the start of the natural growth cycles.
bonemeal mixed into planting hole.

Best fertilizer I use for iris is alfalfa pellets in the form of horse or rabbit feed. They are clean, economical,easy to use, safe for all pets, humans and wildlife and obviously 100% organic. I sprinkle them into the hole and mix with the soil when planting new rhizomes or around existing plants and lightly dig into the soil. :)

hey chad.. if i fert'd in fall .. in my z5.. there is a good chance i might rhiz rot plants that are in lush growth when the first frost hits ... plants need to be going dormant as ground freeze/winter comes.. not growing like crazy ....

be careful taking advice from peeps in zones dis-similar to your own ... though dont get me wrong.. i find the divergent methods fascinating ....

ken

Ken, so if not in the fall then, at planting time or 2 weeks afterwards ? It seems to me you want to encourage some root growth etc. before winter ?

In my experience fertilizer (not too excess, especially N) does not drive growth, it just assists it. Growth in Iris is more triggered by average temperature and moisture levels. Hot and dry = dormant.

In my zone, rot issues arise from water-logged soil which can be bad drainage or a suffocating mulch.... what is this frost you speak of :)

Chad

i suppose in your z9 ... a windbreaker is considered a winter jacket.. lol ...

never fear.. a LITTLE of this or that is most likely never going to be a problem ...

its the MORE IS BETTER attitude of too many peeps.. that if a tablespoon is good.. then 2 handfuls has to be that much better ... and frankly.. it never is ...

ken