Hostas, lilies and bearded irises are often very hardy. You probably won't have many hardiness problems with these, unless you happen to have some less hardy, unusual kinds of lilies or irises.

OHhhhhhhhhhhhhhh...can I move out there with you?? I love Montana!! I used to live in Billings for a short while, back in '89 - and I remember that winter was pretty harsh, especially with the wind chill factor. I'd say your plants would survive the winter if you set them a little deeper than normal, and heap a lot of mulch on top of them when DEC rolls around.

Well gee , your probably already here,, and now realize that all three of your picks do fine here,, I am not in Helena,, but further north,Montana is a little differnt the climates can change with in 10 mile in any direction,, however if you stick to plants rated for zones 4 to 5 you are fine,,luckily the snow cover adds alot of protections from the winter cold,, working as insulation,, mulch alos helps on the more tender plants,, the three you have listed grow well in my garden with out winter mulches,,
hope your enjoying the state,,
jwj

forgot to add, now that your here drop by the rocky mountains forum,, hmmmm

I just stumbled upon your post, and wish I'd read it earlier. I live in Helena, am a landscape designer here and will tell you irises, hosta and lilies are almost definitely hardy here. I've had great luck with all three. You don't have to treat them any different than other hardy perennials. Whereabouts in Helena are you living? Weather conditions in the Helena valley are very different from those on the sides of our foothills (in town or just on the edge of town) and it's different if you're living in one of the outlying communities like Montana City or Clancy. We go from a solid Zone 3 to a zone 5 in certain sheltered microclimates. You can email me directly is you want to. Glad you're here!
Maureen