Weak stalks Cold damage?

Hi knowledgeable ones! I'm new to the forum and have really beem impressed with the posts. I need your help. I have several TBs that were putting up stalks, only to have the top six inches or so suddenly get weak, and flop over. We did have some low 30's a few days ago, but others have done fine, and some were blooming right through the cold. The ones in question didn't get very tall - about a foot - as if development was arrested. Lady Friend, an EE, and Stairway to Heaven, also very early, were the first, but now I have noticed it on three others. I have assumed it was the cold temp, but also wonder if the caster oil based vole repellant I put down when they were planted affected them. Saw no warning that it would. I would assume I didn't plant them early enough and they didn't get their roots established well, except that two cultivars were two years old. Any ideas?
Also - Immortality (a new one from the garden center) tried to open two bloom at the base of the plant! No stalk, just blooms! TIA any help.
Ragan - North Carolina Zone 7

Sounds like your stalks froze. A late frost got a couple of my iris too. Mostly discolored leaves but a few browned buds. Cut off the damaged portions, & wait to see what happens.

I wouldn't think the vole repellent would be an issue.

As for Immortality blooming at the base of the stalk--wierd!
I don't know what to make of that.

They were discussing a similar phenom on the bulb forum regarding hyacinth and daffs. Someone said that it was from the temperature. They bloom at a certain temp even if they haven't attained full height. The 30's then 70's then 40's then 80's will do that. Maybe it does it to irises as well.

LimeyLisa Kay