Butterfly bushes

I have ten or so butterfly bushes that I started from seed three years ago. They have been awesome the last two years and grow so much during the summer I have to trim one from blocking a fence gate. I trimmed them back pretty hard last fall, only left 6" or so above the soil, and this year some of them are getting new growth yet. Did I trim them at the right time? I could have sworn I read to do it in the fall but now I see spring when I google it. I live in zone 6 (central ky) and we have a cold winter for us, are they just being slow or could they have gotten killed? They were really hardy last season and never get watered. Appreciate the help in advance.