I recently read in a well-respected garden magazine that buddlia could be cut back in late fall or spring. Everything I've heard prior to this stated to cut back buddlia in spring.
In our zone, it is preferable to wait until spring, as this helps increase hardiness. At least, that has been my experience. :)
I have never gotten one to come back, but I have one in a pot this year I am going to bring into the garage to see what happens.
I was surprise that the two I have did come back. They are one of the latest plants to show life in spring, but once they got going, they really took off. The Green Bay Botanical gardens has a huge one - I wonder what they do to protect it over winter.
I wonder why mine came back and not yours. Strange.
I had mine in the very backyard with a pine tree on one side and the rest all open. It may have been too cold for them. A friend of mine has one that is close to the house and is huge!
Try this link and ask them how they winter there Buddelia
Here is a link that might be useful: GreenBay Botanical Gardens
Greetings------ This is the response I got from Mark Konlock Director of Horticulture at the Green Bay Botanical Gardens. To overwinter, last year we mulched with wood chips by mounding chips 12-15" high around all of the stems . Make sure to get chips between all the stems . In the spring we uncovered and cut back to outward facing buds about 6" above the soil line . the results are visible in the Color Foliage Garden .I hope this helps----Upnortdareh
Thanks for calling and asking the question! :-)