Just talking about this with one of the Detroit Zoo volunteer gardeners:
Here is a picture of the butterfly bush that we rescued from the zoo on our last zoo day back in 2014 -- when we were moving everything out of the way for the new Wolf exhibit. I couldn't believe it when it overwintered last year, and the little stinker is already growing this year.
I know you said last week not to worry about covering the plants now that it's gotten cold again, but the low temperatures around my house are dropping into the lower 20's so I finally broke down and covered a few of my plants last night.
I haven't been gardening that long, but I do have to say that this is the first time I have had to protect plants from a frost during the winter. Ok spring starts tomorrow, but it's still technically winter, right?
- G -
Here's what I sent back to G. I hope it prompts some tales of what butterfly bushes have done elsewhere and elsewhen (and just to put into perspective the invasive-plant warnings this may generate: in our region Buddleia is not generally invasive. We do see seedlings but not the plague seen in drier, longer-growing-season zones).
You're so right. It was still technically winter when you emailed. And in practical terms it may be winter yet again hereabouts, a few more days here and there!
I think your plants would have been fine. Although many started to grow in the warmth a while back, that growth arrested. And it rained, so there was moisture in the soil, so it is more of a heat sink than before. The plants didn't extend themselves out of the safety zone. They are close enough to the ground that the ground protects them. So try not to worry in what's to come!
Butterfly bushes, now. Buddleia davidii. They are tough. They grow in every break in the weather. Even in January I've seen them start to grow like this, like it's spring, only to get that growth killed back. They can put up with a ton of kill-backs, so long as they are where the soil's well drained. (So many times we've told people who said "I cut it and I killed it" that you cannot kill a buddleia by cutting it. If you cut it in spring and it did not grow back, it was dying or dead before you touched it, of poor drainage during winter. They cannot handle slow drainage while they're dormant.) Since yours made it through one winter in its current place already, it must be decently drained there.
If you believe in psychologically protecting plants (like me talking to things as I transplant them, encouraging them with descriptions of how good they're going to have it, and assurances that I'll let them stay put in that new place, etc.), here's another tactic. Tell it how lucky it is to be in your garden. Tell it if it was in my garden, very shortly now when I could get out to work there it would be faced with me saying to it, "Nice new leaves! But it's time for your annual cut, anyway, little bush." I'd say that then I'd whack it back to an inch high. Tell that to your plant and it might reach 12' in gratitude...