From what I've read it's best to leave them until spring and then cut down the old growth to about 8-10 inches from the ground. They will regrow up to 5 ft. again by the middle of the summer. Hopefully someone with more experience will chime in but that's what I've done with mine the past few years and they're growing nicely without overtaking the bed I designed for them.

I've already cut mine back to 10"-12" & have for the past several years in the winter...they bloom from new wood. They are hardy to Zone 5.

I think it's preferrable in colder climes to wait until spring to trim them back. You may be OK with a fall pruning if it doesn't stimulate new growth. I do know for a fact that if you trim them in the spring and get a freeze afterwards, you can pretty much count them dead. I learned this from experience. So now I trim them late spring - like late April or early May.

Sandy

Thanks for all the advice. I'll wait till it warms up in April or May to trim them back.

Hello Islander, here in zone 5/6 we are on the borderline of hardiness for most Buddleia. I agree that in cold zones it's best to wait until Spring to cut it back. I cut mine down after they start to show new growth, which is usually sometime in April, cutting the stalks back to new green shoots between 8-16 inches high.

Unfortunately your plant that is in a pot is most likely toast, because the roots of the plants in pots are much more vulnerable to freezing ambient temperatures.

Here is what the 'Black Knight' looks like after I cut it back in late April. I also thin out some of the overcrowded shoots near the base of the plant.
{{gwi:194802}}

Looks like a really healthy plant, terrene. Mine are smaller, younger bushes. I hope they make it through the winter okay. Maybe the potted one will surprise me but I won't hold my breath.

Yes, that butterfly bush is mature as it has been there about 6 years. It gets about 7 feet tall each summer, and I think it remains attractive through most of the winter. It also acts as a nice wind screen in that location!

The one in the pot is not likely to make it in our zone. The rule of thumb I've read is that plants in pots have to be at least 2 zones hardier to survive the winter.

Hello islander, you could try planting that potted plant in the ground and hope it makes it.

BB is super easy to grow from seed via winter sowing. I grew them last year (my first to WS) and one grew large enough to set a couple of flower spikes its first year. If your potted BB doesn't make it, replacing it shouldn't cost much if you can grow another from seed. I harvested some seeds from my plants and could send you enough to grow a few new ones.

TY gardenweed for the offer but I'm going to decline. I planted about 8 bushes this summer. I waited mostly till they went on sale. I would be interested in knowing how you harvest and propagate the seed from them though? I know(?) you're supposed to dead head them to keep them blooming. Do you just dry the spent spikes? What about pollination? Sounds interesting.

Thank you group for all the helpful responses.

Joan :)

Hah!! I didn't know about dead-heading them until this past summer when my neighbor's mother walked through my gardens with me and started pinching. So after that, I pinched until it got late in the season. I left the last of the flowers on the bushes to set seed. It's a long wait before they ripen. When the tiny pods begin to look dry, I cut the seedheads off and shake them inside a paper bag to get the seeds, which are as tiny as dust motes. I keep the seeds in the open bag until it's time to winter sow them. Germination rate is very high. They're butterfly magnets--pollination is not an issue.

Wow, I didn't know it was that easy!! I felt pity on my potted unplanted orphan and brought him in some where warmer. When I deadheaded the few spikes left a lot of "seeds"(?) fell off of them. I'll see if they germinate. It must take forever for them to reach full size.
I have a whole nursery of amaryllis bulbs now sprouting on my front window sill but I still have a little room. Very interesting! Thanks for the info!

I'm in zone 6b, PA, and I wait until I see the first new growth in Spring. Then I cut just above a set of new, healthy green leaves that are about 6 to 8 inches from the ground. It seems harsh, but they respond quickly by sending out fast-growing, new growth.
Kim

Here is a link that might be useful: My Garden

islander_mi - they grow so fast from seeds it won't "take forever for them to reach full size." The ones I grew from seed grew over 2 ft. tall last year. I expect they'll get up to 4 ft. of new growth on them this time.

I can't vouch for how they will germinate when grown indoors. I sowed the seeds in recycled milk jugs & set the jugs outside in December. The seeds sprouted April 4 according to my sprout chart. If you need more information, do a search on the Winter Sowing forum.

Well the frozen potted bush I brought inside has come to life. I see zillions of little baby seedlings in the pot, and leaves sprouting from the butterfly bush. It's alive...!!! :)