Mosswitch I have both plants but usually don't cut them back until parts are dead. Both my plants are fully leafed out, are yours dormant when you cut them back? How far do you cut them back? What media do you use to start Brugmansia? Dealing with big plants and whittle them down as winter progresses is what I did last year. It's less dignifies then your plan.

I have to cut them back to get them inside, and no, I don't wait until they are dormant as they don't really go dormant here since they are grown as a tropical potted plant.

I used to be afraid of cutting them back, but one day I was in Kansas city at Powell Gardens, and they had two huge ones that had just been brought in. They were whacked back to about 4' tall, and that had obviously been done in previous years. So I don't think twice about it any more. Same goes for tropical hibiscus. They will just leaf out again in no time. I root both the begonia and brugmansia cuttings in water then pot them up when they have good roots. I'm still waiting for my pink brugmansia arborescens to finish blooming, I can't bear to cut the flowers off! Hope the weather holds for another week!

Aren't brugmansias hardy in your area? Do I read that right, you are in CA?

So how tall is your brugmansia after whacking? And how tall is your begonia after whacking? Yes brugmansia are hardy here in the ground. Mine are in pots because tree roots are so greedy. Pots don't provide any resistance to cold. But what mostly gets plants is the wet cold soil. Our winter is usually just rain and plants can sit in cold wet soil for only so long. Last winter I lost one brugmansia that I didn't take in. I have a cool greenhouse that I drag plants into. Out of the wet on the brick floor I had impatiens come through the winter but outside impatiens go to jelly. Last year was the first year I had a greenhouse and there was lots of room. This summer I was very greedy and many plants have to go in. Whacking back is the only way it will all fit. Next year will be worse. My best and most favorite plant this summer was mandevilla Alice Dupont. DH says I have no limit when it comes to mandevilla. Will be Peggie at the trough next year!
I grew up in Missouri and my sister hangs out on this forum.

The best place to prune a brugmansia is above the Y. It will have to grow less next year before it starts producing flower. But since a brugmansia produces blooms on new wood it will have to grow a new Y before it can start blooming next year. Mine died to the potting soil last year and that may not have been all bad. I had 2 trunks last year and it had 4 trunks this year which looked pretty perfect. I took some tip cuttings. This is a variegated plant which I guess is even more likely to rot and greenwood is more difficult anyway. So I am using chopped sphagnum moss and Turface, equal parts as a starting media and putting the cuttings in a covered pot together. If that doesn't work starting in water works for a lot of people. I don't like the long transition to soil. I think these plants might be water hogs. Last year this plant was in a different garden with an eastern exposure and was misted each morning. I had lots of leaves and if a worm ate a hole I just pruned off the leaf and a small leaf grew big to cover the spot in a few days. This summer it got plenty of water but no misting and the trunk did not re-grow it's leaves. It's a beautiful plant either way. As a multi branched tree there is room for bellowing Diamond Frost euphorbia and calibrachoa.
Begonias are so easy to start I think I will just start over next year with the cuttings I took.