I have an old Black Knight that is sited in my front garden near the driveway and street growing under pretty challenging conditions - full sun, dry sandy-ish soil, exposed to severe northwest winter winds. The thing is doing great! I am very pleased with how this particular cultivar performs as we are close to the borderline of hardiness for Buddleia in our zone and this is not a sheltered location at all. The Black Knight grows about 7 feet tall each season, which is tall enough as it is - can't imagine how large it would grow if it wasn't cut back. I cut it down sometime in April when the new shoots appear from the old wood. Just above the green buds so it's about 12-16" high. I also thin out some of the weaker or dead stems and maybe even remove some of the shoots. Btw, I also have a Pink Delight which is not as vigorous so I don't usually cut that one down as low as the Black Knight. Here's what it looks like this Spring after it got its pruning -

There's some good info in the link below, says they are hardy to zone 10 and they like well draining, loamy soil. Maybe your soil was the problem on the one you lost. Good luck, I love this plant but don't have one - yet. Here is a link that might be useful: Butterfly Bush - Buddleia davidii

Jeanne, I would be concerned with pruning in December in your growing zone. You all down there in Houston stay much warmer for much later in the season than we normally do here in NC. I would also be concerned about doing this early pruning in other warmer zones. In those zones where it gets cold and stays that way, then early pruning shouldn't be an issue since the plant is dormant and doing so won't stimulate growth. This year I will play it by ear since the weather has been so weird. If it gets cold early enough and seems to be staying that way, then I will prune in late December again. If the weather is vacillating and can't make up its mind, I might hold off until mid January. In my zone, I am more concerned about coercing the clematis to come up too early by pruning in late winter/early spring. This past year, we had an unusual warm spell that put a lot of things into growth mode. This was followed by an extreme cold snap with low temps around Easter that bit everything back. If I had pruned in late winter/early spring when it warmed up, I would have put the clematis in major growth mode and they would have been really zapped by the temperature turnaround.

Sorry, I never found Asclepias rubra. If only! But I have located a seller of Asclepias quadrifolia in New York state, and have exchanged emails with the nursery owner. So, that is to say, I have given up on rubra and have plans for quadrifolia, but in the future I will definitely be able to trade seeds fro quadrifolia. What milkweed species do you grow currently? And, I am not sure if this is too specific a question, but do you know anything about rare asclepias syriaca varients (such as white common milkweed)? I am very interested in them and can trade or pay.