I have planted lots of acanthus in varying degrees of filtered shade, but none of them are thriving. I dug up three that were robust when I bought them a few years ago and moved them to a planter. So far they don't seem to like the move. I've routinely watered some, while neglecting others, yet they all look the same - exhausted and puny. Are our summers too hot for them to thrive? Ditto for calla lilies.
Donna, I have had the same experience with my acanthus and have wondered the same as you. I had one "Summer Beauty" which did fine for 2 seasons; well, it bloomed and grew a bit, but this year it's very puny and pale. The other two that I have are supposedly a "southern" variety, though I can't recall the name. They have struggled for 3 years. I have one in full shade, and the other two in part shade. All the same.
I've had better luck with Callas, though some varieties have done better than others. The ones doing best are in a very wet, boggy spot in soil with lots of clay. They get high shade in the morning, a little direct sun at mid day and heavy shade in the afternoon.
I didn't know there was a southern variety of acanthus, although your experience sounds like it's no better. I think they might be happier in the pacific NW. I planted my calla lilies 4 years ago in sandy soil and bright filtered shade. They would emerge in late feb and I'd get a few small blooms, and like my hostas, they would disappear in July. I moved them to a large planter where they get a few hours of full morning sun followed by deep shade. I'm keeping them well watered, but they don't look happy, and I've gotten no blooms. I will continue to move them until I find their preferred site .