I have a butterfly bush that has been beautiful in the past. The leaves are all yellowing and falling off. It is in bloom but looks bad because it is almost leafless. We have had a lot of rain in E. Texas lately, I mean a lot. Not complaining about the rain just wondering if this could be the problem. It is fairly moist around the base of the plant and has been that way for some time now. Would appreciate words of knowledge.
My bet is too much water. Not much you can do about that. If you have mulch around the base of the plant, I'd pull that back, but aside from that you'll just have to weather the storm.
Mine are doing the same thing! Only mine are still really small I just planted them this year. They seem to have new leaves coming where the old ones are falling off but they are not really growing like I thought they would. Maybe next year they will take off, I'm just glad they are still alive.
If you have had a lot of rain, the roots could be waterlogged. Also, the rain could be washing all the nutrients out of the soil.