Too much water or too little nitrogen can cause yellowing leaves.

Here is a link that might be useful: My butterfly gardens

Thanks for the info. At first, I thought I was overwatering mine (causing yellowing leaves and brown leaf tips) but now I'm thinking it's more a case of they don't like to dry out completely at all...

I lost 2 butterfly bushes this summer that were new this year.(I currently have 10 healthy bushes around in differnt locations) I think mine were stressed from our heat & then I overwatered them & "drowned" them. I think they prefer good drainage & don't like "wet feet"! Did you amend your soil where you planted it? Add rooting stimulator to help it get established?
I also tried several times to grow one cultivar 'Blue Chip' in containers, in different locations & for some reason...never could get it to thrive like the other bushes have. Luckily I had purchased them from a local garden center that has a 1 Year warranty on their plants. I had wanted a small cultivar & now have 2 of the 'Flutterby Series' that I've just recently planted. Hopefully, they will do better!
Good luck,
Lila