I just got these two plants in May and I've had zero experience with butterfly bushes in the past. I don't know what is normal as far as how long it takes them to get established, etc. I am having a problem with the leaves turning yellow and developing brown spots. This is happening to the more mature leaves, closer to the bottom of the plants. I am attaching 4 pics... one of the problem leaves and one of the new growth (which looks good) for each plant. Am I possibly over or under watering? Help I don't want them to perish!

Image link: