I have struggled with spider mites on all kinds of plants over the 7 years I've been gardening in Kansas---forsythia, butterfly bushes, marigolds, dwarf blue spruce, and more. This summer when we came back approximately a monthy ago from a week's trip, I discovered that both of my butterfly weed plants that I had planted just last summer were looking distressed. Then within a few days, they looked so bad that I took a closer look and discovered that they were infested with spider mites. They were in bad shape by then and I was afraid I might loose them. However, I sprayed them with a product I bought last year and found very effective and they have made a marvelous recovery now with lots of new blooms. The product is Bayer 3-in-1 Insect, Disease & Mite Control---for use on roses, flowers & shrubs.
I am not positive if the product is still available---I had to buy it at a garden center in a larger city last year. However, it does work!
Glad to hear your plants made a good recovery! Our rose has some new blooms on it now, but still looks bare. Hopefully it will fully recover by next season =) We had a strong downpour of rain yesterday and a little more today. The rain will help too.