Do you have a photo? My astilbe have never had daisy like blooms so I'm wondering if the false dandelion weed has grown near the plant.

Here is a link that might be useful: false dandelion

Well, unfortunately I finally just cut them out and destroyed them, Corrine. They were really weird. The Astilbe flowers were about 18" high on very normal-like stems. These other things were about 2.5' high, on very large stalks, with the yellow flowers at the tops. While the photo of the False Dandelion looks similar, nothing else about that plant resemebles what I've had.

Astilbe are tough plants! In my mind the charm of Astilbe is it's long lasting presence even when the blooms fade; however, the foliage is still a lovely accent to large leaved shade plants.

I've read that for good blooming Astilbe like composted manure in the fall. Alfalfa pellets also works!

Sounds like a weed seeded itslf in the crown of your astilbe and grew up in it. That happens, sometimes you have to dig up the plant and separate the weed out of the center of it. Don't give up on astilbe, they are wonderful plants!