You know, I don't think there are any hard and fast rules. I usually dig the clump up in early spring and divide it in to sections that show several new leaves emerging (say, at least 4 inches across each, but larger is better) and then replant in good soil and keep well watered.

I've done it in mid-fall too with no ill effects that I can see. Either way I'd wait until the rainy season, but astilbes tolerate division really well so I'm sure your plant will be fine. Let us know when you do it and how it goes.

Take care,
Grant

Thanks so much...... I'll try as soon as the weather gets cooler..... it is still very hot and dry here.

You're very welcome. Once it's rainy you and your astilbes should be fine. They really are lovely in bloom, aren't they? Good luck!