I would be tempted to winter sow them in your area. My seed book says for Astilbe:
Germination: 42 to 56 days @ 18ºC (65ºF)
Do NOT like dry or alkaline soil, baking sun or high winds
Check put the Help Web Link to the right: Tom Clothier
It suggests very low temps if it doesn't germinate in within 4 weeks, so I think wintersowing would be the way to go
