Can I trim Astors?

When they are growing to the height you like, pinch the terminal apex (top) leaf pair and it will cause the lateral buds on the sides to sprout and grow bushy. You want to do this when the leaf pair first emerges.

I would trim them, when they reach about 2' or 3' at the most. Just see if this will help you. Best wishes, J.

I love aster and mums, but my mums always freeze out. I understand that it is best to prune them back through July to force them to fill out foliage, then let them concentrate on creating their blooms for fall.

http://homeguides.sfgate.com/cut-back-asters-81861.html

You can after a few feet high- best of luck :)