Erigeron

Horticultural advice please.  A friend gave me a small clump of this pretty erigeron for our rockery and it is in danger of taking over.  Can anyone advise if we can safely split it and when we should do that?  Thank you.  Which comes first, the white or pink flowers?

We've had a quiet day.  We rose later than usual and had a leisurely breakfast before settling down to watch the Orienteering World Cup in Davros.  Unfortunately, the British team were well down the placings, but it was still quite exciting at times and good to watch the elite competitors move at speed through the Swiss alpine forest, most with apparent ease.