I divide mine in Spring and Summer. I have done it in the fall, with mixed success. They seem to take quite a while to re-establish themselves, and half the divisions made in fall didn't survive winter. You're in zone 5, and they aren't reliably hardy in your zone, so I'd do it early enough they have plenty of time to settle in before winter.

I've had the best luck dividing mine in the spring. I do most of my moving and dividing in the fall, but as with donn, I've had mixed results with shastas in the fall. I've got 'Becky' which is very hardy for me.
Brenda

Thank you - I will hold off until next spring to divide.

Deanna

I divide mine early and often.....

To be honest I have no experience other than doing it in the spring but I have never had a problem with this. I have a very vigorous culivar that needs it every year. I just gouge out the sides. Soon I will probably have just a shasta daisy gaden with all my splits.

The above poster mentioned dividing by "just gouging out the sides." I was just wondering about this...does it work well on the Shasta "Becky?" Or do I have to dig the entire plant out of the ground then divide?

D'Sabato's book The Well Maintained Perennial Garden warns not to divide them in the fall. Suggest not to even plant them in the fall. Surprised to see she reports they are not reliably hardy in zone 5. Division every 2-3yrs.

I divided mine in September, and gave several to my neighbor. They're getting new growth right now, hers are too. I planted them three years ago, and they needed to be split up, didn't seem to affect them at all.
I dug mine up and split them with a big bread knife.Planted some in the same area and moved others to other areas.