They should be transplanted in spring.

I transplant almost all our perennials in fall. That includes our fall asters, which are very largely smooth, calico and especially New England asters.

The major reasons are (1) to not disturb (including walk on) parts of the garden in spring (bulbs, then primulas, etc. are flowering) and (2) it can be very difficult to identify/locate individual summer and fall blooming perennial cultivars in the ground in spring, especially with close plantings.

Fall transplanting works here.

Another vote for spring planting. Asters never really have time to get a good root system re-established in an English autumn whereas they grow away in spring uninterrupted all summer to blloom well in September/October. As a general rule of thumb, spring planting works best for the late bloomers (any time after summer solstice) while early summer flowers such as daylilies, pulsatillas and delphs always do well for me with autumn division or transplanting.

The rule of thumb is transplant fall bloomers in the spring, and spring bloomers in the fall.

I have done the opposite (making sure to keep watered in) many times, with great success.

If you decide to move, make sure to cut it back by half. This makes it easier for roots to get reastablished.

Evidence for fall:

(Picture: Sept 29, 2013): both the 'Alma Pötschke' and the 'Bluebird' were divided and divisions replanted during October 2012.

As Campanula indicates location, climate, etc. are important factors.

Compared with southern England (where I once lived), winter is more severe here (southern Ontario) and spring comes later. We go relatively quickly into a hot, often humid summer, which can be hard on plants if attention is not paid to watering requirements. Watering in the evening avoids loss to evaporation in the heat of the day. Our falls are typically long, mild and wet.

Are you "Sunny Borders" of New Britain CT fame?

I didn't even know of it Emerogork2.

On-line: looks like a pretty impressive place:
3,000 different perennial species and hybrids for sale!

Oh WOW!, look at that 'Alma Potschke' what a stunner. I lost mine a few years back along with a lot of the others I imported from Old Court Nurseries in the UK. I still have Davy's True Blue, Blue Eyes, Anita Ballard and a couple of others, I'd love to replace some of the ones I lost, not having much luck tho :(. Anyone know of a Canadian mail order source for Michaelmas Daisies?
I have divided and replanted in the fall but I've found with the New England asters it's better to wait until spring here.

Annette

I am amending my suggestion here since transplanting, rather than division, can be done (carefully) at any time. Mucking about with pruning saws and trenching spades is a different proposition to merely relocating an entire plant......although even this would be far more than the amellus types can bear - spring is the ONLY time for dealing with either amellus or frikartii types.

I love 'Alma Pötschke', AM, not least because I take the colour to be magenta and I don't take kindly to people trying to ban particular flower colours from other peoples gardens (as per Gertrude Jekyll).

Interesting comments, Campanula.

"When do I transplant Asters?"

Perhaps the first answer should be, as you indicate, "Which asters"?

There's evidently more latitude with some than with others.

I have used both New England asters and Frikart's asters for years. The former is extremely hardy here and the latter (at least in my experience) is definitely not. Individual Frikart's aster plants may last five or more years, but the large majority I've planted lasted a notably shorter period of time.

Wild New England aster is extremely widespread in North America (east of the Rockies) and it has also naturalized in Europe. On the other hand, Frikart's aster is a hybrid between Italian aster (Aster amellus) and Thomson's aster, both plants from mountainous regions of Europe and western Asia