The rule of thumb is :

Spring bloomers divide in fall

Fall bloomers divide in spring

I personally have done it any time of the season with great success.

I divide spring bulbs when they droop. Otherwise I skewer them.

I have mercilessly divided hosta this late in the season and transplanted (like divided it into over 10 sections because it was huge). It was also a warm fall. Didn’t let them dry out and they survived wonderfully. Seems like we are some time away from a hard freeze so you could try it. What do you want to divide?

I have recently changed my dividing practices...and now do it when the plants are actually in full summer growth (thank you, Jimi Blake of Huntingbrooke gardens). This is absolutely failsafe - the plant wants to grow and thrive and is in peak condition...but, the downside requires removing all the flowering top growth so you will lose that seasons blooming...but the upside is guaranteed success at dividing perennials.

When you have the time and the ground isn't frozen.

Camp, that's a big bonus to be able to divide then. For us, it is guaranteed as long as the plants don't stress. For example, trying to divide in the SE USA during summer growth would be giving plants a death wish because of heat stress and water issues. Up here I can divide in summer (and I did--in July and August) but it requires quite a deal more work. Our dry summer this year meant I was doing far more watering than if I had divided in spring or fall, and it taxed both my time and our well, unfortunately. Had it been a summer with normal rain, dividing in summer would have been easier, but still more work than fall or spring for us. Although I have not been in this area for too many years, it seems our falls are long and gentle. Even safer than spring, which can be schizophrenic if there is a late frost, or weirdly fluctuating temps, or like this year with massive amounts of rain that just plain ended come the middle of June. Very wet spring, terribly dry summer. On the coast the ocean tempers our summer heat and delays the onset of frost. That means very friendly fall temps for dividing, as long as the plants don't dry out! This fall we have had periodic deep rains (thank you!), and are expecting another one tomorrow. This year has been an ideal fall for transplanting.

GLE, try to get some transplants in the ground before this rain starts tomorrow! I'd love to do the same. It's turning out to be such a friendly fall I'm thinking I can move some of the spring transplanting up to now. Thought I would run out of time to do it this fall, but maybe not!

I do it whenever the sun and moon tell me to. No joke...it's basically whenever the mood strikes me. I did as Campanula described, dividing my helenium when it was in full growth this past August...mainly because I could see how it was at its peak and I knew I had to move it to a better placement. And pretty instantly it stopped flowering - but within weeks it was sending up new shoots from the root base.

I have transplanted in every month of the year. Of course, the plantings in Dec., Jan., and Feb were unusual years in that the ground wasn't frozen. The same goes for planting in the heat of the summer. It takes extra care of the new plants. I planted 7 perennials in Dec one year and the only one that I lost was a Sweet William. I have transplanted huge garden phlox plants when they were in full bloom and they never even wilted.

Definitely, Deanna - Huntingbrooke is situated in damp and fertile Ireland - I can see a lot of cool weather plants go into a sort of temporary dormancy in the heat of a US summer. It would be a risky business doing summer divisions in the woods too...where I have no irrigation - even a spring planting is too chancy with only rainfall which is less than Austin, Texas (in east Anglia)...but in my garden and allotment (hoses!), the summer split has been an outstanding success.

I was always hopeless at getting onto it in spring and by the time we got to autumn, it was the 'wrong' time to do all the plants most recently in flower and I had forgotten what needed dividing back in spring...so I had a fair bit of attrition. I have always split my auriculas in midsummer so it didn't seem too alien but the best thing was being able to see what needed it (so hard when everything was still tiny in spring) and guiltlessly go in hard with a border spade .As opposed to the futile nipping, faffing and thinning, my lack of timely divisions always required by the time august rolls around (in short, during peak chaotic mess when a savage digging is most satisfying and effective).

eta - transplanting is different to dividing though. Taking the biggest possible rootball, nicely damp is easier (more successful) than hacking, sawing, pulling apart into numerous splits and then replanting. Bare-rooting in dormancy is even easier. Imo, my success rate definitely varied with autumn splits having only an evens chance of failing/growing, whereas I can see this years divisions have already established - feel they are going into winter in good fettle.

Camp, I had no idea you all were THAT dry! That is a challenge. I thought all of England was rainy, like Seattle. ;-)

I did transplant two hostas this fall, and I got as much original root and dirt I could. It felt so liberating to put something in the ground that didn't have mangled roots, and to know that it was just 'merely miffed' for a while and not in full recovery mode.

Camp, I'm with you on the time thing. Spring is finishing up school, fall is beginning school, and both spring and fall are my kids' sports seasons. I can get so frustrated. It seems there's no time to do anything at all when I have so much to do. Thankfully plants can handle some neglect. I'd be toast otherwise.

"I thought all of England was rainy, like Seattle. ;-)"

And that is a very common misconception as well. Seattle is really not all that rainy........we too get far less annual rainfall than a great many other locations in the US, inlcuding Chicago, New York, Kansas City, Atlanta, etc........and we are about on par with Austin, TX.

It is just that rainfall in Seattle and the Puget Sound area comes in very light amounts (really just a drizzle most times) over an exended period - anytime from October through June. But summers here are dry.......very dry!! This is a recurring summer drought area.........we received no measurable trace of rain (so less than .05") for more than 105 days this summer.

Well, gardening AND geography lessons! I thought Seattle was a drenched rainy place, figured you had lots of marshes around. That completely changes the picture of what your native flora are like if summers are dry. Live and learn at every age!

Native PNW flora is for the most part extremely drought tolerant. Think California coastal native plants and you will see many of exact same species. There are a few marshes around but mostly restricted to river estuaries or locations with a very high water table, otherwise not common at all. And even streams and creeks are dry in summer or very low. All the local tree frogs migrate into my garden in summer from the surrounding greenbelts and woodlands, as they appreciate the frequent irrigation they miss in the woods :-))

Cambridge is the second driest and warmest place in Great Britain...with less than 2 feet of rainfall every year. So yeah, dry...compared to Western Scotland or Wales on 10 feet of annual water (and even 12feet).

OK, so we get over four feet of precipitation here annually on average, or 130 cm. I did NOT know you all were so dry. I visited Cambridge once in the summer. It was lovely!

I'm thinking about my climate, and I'm happy. I really do love where I live, which is a nice bonus. I love the mild summers that make so many plants happy and make it so comfortable to do gardening chores. I love the short growing season because that means before I'm worn out everything, including my back, snuggles in for a nice winter. I love the winters with the snow and how the colors on the water seem both layered and translucent. Now I'm feeling happy!

(Edit: I am absolutely NOT inferring negative things about somebody else's climate! Merely saying that as these posts make me think about my neck of the woods, I find that it's nice to like where you are. Snow, late frosts, heating and plow bills, and all.)