I do a lot of division about this time of year until the temp hits 70 or so. I don't like to divide them when temp climbs too high. I don't like to do much division in late fall either because of the unpredictability of hard frost. The heat wave we are experiencing is throwing me off kilter, though. This is way too warm too soon. PS. The ground below the surface is still soggy, though. You may need to wait a bit until it dries up some more.

Oregano can be divided anytime, but preferably spring as new growth emerges or fall as you are preparing your beds for the winter. Take a sharp spade and first dig out one half of the clump, then cut the remaining clump in half again...or not. The idea is to remove the old growth. Chives are best divided in spring, also as new growth emerges. One way to thin them out and rejuvenate at the same time is after they bloom and before the flowers go to seed pull out the flower stems from the crown removing the entire stem and roots. Then dig the clumps you want to remove. If lavender is to be rejuvenated it is best done early spring just as new growth emerges. But you can start the process as soon as it finishes blooming by first trimming back the flowers, then prune only old or dead growth by pruning out no more than 1/4 of the plants current size, shaping it into a mound. I've read where you can prune it back again in November but I prefer to wait until early spring. You may want to verify the mature size of the variety you have because some lavenders, like grosso, are large plants. After you have divided the oregano and chives you can pot up your excess and sell or donate the plants to any organization that may take them.

It depends on the plant :-) I've divided some in the middle of winter (eg: hostas, daylilies, Siberian iris) with no ill-effect. OTOH, both agastache and penstemons tend to prefer really good drainage and sometimes have issues with our winter weather. Agastache in particular I would tend to leave be until spring and drier conditions. FWIW, the rule of thumb in the timing of dividing perennials is to divide early blooming perennials in fall and later season perennials in spring. But that's pretty general and there are always exceptions.

fall is a great time to divide all you have (and most things) because it's cool .your bleeding heasrt will die back after dividing but they are tough as nails and will return next spring. hosta can be divided ANY time. they are toughest of all perennials i think. after you divide flowering plants that have finished flowering, cut them back hard so enrgy goes into roots and not flowers/foliage. But Tracy Di Sabato's book The Well Tended Perennial Garden for charts of plants and timing for their pruning, dividing, etc. Best investment you can make. best, mindy