Here is my experience in my growing area for plants that I have personally divided:

Anthony Waterer Spiraea -- A shrub that does not need splitting. If you want offspring, I would recommend "layering."

Threadleaf coreopsis -- has been extremely hardy for me and survives ruthless division. In fact, it needs division every 2-3 years to keep it blooming and to keep the size manageable.

Autumn Joy sedum -- is a fall bloomer, hence the name. Sedum blooms are not particularly showy, more a case of the seed head slowly changing color. Best to wait 'til it's done blooming so you can enjoy what color it does make. This being said, sedums tolerate ruthless division (down to a single stem) as long as the temperature stays moderate, they receive enough water, and your soil is decent. With your Purple Emperor, I am guessing it's bloomed by now. OK to divide it now, but it's best if it can get good water, have decent soil and not get blasted by heat.

Amsonia - would love to split mine, but in 6 years, it has stayed a perfect manageable size. If only all perennials were so easy...

thank you

i wish my amsonia would stay small, this year it flourished.

i will layer that Anthony Waterer then I love the bush and would love to Make more.

anyone else on the others??

I don't think you can hurt BES. Ideally, you should wait until they go dormant. But if you take care to dig, split, replant, and water all within a short time (the same day) I think you'd do fine. Mine grow like weeds no matter what I do to them.
Martha

Russian Sage - You can propagate through stem cuttings and you don't need rooting hormone. Not sure about splitting - mine is still young. You can move anytime as long as you wzter your plants so they re-establish their roots.

BES - I leave them be, and let them reseed. It's easy to locate the new plants in the spring with their fuzzy leaved seedlings. And new ones will flower in their first year. You can split and relocate, but if you like the clump as it is now, well, I'd leave it. One of my favs.

Silvermound - You can split this one in as many sections as you'd like, but the smaller the split, the smaller the plant you will get next year. Take a knife with you - the roots are sometimes 'woody', and make sure you plant in a well drained area.

Threadleaf Coreopsis - Just split one a couple of weeks ago. This was just as it was starting to bloom. The splitting and moving had NO effect on it, and all are blooming heavily right now. Had I known this plant was this tough, I would have split it into more sections.

Sedums - I'd wait until they finish blooming to split. And again, if you like their size and want to keep the present plant, just cut off a stem and stick it in the ground and keep the area moist. My dog breaks pieces off every season, and I now have quite the number of sedums. If she breaks them off very late in the season, I bring them in and pot them up as house plants for the winter, them place them in the garden in the spring...

Spiraea - Trim the present plant you have and take cuttings and propagate in pots. You can also do the layering.

Nicole.

Have some beautifur clematis that I would like to split.
Is this possible and how and what time of the year. Thanks
for any help.

I have a Spiraea Bumalda Crispa. If this is the plant you are referring to, a botonist told me not to split it, however, he did say to deadhead it and I would get another bloom out of it, maybe even a third.
My blooms were a bright fushia. Today is Thursday Aug. 26/04 and this past Tuesday I did not have one bloom left of the original color, they did not fall off just turned a brownish color. Not worth keeping like it was. I remembered that when he gave me the advice he told me to deadhead it very hard.
I pruned off almost 1/4 of the plant on Tuesday and when I looked at the plant this morning, there were about 10 new clusters of that beautiful fushia color.
Good Luck.
Punky