Question
Can I divide a clump of peonies?  If so when and how.

Answer
I don't know how long ago you planted your Peonies but new Peonies take a year or two to bloom.  Subsequent seasons should give increasing numbers of flowers (unless there's a problem).  Young Peonies do not get divided -- hopefully you are asking about established, possibly maturing Peonies.

The North Carolina Cooperative Extension Service posts a short report on Peonies:

www.ces.ncsu.edu/pitt/ag/hort/releases2002/newsoct26.html

They note: "Some mail order catalogs provide a rating range from 100 to 300 chilling hours per winter for cultivars." In addition, they warn, 'Newer Peony cultivars do not perform well in the south because they were bred and selected in northern nurseries and do not receive an adequate amount of cold weather in North Carolina.'  If you did not plant these yourself, you may be looking at some beautiful Peonies that have never performed in the garden the way we both know Peonies do on Long Island.

Now, let's suppose these Peonies are hybrids that do well in your region (which you have not identified, so this is guesswork).  Let's assume that your local weather is just right for all your Peonies.

Do they need to be divided?

Mature perennials often begin to languish when the crown gets too big to support the growth.  That's why we divide them.  Root division is aa frequently used method of propagating favorite Peony cultivars.  But as a maintenance measure, some gardeners do not think they should ever be divided.

The experts at N.C. Extension have this advice about the subject: 'Peonies do not respond well to transplanting and reestablish slowly.  Divide and replant only after they become crowded - usually after 10 to 15 years.'  They post helpful instructions for this procedure.

The National Gardening Association recommends dividing when you notice 'swelling red growth buds coming from the crown of the bush.'  Here's the URL:

www.garden.org/regional/report/arch/inmygarden/96

Why am I telling you all this?

Because a divided Peony can take YEARS to recover before it blooms again.

You should wait until Autumn to divide any Peony, anyway.  Be sure to not set the divided crowns any more than an inch below grade.

Colorado State University Cooperative Extension also has a section covering Peonies, including a part called, 'When to Divide Peonies,' which they recommend only when necessary 'when you notice the plants are not blooming as profusely as they once did.'  You can read the whole report here:

www.colostate.edu/depts/CoopExt/4DMG/Flowers/Perenls/peonies.htm

The Bachman's website considers the 'Divide' solution when Peonies set few or zero buds:

www.bachmans.com/retail/index.cfm

Bottom line: Can you divide Peonies?  Sure, you can.  When?  Autumn, and ONLY if needed.

THE LONG ISLAND GARDENER