Yes, fall is the right season for dividing herbaceous peonies (Paeonia lactiflora and others), but I have to stress right from the start there is no obligation here: peonies don’t need division. They can easily live for 100 years or more in the same spot and will be only become more beautiful with time. Nor do they appreciate gardeners fiddling with their roots. In fact, they hate it and will take years to fully recover from the shock. So if your peonies are doing fine and you have no pressing need to move them, just leave them alone.
That said, if you want to propagate a herbaceous peony, division is pretty much your only choice. Yes, you can also multiple them by tissue culture, but that method really isn’t within easy reach of the home gardener. And peonies grown from seed will not be true to type and will take years to bloom. So if you want more peonies without having to pay for new plants, you have little choice but to divide them.
Divide and Transplant
Peonies dislike transplanting even more than division. If you dig up a mature clump and simply move it, it can take years to start to bloom again, if indeed it ever does. So again, my best advice is if you don’t absolutely have to move a peony, you’d do best to leave it alone.
Of course, there are all sorts of legitimate reasons why you really do have to transplant a peony: the conditions at the site have deteriorated (notably, full sun has become deep shade as nearby trees grow), the garden is being redone for whatever reason, you’re moving and want to take a Grandma’s old peony with you, etc. If so, don’t just transplant, divide. Dividing is ultimately less of a shock than transplanting and has the effect of rejuvenating the plant, allowing it to grow back gradually and soon reach its former glory.
How To Divide a Peony
By the end of August and into September and October, your peonies are either dormant or close to being dormant and won’t suffer as much trauma when you dig into their roots. Their leaves haven’t put on any growth in months and will soon be losing their green coloration and turning sometimes lovely shades of purple, yellow, red, and orange (although some varieties go straight to brown), if indeed they already haven’t started. A change in leaf color is a sign their growth is over for the year.
Start by cutting the leaves back to about 6 inches1 (5 cm) from the ground so you can better see what you’re doing. Leaving the leaf stalks short but in place isn’t absolutely necessary and some gardeners cut them right to the ground, but they can make great handles for handling the plant.
Unlike many perennials you can divide by just digging out a chunk from the outside of the clump to move elsewhere, thus causing minimal disturbance to the mother plant, when you divide peonies, it’s wisest to dig the whole thing up.
You’ll find the clump is huge and heavy, with long carrot-shaped roots reaching irreguarly out in all directions. You’ll inevitably have to slice off the tips of some of the longer roots, but do try to keep as much root length as possible. You’ll have to dig all around the rootball and try to lever it free, cutting underneath as well. That’s a lot of work and you might want to bring in someone to help.
Now rinse thoroughly with water to remove the soil, otherwise you won’t be able to see what you’re doing.
Move the clump to a spot where you can give it a thorough look-over. You’ll notice that what appeared to be a solid rootball as actually composed of several growing points, each with several roots, attached to each other by a bit of underground stem. Your goal will be to figure out the best places to cut. Try where possible to divide between sections that are clearly separate.
Unless you want to produce a large number of plants and don’t care if they don’t bloom for several years, don’t divide the plant into small fragments with only one or two eyes (buds). Prefer divisions with three to five eyes. They’ll recover and start to rebloom much more quickly.
Slice between sections with a sterilized knife (dip it in rubbing alcohol between each cut). Remove and compost any dead roots.
You can plant the divisions right away or wait a week or two. The plant, being essentially dormant, won’t suffer from a relatively short exposure to air. In fact, I suggest letting the roots dry out at least for a few hours, until they soften up a bit. That way they’ll be less brittle and likely to break when you manipulate them.
Where to Plant
Choose a location with rich well-drained soil (even clay will do as long as the drainage is good), if possible in full sun. Partial shade is acceptable, although that will reduce future bloom somewhat. Avoid spots dominated by tree roots.
Whatever spot you choose, plan ahead. Peonies are permanent plants and should be placed where you won’t have to disturb them for the next 40 years!
Now plant the divisions, digging a hole to a depth and width appropriate to the size of the division. It is important to ensure that the eyes are not buried more than 2 inches (5 cm) deep, otherwise the plant will struggle to bloom. Planting too deep can leave the plant “blind”, that is to say it will produce only leaves and no flowers.
Now, fill the hole with soil (you can amend it with compost or fertilizer if you prefer, but if the soil is naturally of good quality, that won’t be necessary), tamp lightly, mulch and water well.
And there you go: it’s no more complicated that that.
Slow But Steady
In general, freshly planted divisions sometimes flower lightly the following spring, sometimes not, but certainly should the second year. After 4 or 5 years, your peony will have pretty much recovered and should flower quite abundantly, although it will still take it another 4 to 5 years for it to reach its maximum bloom.
So, gardeners, off to your shovels: you’ve got a bit of dividing to do!