How to Divide Your Perennials for Fall

This time of year, tons of free plants are waiting for you to claim them-and they're sitting right in your own yard. It's one small upside to fall yard cleanup: All those perennials you've been nursing along for years have been dutifully growing, and they occasionally benefit from being divided. Here's what you can and can't divide, what you should look for, and how to do it.

Which perennials you can divide

Here's a basic rule of thumb: Never divide a plant while it is still flowering, but a plant that is post-bloom is ripe for the taking. Right now, we're dividing spring- and summer-blooming plants. There are three main reasons to divide: Either a plant has outgrown the space, it had a poor showing this year and could use a refresh, or you simply want to double down and split it so you can move a division somewhere else.

This includes irises, phlox, lily of the valley, lilies, hostas, veronica, salvia, coreopsis, and the whole daisy-type flower club: black-eyed susan, shasta daisies, coneflower, and gaillardia. You can also use this time to divide perennial vegetables like artichokes and horseradish. When in doubt about whether a plant should be divided, take a moment and look it up.

What you can't divide

Some shrubs and flowers don't do well in transplanting, and also don't do well dividing. These include poppies, woody shrubs like lavender, and flowers like lupine, butterfly weed, red hot poker, and candytuft. The good news is that in a few cases, like butterfly weed and lupine, these plants will do the work of self-seeding for you. In the case of woody shrubs like lavender, you will discover that some lower branches root themselves on their own, and those divisions can be separated and transplanted.

How to divide your plants

There are three tools I use for dividing plants: a long, skinny trenching shovel, ahori hori, and a good serrated bread knife. For flowering bushes (coneflower, daisies, veronica, salvia, coreopsis, etc.), you'll use the trenching shovel, as vertical as you can, to slice the plant into divisions. You can step on the shovel, but the goal is to go straight down, slicing through the root. Once you've made this initial slice, cut around the division using the same up and down slicing motion with the shovel. You should be able to easily remove the division once you've gone all the way around, and it can then be transplanted into a hole three times the size of the division.

For plants such as irises that spread via rhizome-that is, a root that sits just above the soil line-most of the dividing portion is just below the surface of the soil, and your hori hori is your best tool. Use it like a knife to cut through the rhizome. If you need to dig a big piece of the rhizome up to access it, you can use a regular spade to do so, cut it into divisions, and replant however much you'd like to in the same spot. Irises are incredibly hardy.

For artichokes and horseradish, you'll go back to the shovel. Artichokes do most of the work for you: You'll see pups" in the fall-small new formations of artichoke plants at the base of your existing plant. Use the slicing method detailed above to slice the pup away and plant it immediately; it will have very shallow roots. Horseradish, on the other hand, is all root, and the roots are how horseradish spreads. Dig deep with the shovel and pull out entire roots. If the carrot-shaped roots hang on via horizontal roots to the main clump, get in there with your serrated knife or hori hori to separate it.

Your serrated knife is a last resort, but it works magically. Take, for instance, a huge clump of common iris, which will have a big rootball, despite the rhizomes. You can stick the serrated knife into the dirt and start sawing away at the root ball.