How to Divide Perennials for Autumn
There are tons of free plants waiting for you this time of year, sitting right in your yard. There’s one small benefit to cleaning up your yard in the fall: all those perennials you’ve been caring for for years are growing obediently, and sometimes it can benefit from dividing them. Here’s what you can and can’t separate, what you should pay attention to, and how to do it.
What perennials can be divided
Here’s a basic rule of thumb: Never divide a plant while it’s still blooming, but once the plant has bloomed, it’s ripe for harvesting. Now we separate spring- and summer-flowering plants. There are three main reasons for dividing: either the plant has outgrown the space, has performed poorly this year and needs to be refreshed, or you simply want to double it and divide it so you can move the division to another location.
This includes irises, phlox, lilies of the valley, lilies, hostas, speedwell, salvia, coreopsis and the whole club of daisy flowers: black-eyed suzanna, Shasta daisies, echinacea, gaillardia. You can also use this time to separate perennial vegetables such as artichokes and horseradish. If you’re unsure whether to divide a plant, take a moment and look.
What you can’t share
Some shrubs and flowers do not tolerate transplantation or division. These include poppies, woody shrubs such as lavender, and flowers such as lupine, butterfly grass, red hot poker and lollipop. The good news is that in some cases, like butterfly weed and lupine, these plants will do all the self-seeding work for you. In the case of woody shrubs such as lavender, you will find that some of the lower branches will root on their own and these branches can be separated and replanted.
How to Divide Plants
I use three tools to divide plants: a long, thin spade , a hori-hori , and a good serrated bread knife. For flowering bushes (echinacea, daisies, speedwell, salvia, coreopsis, etc.), you will use a trench shovel as vertically as possible to cut the plant into pieces. You can step on the shovel, but the goal is to go straight down, cutting through the root. Once you’ve made this initial cut, trim it down using the same up and down motion using the shovel. Once you have gone all the way around, you can easily remove the septum and then it can be transplanted into a hole three times the size of the division.
For plants like irises that spread through the rhizome, which is the root located just above the soil line, most of the dividing part is just below the soil surface, and your hori-hori is your best tool. Use it like a knife to cut the rhizome. If you need to dig up a large piece of rhizome to access it, you can use a regular shovel to do this, cut it into pieces and replant as many as you like in the same place. Irises are incredibly hardy.
For artichokes and horseradish, you’ll come back to the shovel. Artichokes do most of the work for you: In the fall, you’ll see “puppies”—small new growths of artichoke plants at the base of an existing plant. Use the cutting method above to cut the puppy and seat it immediately; it will have very shallow roots. On the other hand, horseradish is the entire root, and the roots are how the horseradish spreads. Dig deep with a shovel and pull out the entire roots. If the carrot-shaped roots are hanging down through the horizontal roots with the main clump, reach in with a serrated knife or hori hori to loosen it.
A serrated knife is a last resort, but it works magic. Take, for example, a huge clump of common iris, which, despite the rhizomes, will have a large root ball. You can stick a serrated knife into the soil and start sawing away at the root ball.