Division is one of the coolest magic tricks a gardener can pull off.
It involves digging up a single plant, breaking it into two or more pieces, and replanting them to multiply all by themselves - until you decide to divide again.
So, really, plant division is multiplication. (Mind blown.)
Why and what to divide
Dividing a plant not only creates “new” plants, it also rejuvenates old, congested plants, and allows you to pull out any diseased portions or weed roots in the process.
Best of all, making new plants from your existing landscape plantings will save you a lot of money, especially if the plants in question are expensive or hard to find.
Any perennials, grasses, bulbs, or suckering shrubs are good choices for dividing, as long as each division has a sturdy, rooted base that can grow independently.
However, there is a handful of plants that don't like to have their roots disturbed, including lupine, poppy, false indigo, butterfly weed, peony, and sea holly to name a few. A good rule of thumb is to avoid dividing plants with a taproot (a downward-pointing root that resembles a carrot).
When to divide
Some gardeners (the author of this article included) choose to divide whenever it's convenient.
For example, if you're digging up plants while rejuvenating a garden bed, you might as well divide them before replanting. If you're repotting an overgrown houseplant, it makes sense to break it up into smaller plants while it's out of the pot.
Generally speaking, the best seasons to divide are in spring or fall. If you’re dividing a hardy plant, do so in fall so the roots have plenty of time to grow, giving the divisions a hearty head start.
Divide tropical plants in summer when they're most active, but do it when rain is in the forecast so that they'll become better established. Divide cold-sensitive plants in spring, especially if you live in a particularly cold climate.
Divide plants on overcast days if possible, or wait until the late afternoon so that the newly exposed divisions won't be scorched by direct sunlight.
How to divide plants
Start with a plan for your divisions so that they can go right into the ground without drying out. Proper preparation will allow the new divisions to become established and thrive.
Prepare the new planting site by pulling all weed roots, removing spent mulch, fluffing up the soil with a cultivator, and adding some well-rotted compost and topsoil.
Now it's time to remove the “mother” plant. Cut a circle around its dripline (the soil beneath the outermost leaves) to loosen up the soil. A small transplanting shovel is ideal for most perennials, but a full-sized shovel will also work.
Gently lift the entire plant out of the soil by rocking your shovel like a lever at different points around the plant. Remove the plant with your hands and place it in a shady spot to begin dividing.
Dividing is often as easy as loosening up the soil and pulling apart the plant with your hands, but some plants require more effort. Pull apart the uprooted plants with the tines of a cultivator and slice right through the roots with a clean, serrated knife, in a sawing motion. You may also cut through the root ball with a sharp shovel.
To prevent moisture loss and help the new divisions get started, cut off the upper two-thirds of their leaf growth. New growth will quickly return, and the plants will fill out entirely within a year.
To plant, lay out the divisions in the new planting site, giving them ample room for a few years’ growth.
Dig a hole for each division, leaving a small mound of soil in the center of each hole. Hold the divisions over the mound so that their crowns (where the plant originally emerged from the ground) line up level with the surrounding soil, and fan out the roots over the mound.
Replace the soil with a hand trowel, and use the back of the trowel to pack the soil in place. Finally, water the divisions deeply to ensure there are no air pockets that will dry out the roots.
Once you've planted each of your divisions, add a 1.5-inch layer of mulch to your bed to keep the soil moist and prevent weeds while you wait for the plantings to fill in.
This is the important part: Water newly planted divisions regularly until they're firmly rooted and self-sufficient. A tell-tale sign that a plant needs water is when the leaves start to wilt or dry out.