There's a lot of math going on in every successful garden. Adding new plants. Subtracting weeds. Multiplying your perennial population. And there comes a time for division, too. When plants look overcrowded or cramped, it's time to divide (generally, every three to five years). Or, you can divide to add your favorite perennial to a different spot or share it with a fellow gardener.
Remember: If the plant blooms in the spring, divide in the fall; if it blooms in late summer or fall, divide in the spring (midsummer-blooming plants should be divided in early spring).
Dividing is simple. Follow these tips, and you'll soon be a master at easy division. It doesn't get any easier than this.
3. Place cutting in a pot of moist perlite or vermiculite. Put in a sunny spot or under a grow light. Slip the pot into a clear plastic bag, allowing the condensation to keep it moist. After one month, gently tug the stem to feel for resistance. If you get resistance, roots have grown and you can transplant to potting soil.
Rhizomes are plants that have a thick underground stem; iris is one example.