Hardy bulbs, like irises and cannas, will produce flowers year after year when planted in the right growing zones. However, as bulbs grow, they can get overcrowded and need to be divided to protect the plants and keep them blooming. Keep reading to learn more about when to divide bulbs and how to do it.
Why Dividing Bulbs Is Important
Cannas, irises and other rhizomes are types of bulbs. Like a potato tuber, they're actually underground stems that grow horizontally. They're long and firm, with multiple growing points or buds along the sides. These grow upward and out of the ground as new stems. As the years go by, the number of rhizomes and their size increase, producing a mass of rhizomes that needs dividing.
When to Divide Bulbs
The best time to divide bulbs is in spring, as the new buds are starting to grow. It's best to wait until about two weeks before the last frost. For plants that are hardy through the winter, dig the whole clump up as they break dormancy.
In growing zones where the plants aren't winter hardy, you'll want to pull up your rhizomes in late fall. Wait until after the first frost, but pull them before the ground freezes. Remove the dead stems and dig up the rhizomes for storage over the winter, where they can be left until spring to divide.
How to Divide Bulbs
With larger groups, the rhizomes will fall apart when you dig them up, making that first division for you, but you may require a gardening knife to help break apart smaller groups of rhizomes. Gently move the roots apart and identify the growth points, just as you can see eyes on potato tubers. With both cannas and irises, you can see the growth points easily, and there should be multiple "eyes" on each rhizome. Select rhizomes that have both a stem and roots attached, bypassing any that don't have any eyes or buds.
Before you start, rinse away any soil and inspect the rhizomes, removing any soft or decaying areas. Be gentle when handling the rhizomes to avoid damaging them. After rinsing, dunk the rhizomes in a disinfecting bath of 1 part bleach to 9 parts water. Using disinfected clippers, prune back the roots.
Irises are particularly prone to disease, so it's essential to disinfect the blades of your cutting tools both before you start dividing the group, as well as before working on another group of irises. A solution of 1 tablespoon of bleach in a gallon of water should be strong enough to cleanse the blade. Rinse the bleach off the clippers before cutting the rhizome.
Planting Divided Rhizome Bulbs
When your new divisions are ready, plant them just below the surface of the garden bed. Don't plant the rhizome too deep as they prefer to almost sit on top of the ground. A 2- to 3-inch deep hole is just right. Place the rhizome inside, spread the roots out and tamp soil around it. Be sure to only partially cover it. Water well to fully saturate the roots and ground.
While hardy bulbs, like cannas and irises, are pretty low maintenance, taking a little time to divide them can help ensure you enjoy a robust bloom each year.
Check out Burpee's guide to cannas to learn more about planting and growing cannas.