Do the Math: Divide and multiply your perennials

Perennials have much to offer gardeners. Many, such as black-eyed Susans, ornamental grasses, daylilies, hostas and ferns, are long-lived, returning every year to add color and texture to beds and borders. At some point, however, these plants and others may need to be lifted out of the ground and divided. Many plants can be divided in late summer, but spring is a great time for this task because they then have the entire growing season to recover and produce a sturdy new root system.

Dividing perennials is beneficial for several reasons. It helps rejuvenate plants such as Siberian irises, which produce large clumps of leaves. Dividing is used to control aggressive, spreading perennials, such as yarrow (achillea) and monarda. It’s also an easy way to increase plants for your garden. 

Handy tools for dividing plants include a spade with a sharp edge, a gardening fork, and, my favorite, an old bread knife with a serrated blade. After planting the new divisions, water them with a diluted liquid transplant solution, such as Miracle Gro’s Quick Start. These products promote root growth. 

Divide and multiply

Hostas are among the easiest plants to divide in spring. When the leaves are a few inches tall, use a fork or a spade and dig up the entire clump. Brush or wash away the soil so you can see where the clusters of fleshy roots join the leaves. Each cluster is a separate plant. Carefully tease the individuals apart, so that each one has a few leaves and a good clump of roots. Set the new divisions in the ground at the same depth at which they had been growing and water thoroughly. This method works on daylilies, pulmonaria, coneflowers, and other clump-forming perennials. 

Donut hole, anyone? 

After a few years in the garden, ornamental grasses, such as fountain grass (Pennisetum), Japanese silver grass (Miscanthus), and switchgrass (Panicum), develop a dead spot in the center of the clump. Marcy Stewart-Pyziak of The Gardener’s Tutor in Manhattan, Illinois, calls this a donut hole.  “The plant grows in a ring, and the inside is basically dead,” Pyziak said. “The same thing often happens to Siberian iris. When it does that, you know it’s time to divide the plants.” And divide you must, otherwise the grasses become floppy and the Siberian irises produces fewer, smaller flowers. 

For Siberian irises, dig up the entire plant with a fork or spade and set the clump on a tarp or the ground. This is where an old bread knife or a small handsaw, set aside just for this purpose, can cut through the emerging leaves and rhizomes (thick underground stems that produce the shoots and root systems of new plants). Each division should have a hefty rhizome or two. Discard any woody parts and replant. Siberian irises (and other types of iris) benefit from division every three years or so. As with the hostas, set the plants back in the ground at the same depth they were growing, add compost or other organic material, and water them well.  You can do the same thing for bearded irises (shown at left).

When it comes to rejuvenating large ornamental grasses, they are usually too big to dig out of the ground. And, most ornamental grasses do not produce green leaves until early May. But in March, I use long-handled hedge shears to cut the old leaves as close to the ground as possible. That’s the easiest way to see the “donut” that has developed--the inside of the clump is usually soil or debris. Next, I use a sharp spade and slice a few sections from the outside of the “donut.”  I dig up these sections (even though there are no green leaves yet) and move them elsewhere in the garden or toss them in the compost. Some, like Morning Light Miscanthus, I plant in large containers with annuals. 

Divide and conquer

Some runaway perennials, like asters, yarrow, helianthus (summer sunflower) and monarda spread by underground runners and may need dividing every year to keep the plants healthy and prevent them from running wild. When the leaves are just a few inches tall in spring, it’s easiest to slice a few sections with a spade, lift them out of the ground, and use them elsewhere or compost them. This technique also works on coreopsis, epimedium, lysimachia, lily-of-the-valley, bishop’s weed, and sweet woodruff (Galium). 

If you didn't get around to dividing your perennials this spring, you can do it in late summer. A good rule of thumb is to get the divisions back in the ground by September 15 so they have a month to get their roots established before the first hard frosts hit the area.  As with any transplants, keep them watered, especially during dry periods.

Here’s a general guide to some common garden perennials and the best time to divide them.

Plant Spring        Aug/Sept

Achillea                   X           X

Carex (sedge)         X

Chrysanthemum     X

Coneflower             X           X

Coreopsis               X

Epimedium             X           X

Ferns                      X

Helianthus              X

Hemerocallis           X          X

Heuchera                X          X

Hosta                      X          X

   

Iris                           X          X

Ornamental Grass    X

Pulmonaria            X          X