Ornamental grasses are reliable, beautiful and pretty much carefree. Just remember to divide them regularly.

With a nod to garden designer Piet Oudolf, we planted this 50-foot border of perennial grasses, mixed with rudbeckia and echinacea for extra color. If you want a dramatic, easy-care display of plants, it’s hard to beat ornamental grasses. For more photos, see the slideshow of this garden.
Grass that needs to be divided

The doughnut-shaped growth pattern in this clump of switchgrass indicates that it’s ready to be divided.
Dividing ornamental grass

Sturdy boots and a sharp-bladed shovel are required equipment for dividing grass clumps.
Ornamental grass

A healthy division of feather reed grass (Calamagrostis), ready for re-planting.

Ornamental grasses are among the most carefree and graceful of perennials. They ask so little and manage to look wonderful over a long season. The only big chore is dividing. By doing it every few years, you can keep your clumps healthy and vigorous. What’s more, you can share extra clumps with other gardeners.

There’s no rule about when a grass clump needs dividing. I usually do it when the clump is crowding out other plants. With miscanthus cultivars, this often happens after two or three years. I usually reduce the clump by half or a third.

Some grasses will grow into a “doughnut” pattern that’s especially noticeable when the grass has been cut back in spring or fall. I see this a lot in the switchgrass cultivars (Panicum). At this point, it’s best to divide the grass to ensure renewed vigor. You’ll notice this growth pattern in other perennials, such as Japanese irises.

You can divide grasses in the spring or fall. I prefer to do it in the spring because I like to leave the grasses standing in fall and winter. Once the soil is free of frost in early spring, I get to work because some grasses, such as calamagrostis, start pushing out new growth pretty early. In gardens with lots of ornamental grasses, a gas-powered hedge trimmer is the best tool for the job. It’s especially helpful with miscanthus, which have thick, woody stems. A hand-powered hedge shears works fine, too.

Once the grasses are trimmed to the ground, I can see any “doughnuts” and start dividing. The key tool here is a sharp shovel with a sturdy handle that you can use as a lever. I like to cut a circle around the clump and then lift out pie-shaped slices. Even with a sharp shovel, it can be difficult to slice through the clumps. But, with regular division, the clumps rarely grow too large.

Choose your best clumps for replanting, and give the rest away — or find new homes in your garden. In some situations, I replace a doughnut with a trio of small clumps. That way, the grass will still have the same mass in the garden with renewed vigor.

Before you put away your tools, make sure you water your transplants. Set your hose to a trickle and let it sit at the base of each new transplant for 10 minutes or so. Don’t skip this step! It gets your divisions off to a good start and eliminates large air pockets in the soil. Finish the job with a top-dressing of slow-release, granular fertilizer.

David Grist, Online Content Coordinator, Gardener’s Supply