Dividing ornamental grasses rejuvenates and propagates plants by separating the dense root crown of a mature plant into smaller, healthier divisions. While most gardeners prefer less stressful seasons, summer division is possible but generally advised against due to the high risk of transplant shock. Success depends heavily on the type of grass and the gardener’s diligence in providing intensive aftercare and a low-stress environment.
Why Ornamental Grasses Require Division
Ornamental grasses are clump-forming perennials that require periodic division to maintain vigor and appearance. As the plant matures, new growth occurs around the periphery of the clump. Over several years, this pattern causes the center of the crown to become woody and die out, often visible as a hollow patch. Division removes this dead material, allowing the healthy, younger sections of the crown to thrive.
Division is also necessary to manage the overall size of the plant. Large clumps can encroach on neighboring plants or pathways. Separating the plant reduces the size of the original plant and provides new starts for the landscape. Regular division, typically every three to five years, ensures strong foliage and healthy root systems.
Ideal Timing for Dividing Grasses
The optimal time to divide ornamental grasses is when the plant is actively growing but not under the stress of extreme temperatures. This timing allows newly separated roots to establish quickly before facing environmental challenges. Grasses are categorized into two main groups, each with a distinct preferred division window.
Warm-Season Grasses
Warm-season grasses, such as Miscanthus and Pennisetum, begin active growth when the weather warms, typically mid-spring or early summer. These varieties are best divided from mid-spring up until mid-summer, before they begin to flower. Dividing them during this period ensures their root systems recover quickly during their peak growing season.
Cool-Season Grasses
Cool-season grasses, including Festuca and Calamagrostis, grow primarily in the cooler temperatures of spring and fall. The ideal time for dividing cool-season varieties is either in early spring, just as new growth emerges, or in early autumn. Fall division must occur at least six weeks before the first hard frost to allow sufficient time for root establishment before winter dormancy.
Technique for Successful Division
Preparation for division should begin a day or two prior by thoroughly watering the clump. This softens the soil and hydrates the plant, reducing moisture stress and making digging easier. Before lifting, cut the foliage back to a height of four to six inches to minimize moisture loss through transpiration once the roots are disturbed.
To lift the plant, use a sharp spade to slice a wide circle six to eight inches deep around the clump, positioning the spade outside the visible crown. This captures the majority of the root ball and minimizes damage. For very large or tough clumps, it may be easier to divide the plant in place using a sharp spade or garden saw.
The root ball must be divided into smaller sections. Each new division needs a healthy mass of roots and at least one viable growth bud. Dense, fibrous root systems may require heavy tools like an axe or specialized root saw. Discard any woody or dead material from the center. Replant divisions immediately at the same depth as the original plant and water deeply to settle the soil.
Mitigating Risk When Dividing in Summer
Dividing grasses during the high heat of summer carries a high risk of transplant shock, especially for cool-season varieties. The primary challenge is that severed roots are stressed when the foliage is losing moisture rapidly. To reduce this stress, the foliage on new divisions must be sheared back severely, leaving only a few inches of growth. This action dramatically cuts the plant’s water needs, reducing strain on the damaged root system.
After replanting, intensive aftercare is required:
- Provide immediate and consistent deep watering to encourage new root growth.
- Keep the soil uniformly moist for the first few weeks, avoiding waterlogging.
- Apply a two-to-three-inch layer of organic mulch around the base to regulate soil temperature and conserve moisture.
- Perform division on an overcast day or provide temporary afternoon shade for the first week.
This simple step significantly lowers the ambient temperature and reduces direct sun exposure, which contributes heavily to transplant shock.