Rubrum grass is the common name for the ornamental plant known botanically as Pennisetum setaceum ‘Rubrum’, or Purple Fountain Grass. Whether this plant is a perennial or an annual depends entirely on the climate in which it is grown. Botanically, it is a tender perennial, but its widespread use in gardening often dictates its functional classification. Understanding this dual identity is important for gardeners planning its care throughout the year.
Defining the Lifecycle: Annual vs. Perennial Status
The Purple Fountain Grass is botanically classified as a tender perennial, meaning it naturally lives for more than two years in its native habitat. This perennial status is maintained only in tropical or subtropical climates, specifically within USDA Plant Hardiness Zones 9 through 11. In these regions, the plant enters a period of winter dormancy, but its root system remains alive, allowing it to regrow vigorously each spring.
A true perennial is defined by its ability to survive the winter and return in subsequent growing seasons. The ‘Rubrum’ cultivar cannot tolerate sustained freezing temperatures; prolonged periods below approximately 20 degrees Fahrenheit will cause the death of the root crown. Gardeners in Zones 8 and colder, where winter soil temperatures drop significantly, must therefore treat the grass as an annual. An annual completes its life cycle in a single growing season before dying, meaning the plant will not survive the winter outdoors.
The inability to withstand hard frost dictates its use across different regions. Even in marginal Zone 8, while a mild winter might allow the grass to return, its survival is not reliable, leading most gardeners to replant each year. This distinction highlights the difference between the plant’s inherent botanical nature and its functional role in a given landscape setting. The rapid growth rate of the grass allows it to quickly achieve its full ornamental effect.
Key Features and Ornamental Value
The widespread popularity of Purple Fountain Grass stems directly from its striking aesthetic qualities and graceful growth habit. Its foliage provides a deep, rich color, typically a burgundy-red or maroon, which holds its intensity well throughout the growing season. This deep coloration offers a dramatic contrast to the greens and yellows of a typical garden bed or container planting.
The plant is named for its distinctive fountain-like form, where the leaves arch gracefully outward from a central clump. From mid-summer until frost, the grass produces showy, fluffy flower spikes on nodding stalks. These plumes are typically pinkish-purple or rosy-red and possess a soft, bottlebrush texture, adding movement and fine texture to the landscape.
‘Rubrum’ is highly valued as a specimen plant, serving as an eye-catching focal point in garden designs. It is frequently utilized in large containers as a tall, colorful vertical element, or massed in borders to create sweeps of rich texture. Its visual appeal ensures its continued use, even when treated as a temporary seasonal planting.
Growing and Overwintering Techniques
For gardeners in colder climates who treat Purple Fountain Grass as a seasonal annual, cultivation is straightforward, focusing on maximizing growth during the warm months. The plant thrives in full sun, requiring at least six to eight hours of direct light daily to maintain its deep burgundy color and robust form. It requires moderate and consistent watering, especially when first establishing, but is quite drought-tolerant once its root system is developed.
As a warm-season grass, it will not initiate growth until temperatures are reliably warm, and it will begin to turn brown and die back after the first hard frost. Once the foliage has died back, it can be cut down to a few inches above the ground for neatness or left standing for winter interest before being removed entirely. This annual treatment is the simplest method, requiring no special winter maintenance.
Perennial Maintenance (Zones 9+)
In warmer Zones 9 and above, where it is a true perennial, maintenance involves minimal seasonal pruning. Gardeners should cut the plant back severely in late winter or early spring, removing all previous year’s growth down to about six inches. This pruning clears away dead material and encourages a flush of new, fresh foliage as the weather warms. Perennial clumps may also require division every few years to maintain vigor and prevent the center from dying out.
Indoor Overwintering (Marginal Zones)
For those in marginal Zones 7 or 8 who wish to attempt to save the plant, overwintering indoors is a viable option. Before the first heavy frost, the entire clump should be carefully dug up and potted into a container large enough to accommodate the rootball. The foliage must be trimmed back significantly, often to just a few inches above the soil line. This trimming reduces the plant’s need for light and water during dormancy.
The potted grass should be stored in a cool, dark, or low-light location, such as a garage, basement, or cool greenhouse. Temperatures must remain consistently above freezing but cool enough to prevent active growth. Water should be applied sparingly, only enough to prevent the soil from completely drying out, as the goal is to keep the plant dormant. In spring, once the danger of frost has passed, the grass can be gradually moved back outdoors and repotted or replanted into the garden.