Purple Fountain Grass (Pennisetum setaceum ‘Rubrum’) is a highly popular ornamental grass cherished for its dramatic visual impact. This plant features narrow, arching foliage in rich shades of burgundy and deep purple. Its graceful, vase-shaped form is complemented by long, arching flower plumes that emerge in rosy-red or smoky-purple shades, creating a soft, flowing texture. This grass is widely utilized in landscaping as a focal point, a container specimen, or an annual accent plant.
Defining the Growth Rate and Mature Size
Purple Fountain Grass is generally classified as a fast-growing ornamental grass, achieving its full landscape presence rapidly within a single growing season. This quick establishment is highly valued by gardeners, particularly in cooler climates where it is grown as a seasonal annual. Under optimal conditions, the plant progresses quickly from a small start to its mature dimensions after the threat of spring frost has passed and temperatures rise.
The ‘Rubrum’ cultivar typically reaches a mature height of 3 to 5 feet, with a corresponding spread of 2 to 4 feet, forming a rounded, dense mound. This rapid development means that by mid-summer, the grass often reaches its full size, allowing for the emergence of its characteristic red-burgundy flower plumes. The plant’s vigorous nature ensures that even when treated as an annual, it provides a full season of color and texture.
Environmental Factors Influencing Growth Speed
The speed at which Purple Fountain Grass grows is heavily influenced by the conditions in which it is planted. Maximum growth rates are achieved when the plant receives a minimum of six to eight hours of direct, full sunlight each day. Placing the grass in a location with less light, such as partial shade, will result in slower growth and less vibrant foliage color, as the intensity of the purple hue is directly tied to sun exposure.
Soil quality and drainage also impact how quickly the grass matures. While Pennisetum setaceum ‘Rubrum’ is adaptable to many soil types, it performs best in light, moderately fertile, well-drained soil. Consistent moisture availability promotes faster development, especially during the establishment phase, allowing the roots to spread quickly. Although the grass becomes drought-tolerant once established, periods of severe or prolonged dry soil can significantly slow down the growth rate and reduce the overall size the plant achieves.
Essential Maintenance for Controlling Vigor
Because of its fast and vigorous growth habit, specific maintenance practices are necessary to manage the grass’s size and health. Pruning is the most common task, especially when the plant is overwintered and grown as a perennial in warmer climates. This maintenance involves cutting the entire clump back severely to about 4 to 6 inches above the ground.
This severe cutback is best performed in late winter or early spring before the new season’s growth begins. Pruning removes the old, dead foliage, which improves the plant’s appearance and stimulates the production of fresh, vibrant, new purple blades. For plants grown as perennials, the rapid growth over multiple seasons eventually leads to a dense center that may lose vigor. In these cases, dividing the root clump every few years is necessary to rejuvenate the plant.