Fountain grass (Pennisetum) is a highly valued ornamental grass recognized for its distinctive arching, mounding habit and elegant, bottlebrush-like flower plumes. The graceful, cascading form gives the plant its common name. However, the exact mature size is not fixed, varying dramatically depending on the specific cultivar chosen. The final dimensions of a fountain grass clump are determined by its genetics, ranging from miniature mounds to substantial specimens.
Common Types and Their Mature Dimensions
The question of how big fountain grass gets is primarily answered by classifying it into three general size categories based on the plant’s genetic makeup. Gardeners can select a variety whose mature size aligns with the available space in their landscape.
The smallest options are the dwarf varieties, suitable for rock gardens or the very front of a border. Cultivars like ‘Little Bunny’ typically reach a height and spread of only 10 to 14 inches. ‘Hameln’ is slightly larger, commonly maturing at about 2 to 3 feet in both height and width.
Standard varieties represent the middle-ground size, forming the typical specimen seen in many mixed perennial beds. The species Pennisetum alopecuroides generally grows to a height of 2 to 3 feet, developing a rounded mound of foliage. Cultivars like ‘Moudry’ mature at a similar size, reaching 2 to 2.5 feet tall with a comparable spread.
Purple Fountain Grass (Pennisetum setaceum ‘Rubrum’) is a fast-growing, medium-to-large annual option. This plant quickly reaches dimensions of 3 to 4 feet tall, with a spread of 2 to 4 feet in a single growing season. For the largest effect, the species Pennisetum purpureum can grow even larger, sometimes reaching 4 to 8 feet tall, though these are generally treated as annuals in all but the warmest climates.
Environmental Factors That Determine Final Size
While genetics set the maximum potential, environmental conditions determine whether a fountain grass reaches the high or low end of its expected size range. Light exposure is one of the most significant factors, as these are warm-season grasses that thrive in heat.
Planting in full sun (at least six to eight hours of direct sunlight daily) encourages the densest and tallest growth. When placed in partial shade, the plant tends to become leggier and less dense, resulting in a reduced overall size and fewer flower plumes.
Soil quality and moisture levels also affect the final dimensions of the clump. Fountain grass performs best in fertile, well-draining soil, which allows for robust root development and maximum growth. Conversely, poor, compacted, or overly wet soil can stunt the plant’s growth and may even lead to crown rot, significantly limiting its size.
Consistent moisture, particularly during the establishment phase, promotes a larger, healthier plant. Drought stress will cause the plant to conserve energy, resulting in a smaller final size and browning of the leaf tips. For perennial varieties, the local climate dictates whether the plant can return year after year to achieve its full mature size. In cold hardiness zones where the plant is only marginally hardy, it may be treated as an annual, preventing it from reaching its full size potential.
Controlling and Maintaining Established Clumps
Once a fountain grass clump is established, managing its size involves specific physical interventions, distinct from environmental controls. The overall height and appearance are maintained through an annual cutback, which is a necessary practice for warm-season grasses. In late winter or early spring, before new green shoots emerge, the entire clump of old, tan foliage should be cut down to about 4 to 6 inches above the soil line. Tying the old foliage together with twine before cutting simplifies the process and makes cleanup easier.
The horizontal spread of the plant’s root system is managed by division, which is typically required every few years. As the clump matures, the center may begin to die out, forming a less attractive, open ring, which signals the need for rejuvenation. Dividing the tough, fibrous root ball often requires a sharp shovel or even a reciprocating saw to cut the mass into smaller pieces.
For long-term size control, replanting only small divisions, perhaps only four or five inches in diameter, is recommended, as larger sections will quickly return to their mature size. Controlling unwanted spread from self-seeding is important, especially with non-sterile varieties like ‘Moudry’ or the species form. Deadheading the flower plumes before they fully mature and drop seed can minimize the appearance of numerous small seedlings throughout the garden.