Purple fountain grass, frequently sold as Pennisetum setaceum ‘Rubrum’, is a stunning ornamental grass known for its burgundy foliage and graceful, arching plumes. The question of whether this popular garden plant is invasive is not simple, as the answer depends entirely on the specific cultivar and the climate where it is grown. While the purple cultivar is often welcomed for its dramatic color, its parent species is a significant ecological threat in many warm regions. Understanding this distinction is the first step in making responsible planting decisions.
Defining the Purple Fountain Grass Identity
The confusion surrounding purple fountain grass stems from its close relationship with the highly invasive green fountain grass, Pennisetum setaceum. The original species is native to Africa, the Middle East, and southwest Asia, and is a prolific seeder that has spread globally.
The common ornamental plant, Pennisetum setaceum ‘Rubrum’, is a specific cultivar selected for its deep purple-red color and, crucially, its reduced fertility. Horticultural breeding has resulted in varieties like ‘Rubrum’ that are often sterile or semi-sterile, meaning they produce very few viable seeds. This lack of fertile seed production dramatically lowers the risk of the plant escaping cultivation and spreading into natural areas. However, this sterility is not always 100% guaranteed, and some viable seeds can still be produced through cross-pollination with the wild species.
The Mechanics of Spread and Seed Viability
The parent species, Pennisetum setaceum, possesses biological traits that make it a highly effective invader, especially in disturbed and arid ecosystems. A single plant can produce thousands of seeds, which are easily dispersed across large distances by wind, water runoff, contaminated soil, and even on vehicles or clothing. The seeds exhibit a high germination rate, allowing for rapid establishment in new areas.
A particularly concerning trait is the seed’s ability to germinate without light and from depths up to five centimeters below the soil surface. The seeds can also remain viable in the soil for up to six or seven years, creating a persistent seed bank that makes eradication efforts extremely difficult.
The plant’s perennial nature in mild climates further enhances its invasive potential, allowing it to produce seeds year-round. This vigorous growth and high reproductive output give it a massive competitive advantage over slower-growing native grasses. Furthermore, established plants can quickly resprout after being damaged, including following a fire, which is a major factor in its ecological dominance.
Regional Legal Status and Ecological Harm
The ecological consequences of invasive fountain grass are severe, primarily revolving around its ability to fundamentally alter natural fire regimes. The grass is highly flammable and accumulates a large amount of dry, dead biomass, which dramatically increases the available fuel load. In ecosystems not adapted to fire, such as the dry shrublands of South Africa or the native forests of Hawaii, this introduced fuel leads to more frequent and intense wildfires.
In regions like Hawaii, the introduction of Pennisetum setaceum has changed the historical fire frequency from an estimated 500 to 1,000 years to as often as every six to eight years. These hot, frequent fires destroy native plants that are not fire-tolerant, while the fountain grass resprouts rapidly and colonizes the burned, open ground. This creates a destructive feedback loop where the invasive grass promotes fire, and fire promotes the invasive grass.
Due to this significant threat, the species is legally classified as a noxious weed in numerous jurisdictions. It is listed on the Hawaii Noxious Weed List, where its sale and propagation are strictly regulated. The parent species is also regulated or listed as a serious invasive threat in US states like Arizona, California, and Nevada, as well as internationally in Australia and South Africa. Gardeners must check local and state regulations, as planting the invasive species or non-sterile cultivars may be illegal in these sensitive areas.
Non-Invasive Alternatives and Removal Techniques
For gardeners concerned about invasiveness, several non-invasive ornamental grasses offer a similar texture and form without the ecological risk.
Non-Invasive Alternatives
Cultivars of Miscanthus sinensis, known as Chinese Silver Grass, provide a tall, fountain-like shape, though care should be taken to select non-re-seeding varieties. Pink Muhly Grass (Muhlenbergia capillaris) offers a cloud-like purple-pink plume and is a native, non-invasive alternative for many regions. Another excellent choice is Deergrass (Muhlenbergia rigens), a tough, native bunchgrass that provides a strong, mounding texture and is highly drought-tolerant.
Removal Techniques
If you must remove an established, invasive clump of Pennisetum setaceum, manual removal is the most effective method for small infestations. The entire plant, including the central crown and all roots, must be dug out to prevent resprouting. Removal is easiest when the soil is moist, using a pick or mattock to completely extract the large, fibrous root mass. All removed plant material, particularly the feathery seed heads, should be bagged and disposed of as yard waste, not composted, to prevent any viable seeds from spreading. In large infestations, manual efforts are often combined with herbicides, or followed by monitoring to eliminate any newly germinated seedlings.