The grass known as Japanese Blood Grass (Imperata cylindrica) is a complex issue for gardeners and environmental managers. The straight species, commonly called Cogongrass, is recognized as one of the world’s most problematic weeds and is listed as a Federal Noxious Weed in the United States. While the wild species is an aggressive environmental threat, ornamental cultivars like ‘Rubra’ or ‘Red Baron’ are popular for their striking red foliage and are often considered sterile. However, the ornamental variety presents a high-risk scenario due to its potential to revert to the aggressive, invasive wild form.
Identifying the Threat: Species vs. Ornamental Cultivar
The distinction between the species and the ornamental cultivar is based on appearance and reproductive habit. The wild species, Imperata cylindrica, grows tall, ranging from two to six feet, with long, green, sharply pointed leaves. It produces fluffy, silver-white seed heads that allow for widespread dispersal by wind. This is the form designated as a noxious weed.
The ornamental versions, frequently sold as ‘Rubra’ or ‘Red Baron’, are shorter, typically reaching only 12 to 18 inches. These cultivars are valued for their foliage, which emerges green and develops a vibrant, deep red coloration from the tips downward. Although these plants are often sterile and less vigorous, they can revert back to the aggressive, green-leaved species. This reversion quickly reacquires the full invasive characteristics, including rapid spread.
Ecological Impact and Mechanism of Spread
Cogongrass is a successful invader primarily through its extensive network of sharp, vigorous rhizomes, which are underground stems that can reach depths of four feet. These rhizomes form dense, impenetrable mats that displace native vegetation and store vast amounts of energy, making the plant incredibly difficult to kill. A single acre of infestation can contain up to three tons of rhizome mass.
Long-distance spread occurs through wind-dispersed seeds, which are produced in fluffy, white heads. While some populations show low seed viability, this method allows the plant to colonize new, distant areas. Imperata cylindrica is also allelopathic, releasing toxic chemicals into the soil that inhibit the growth of competing native plants. The grass increases the risk and intensity of wildfires because its foliage contains a high silica content and forms a dense, flammable thatch that burns at extremely high temperatures.
Control and Eradication Strategies
Eradication of an established Cogongrass infestation focuses primarily on destroying the dense rhizome mat. Mechanical removal involves repeatedly digging out the rhizomes, but this method is often counterproductive because any small fragment left can regenerate into a new plant. Tillage or disking can be used to bring rhizomes to the surface during a dry season, helping to desiccate and exhaust them.
Chemical control is generally the most effective strategy, utilizing systemic herbicides that translocate down into the rhizome system. The most common and successful herbicides are glyphosate and imazapyr; imazapyr often provides more consistent and longer-term control. Glyphosate is safe to use near desirable woody plants as it has no residual soil activity, but imazapyr can move through the soil and damage nearby trees. Herbicides are most effective when applied to actively growing leaves in the late summer or early fall, allowing the chemicals to be pulled down into the roots. An integrated approach combining mowing or burning to remove old thatch, followed by herbicide application to the subsequent regrowth, provides the best chance of elimination.
Non-Invasive Alternatives for the Landscape
For gardeners who admire the color and texture of Japanese Blood Grass but wish to avoid the inherent risks, several non-invasive plants offer a similar aesthetic.
Non-Invasive Alternatives
- Switchgrass (Panicum virgatum): This excellent native alternative forms attractive clumps and often displays interesting color, such as bluish tinges in summer or copper tones in fall. Specific cultivars provide similar vertical structure and texture.
- Prairie Beard Grass (Schizachyrium scoparium): This clumping, warm-season grass turns a stunning copper color in autumn.
- Mondo Grass (Ophiopogon japonicus): For a lower-growing, fine-textured option, this plant provides a dense, dark green groundcover.
These alternatives allow for similar landscape design elements while protecting local ecosystems from the aggression of Cogongrass.