Armed with pointed tips so sharp that neither cows nor deer will eat it, medusahead (Taeniatherum caput-medusae) is an invasive grass species that seems to have stepped right out of the Little Shop of Horrors. With no enemies, it is spreading rapidly throughout the western United States, outcompeting native grasses and even other grass invaders. Unless steps are found to control its spread, medusahead is likely to turn millions of hectares of grazing land into worthless fields, say researchers in a study that determined why this grass is so successful.
"It is a devilish species because it is absolutely not of any worth," says Seema Mangla, a plant ecologist at Oregon State University, Corvallis, who led the study. "Every animal avoids it."
That's because the medusahead's long, twisting, snakelike seed stems (which give the grass its name) are stiff and pointed like needles. Any animal that leans in for a snack gets jabbed in the eyes and mouth. The grass is loaded with inedible silica, too, providing few nutrients to would-be grazers. As a result, the grass steadily accumulates, forming mounds of thatch, Mangla says. "It's part of a huge change in vegetation structure," as native grasses are overwhelmed by invaders. Other studies have shown that medusahead is spreading at a rate of 12% per year in 17 western states. Although it invaded the United States from the Mediterranean in 1880 and is now found only on more than 1 million hectares, Mangla and others worry that it is picking up steam and may be outcompeting not only native grasses, but even cheatgrass (Bromus tectorum), a more nutritious invader.
Measures to control medusahead's spread—mowing or spraying with herbicides—aren't effective, because they only treat the top of the plants, not the thatch beneath, which protects their seeds, Mangla says. "We need to understand its growth dynamics, what makes it such a successful invader, then we can figure out better ways to disrupt it."
Invasive plants are thought to have especially high relative growth rates, enabling them to rapidly capture water and nutrients. To determine if the medusahead's growth rate figures in its success, in 2008 and 2009 Mangla and her team randomly scattered the plant's seeds on five 1-m2 test plots at the Eastern Oregon Agricultural Research Center in Burns. At regular intervals throughout the growing season, she and her team weighed harvested seedlings. She then compared the medusahead's weight to that of two other grasses growing separately in similar plots: the native perennial bluebunch wheatgrass (Pseudoroegneria spicata), which the medusahead is rapidly replacing, and cheatgrass.
When she averaged the grasses' weights across the two seasons, medusahead came up the winner. Only during 2008, which had below-average rainfall, did the native grass do slightly better. But after the more normal rainfall of 2009, the medusahead raced ahead, growing longer shoots for a longer period of time, the team reported in the 28 October Journal of Arid Environments.
"It's a good study, and shows why medusahead can be so competitive," says Joseph Ditomaso, an invasive plant ecologist at the University of California, Davis. "Since animals won't eat it, medusahead essentially creates its own thatch layer, which is a great tactic for preventing seeds from sprouting, as every gardener knows." The only seeds that can make it past the medusahead's thatch barrier are its own sharp, pointy, inedible ones. "It gives itself every advantage," says Ditomaso, who says the best control right now is simply burning the thatch. Meanwhile, Mangla and her team expect that the medusahead will continue to spread, since climate conditions favoring native grasses are sporadic and rare.