Researchers removed invasive Japanese stiltgrass from 8 Indiana forest sites; native plant diversity rose 24%, while herbicide-treated plots had 123% greater tree seedling density than controls

There's a reason why the forest floor on most eastern hikes in the United States seems to be growing suspiciously less wildflower-y and more uniformly green; it's due in part to the proliferation of an innocuous-looking ankle-high annual, Japanese stiltgrass, which most people walk right over without thinking twice. Ecologists have been warning about its effects for two decades, but as a 2009 peer-reviewed study titled “Invasive plant removal method determines native plant community responses,” in the Journal of Applied Ecology by ecologists S. Luke Flory and Keith Clay demonstrates, removing this grass can have more impact than one might expect.

A grass most people have never even heard of is quietly taking over eastern U.S. forests

While stiltgrass rarely crowds out mature trees outright, it thrives in the shaded understory where native seedlings and wildflowers would otherwise take hold. That tolerance for low light has let it spread through the forests of roughly two dozen eastern states since it was first documented in Tennessee around 1919, likely introduced accidentally as porcelain packing material. Most hikers would walk right past this grass without noticing it: it grows in dense, sprawling mats that blend into the surrounding understory, and even though the plant typically stands one to three feet tall, occasionally reaching three and a half feet or more, its delicate, bamboo-like form doesn’t register as anything unusual to the untrained eye.


What two years of weeding has actually demonstrated

The results of the Indiana removal experiments are the important part, however; the ecologists observed a 24% increase in overall native plant diversity in removal plots compared to untreated reference plots, and in the plots treated with post-emergent herbicide specifically, a 123% increase in the density of native tree seedlings compared to untreated plots. That seedling result is the more striking one: it indicates that stiltgrass invasion was actively suppressing tree recruitment, not simply competing with ground-layer plants for resources. The study authors note, however, that this strong seedling response showed up only with the herbicide treatment, hand-weeding and combined treatments did not produce the same effect on tree seedlings.

This corroborates findings from the same team of researchers in a separate 2017 study titled “Effects of a non-native grass invasion decline over time,” published in the Journal of Ecology. That eight-year study found that for the first four years, invaded plots had 57% lower native herbaceous biomass and 44% lower diversity than uninvaded plots. But by year eight, Microstegium had declined to just 2% of community biomass, and there was no longer any difference in native biomass between invaded and control plots. The authors attribute this decline primarily to natural plant succession, as longer-lived native trees and perennials became established and increasingly dominant, conditions grew less favorable for an annual grass that has to recolonize from seed every year, rather than to tree canopy shade alone simply “shading out” the invader.

Another 2015 study titled “Fire and non-native grass invasion interact to suppress tree regeneration in temperate deciduous forests,” also appearing in the Journal of Applied Ecology, again by the same researchers, found that stiltgrass invasion alone reduced natural tree seedling recruitment by 66% compared to uninvaded areas, and that adding fire on top of an existing invasion reduced tree seedling survival by a further 54% compared to invaded-but-unburned plots. Critically, the study also found that fire increased stiltgrass biomass roughly fivefold the following growing season, meaning prescribed burns, used without first removing the invasive grass, can make the invasion worse rather than better, creating a self-reinforcing “grass–fire cycle.” That’s an important caution for land managers who might otherwise reach for fire as a default restoration tool.

Why those benefits might not necessarily come as easily as one might think

The 2009 study’s authors note that the benefits of removal varied by plant functional group and by which removal method was used, and that combining herbicide treatments did not necessarily outperform a single application. Overall, removal plots still looked ecologically different from the long-undisturbed reference plots.

Why any of this should matter to you

Unless you're a serious botanist, you might not have much of a reason to care about the effects of invasive plants on the ground flora of eastern forests. However, the carbon-sequestration capabilities of eastern woodlands are crucial to the region, not to mention the watershed-protection services they provide, which filter the water before it reaches other ecosystems downstream.

The maple-sugar-bearing trees and the iconic eastern hardwoods provide an economic boon to the region, and can be seen in many a picturesque post on social media; an invasive species choking out the next generation of trees is not something anyone wants to see, now or in two decades. It’s also a reminder that not every environmental problem announces itself dramatically; sometimes it spreads quietly, underfoot, until someone finally measures the difference it’s making.