In 1883, Hawaii successfully introduced African guinea grass for livestock feed; 143 years later, this 9-foot invasive weed can produce 9,000 seeds per plant and has a fire risk score of 0.89.

What was once introduced as useful grass has become part of the growing challenge of invasive species and wildfires in Hawaii. Guinea grass, a perennial species native to Africa, was brought to Hawaii during the 19th century because of its value as forage for livestock.

However, over time, the grass spread beyond managed pastures and became naturalized throughout the islands. Today, its ability to grow in dense stands, produce thousands of seeds, and recover quickly after a fire has made it a major concern for land managers. A historical study, “From Grasses to the Present: The History of Grass Introductions in Hawai’i,” documents a successful introduction of guinea grass by Judge McCully in 1883, and the grass was reportedly first planted on a plantation in Kaʻū, Hawai’i.

The study also records that in 1901, guinea grass was widely used by the livestock industry. The Hawaii Invasive Species Council now lists the species as a high-risk invasive grass and gives it a brush fire risk score of 0.89.

From livestock forage to established pasture

Guinea grass, scientifically known as Megathyrsus maximus, was valued because it grows vigorously and provides fodder for livestock. Its introduction was part of a much broader effort to bring non-native grasses to Hawaii as ranchers and farmers searched for plants capable of supporting growing livestock operations. Historical records show that interest in guinea grass increased during the 1880s.

An 1883 account promoted the grass as a potentially valuable forage plant and an attempt was made to obtain seeds from the United States Department of Agriculture. A successful introduction occurred later that year. The grass later became established on the islands. In 1901, contemporary accounts described it as being successfully grown by dairymen, while cattlemen considered it a useful herb for livestock.

In the following decades introductions continued, including 192 samples of guinea grass imported for grassland research between 1911 and 1964.

A herb that can reach 9 feet.

The characteristics that made guinea grass attractive as a forage also help explain its ability to spread. According to the Hawaii Invasive Species Council, the perennial grass can grow up to 9 feet tall, with leaves reaching about 3 feet long. It can form dense clusters and is capable of growing in a variety of environments throughout Hawaii.

Its reproductive capacity stands out especially. A single plant can produce up to 9,000 seeds, giving established populations substantial potential to expand.

Seeds can be dispersed by both wind and animal droppings. The species can also reproduce asexually through apomixis, meaning that local populations can be genetically uniform. The herb is now naturalized and common on all of the main Hawaiian Islands, and is found from sea level to about 850 meters in elevation.

Why does guinea grass increase?

The biggest concern today is not simply about guinea grass taking over land. Its dense growth can also change the way fire moves across the landscape. The Hawaii Invasive Species Council assigns guinea grass a fire risk score of 0.89, indicating that it is likely to pose a high fire risk in Hawaii. The species can create dense stands in unmanaged areas, particularly when there is no grazing or other forms of vegetation management.

During drought, dry vegetation can provide additional fuel for fires.This creates a difficult cycle. Guinea grass can increase the amount of fuel available, while fire can also create conditions that favor its continued spread. The species is capable of regenerating quickly after a fire and expanding into burned areas. That gives it an advantage over vegetation that may recover more slowly. Once established, new growth can occupy areas opened by fire, potentially reinforcing the grass’s presence in the landscape.

An unintended legacy of pasture improvement

Guinea grass is an example of how a species introduced for economic purposes can have consequences far beyond its original use. In the late 19th and early 20th centuries, improving forage for livestock was an important agricultural goal in Hawaii. Guinea grass offered exactly the qualities ranchers wanted: vigorous growth, substantial biomass, and utility as animal feed. But those same characteristics helped the species persist and spread after it moved beyond cultivated pastures.

The landmark study notes that guinea grass continued to be introduced for grazing purposes long after the initial introduction in 1883. Today, the challenge is managing a plant that remains useful as forage but can also become invasive if left unmanaged. The Hawaii Invasive Species Council describes the species as having strong allelopathic activity, forming dense stands, and increasing fire risk in unmanaged or drought-affected landscapes.More than a century after guinea grass was deliberately brought to Hawaii as livestock feed, its legacy illustrates the long-term ecological consequences that can accompany the movement of plants between continents. What began as an attempt to improve grasses has left Hawaii managing a highly adaptable grass capable of growing 9 feet tall, producing thousands of seeds and helping to create conditions favorable to wildfires.