Invasive Species Management


Before and After: the consequences of alien plant invasions in our native forests can be seen in these images. The left photo demonstrates a diverse native forest ecosystem, with its multi-layered canopy and water-holding ground cover of moss and ferns. On the right is an alien-dominated forest of strawberry guava and kahili ginger. Not only is the native diversity lost, but the forest is missing the essential ground cover species, and can no longer function effectively to trap and hold moisture.


Hawaiʻi is in the midst of an alien species invasion that threatens our islands’ environment, economy, and quality of life. Non-native plants such as miconia, or the “purple plague” grow out of control, producing millions of seeds per tree every year, and threaten to overtake our natural areas and watersheds.

With the steady flow of traffic to and from the islands, the influx of pests is not likely to stop. In fact, 20 to 50 new non-native species arrive in the state every year. Our goal in invasive species management on Kohala Mountain is to:

1) identify the non-native organisms that have the greatest potential to alter the ecosystem functions of our forest (eg. gathering, storing and cleaning rainwater). This potential is based on the current extent of the population and the predicted impact.

2) to prioritize species for control and areas to protect, and

3) to implement a suite of biological, chemical and mechanical control methods to reduce the impact of invasive species on the watershed.

The following plants and animals make up the “Rogue’s Gallery” of priority invasive species for the Kohala Watershed Partnership:

Alien Invasive Plants

Poison Devil’s Pepper (Rauvolfia vomitoria) – This noxious tree is native to central Africa, where it can be found in light gaps in the forest and filling in abandoned pastures and farms. It contains alkaloid toxins in leaves, bark, stems and roots. It grows very quickly, about a foot per month, reaching a height of more than 40 feet. At the rate it is spreading, the “vomit tree” threatens both native wet forest and agriculture in Hawaiʻi. North Kohala is the only place in the world that this plant has been reported as invasive, so we need to learn a lot more from our work there to help protect other places from this threat. If you spot this plant, call KWP right away for help!
Kahili Ginger (Hedychium gardnerianum) – This Himalayan relative of edible ginger was brought to Hawaiʻi in the early 1900’s, but has since become what some call a “garden thug”. The biggest threat from kahili ginger is its effect on the understory. Ginger creates a solid mat of rhizomes on the forest floor, and completely displaces the mosses, ferns, tree seedlings, and shrubs that normally carpet the ground in an Hawaiian rain forest. Not only does the forest lose its species, but it also loses its ability to hold water. As a result, the topsoil washes downstream, eventually killing coral reefs. Controlling the spread of ginger requires cutting and the use of herbicides. We are combatting the weed at our Biodiversity Preserves: Puʻu Pili, Kaneaʻa and Waimanu Bog Plateau.
Kahili Ginger
Strawberry Guava (Psidium cattleinum) – Strawberry guava is native to Central America. It was brought to Hawaii in 1825 as a garden plant, but has since spread by birds and pigs into rain forests across the state. This plant is a threat to the forests of Kohala because it grows faster than native species, and makes complete shade so that other plants cannot get light, and they die. The only way currently to control strawberry guava is to combine both chemical and mechanical means. Cuts are made around the trunk of a tree, and herbicide is painted into the cuts.
Strawberry Guava
Banana Poka (Passiflora tarminiana) – Banana poka is a fruiting vine from the Andes in South America. It was first recorded in Hawaii in 1926. This vine threatens native mesic to wet forest on Kohala Mountain because it grows up into even the tallest native tree canopy and smothers a tree’s leaves. This deprives the leaves of light, and they die. The edible fruits contain many seeds that are spread by birds and pigs. Control of banana poka involves pulling up the roots of the plant. Vines and cuttings will not re-root, so the entire vine dies. Seeds seem to be long-lived, so areas need to be revisited over many years to control emerging seedlings. The Kilohana Stream Biodiversity Preserve is one key place we are controlling this nasty vine.
Banana Poka
Australian Tree Fern (Sphaeropteris cooperi) – Some Hawai’i residents familiar with hāpu’u, our native tree fern, may wonder why an exotic tree fern would be a threat to our forests. In Australia, wet forests are more than one hundred feet tall, and contain thousands of plant species. As a result of this height and diversity of their native forests, Australian tree ferns grow faster and taller than Hawaiian plants. They can overtop native Hawaiian trees and tree ferns, and the shade they produce reduces seedling survival and understory diversity. Cutting off the top of a tree fern will kill it, as will treating the growing fronds with herbicide. Now is the time to replace these noxious pests in your garden before they invade and degrade our native forests! (Photo courtesy of Forest and Kim Starr)
Australian Tree Fern

Non-native animals: Feral ungulates (hoofed animals)

Feral pigs (Sus scrofa) – Wild pigs have a two-fold negative impact on native forest. They not only uproot native vegetation while they are searching for food, but also move around the seeds of alien invasive plants like strawberry guava trees and banana poka vine.

The negative impacts of feral pigs in the forest include the damage to native vegetation like these tree ferns (L), as well as the spread of invasive plants. The image on the right shows multiple banana poka vine sprouts growing from pig droppings.

Feral cattle (Heracleum lanatum) – hundreds of wild cattle roam across the landscape of North Kohala, as they have for more than 100 years. The most significant impact of feral cattle is the trampling and grazing of native plants, which are then replaced by non-native grasses, as seen in the image to the right. The creation of the Kaneaʻa-Ponoholo Biodiversity Preserve involved the removal of more than 200 feral cattle.
Kanea'a feral cattle

Feral goats (Capra aegagrus hircus) – goats are browsers in the dryer pastures and shrublands of Kohala Mountain, where they denude all woody vegetation, and compete with domestic livestock for forage. We have eliminated goats from the 6,600 acre Kawaihae watershed by fencing and animal control, and the vegetation is beginning to recover!

What can I do to help?

1. Know your plants. Learning to identify common species helps us protect our native forest. If you see something you don’t recognize, take a photo and/or a branch and bring it to someone to identify. New invasions usually begin with just a few little harmless-looking plants. At this stage, few people recognize the threat. Be a sentry for our forest!

2. Be choosey about what you grow in your yard and garden. Remove invasive species, and pick natives or non-invasive exotic plants for your landscape. Unsure about what is safe to grow? Contact KWP!

3. Volunteer to help us control invasive species in our forests. Monthly KWP work days focus on planting native species and controlling pests.

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