Question: Something needs to be done about the fast-growing Macaranga tanarius (parasol leaf tree) allowed to grow wild along roadways in Manoa and Nuuanu for several years. Yards and gardens in those two areas are getting hit with an epidemic of this plant. They send thousands of seeds when they mature and blossom. Can’t the city or state destroy these trees before they spread further this summer?
Answer: It’s not just in Manoa and Nuuanu that the invasive plant can be found.
Because it is already well established on Oahu, "there is no possibility of eradicating it here," said Lara Reynolds, outreach specialist for the Oahu Invasive Species Committee, a voluntary partnership of state, federal and private agencies that target the most harmful invasive plants and animals on the island.
"The only action that can be taken is to control the plant locally (in one’s yard, etc.) and remove any seedlings before they reach adulthood and produce seeds," she said. "Unfortunately, it will probably involve an ongoing effort of trying to control this plant in one’s yard, because birds and animals can continue to bring in seeds from any parasol plants that are nearby."
The plant grows quickly, produces numerous seeds, forms dense thickets and tolerates a wide range of soil conditions, she said.
The tree appears to be a problem in wetter valleys, where they grow wild, said Stanley Oka, administrator of the city Department of Parks and Recreation’s Urban Forestry Division. He said he’s not familiar with that tree growing within street right-of-ways or city parks. If they are, "we would remove them as appropriate."
Reynolds said the plant, a native to Malesia (a region between Southeast Asia and Australia), was intentionally introduced into the forests of Oahu and Kauai in the 1920s.
"It is considered an invasive plant species in Hawaii (and in other tropical and subtropical locales where it has been introduced) and is already established on Oahu and Kauai, meaning it is beyond the point of being able to eradicate it from these islands," she said.
Because it is so widespread on Oahu, M. tanarius is not a species that the Oahu Invasive Species Committee focuses on controlling; instead it focuses on high-threat invasive species that can be eradicated, she explained.
M. tanarius can be found in areas with moderate amounts of rainfall from sea level to approximately 660 feet, she said. Localized infestations are found on Maui, as well as in Puna and South Kona on Hawaii island.
ID Invasive Plants
When M. tanarius was introduced here, not much was known about invasive plants in general, Reynolds said.
But now a screening tool — the Hawaii Pacific Weed Risk Assessment — is available to predict whether a plant would be invasive before it gets planted here. See plantpono.org/hpwra.php.
This new resource "is available to landscapers, nurseries and the general public to learn how to ‘plant pono’ (plant responsibly)," Reynolds said. It "can enable all of us to make better choices about what we plant in our islands to prevent a new invader from getting a foothold in Hawaii."
Among the common plants to avoid: African tulip tree, Himalayan (kahili) ginger, lantana, octopus tree and wedelia.
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