Imagine Hawaii without mosquitoes, rats, mongooses, miconia, coqui frogs, little fire ants, coffee berry borers or stinging nettle caterpillars. Once free of these alien species, time, plant sales and human traffic have brought these and thousands more to the islands. We get a new pest here almost every month.
According to Gov. Neil Abercrombie, “Invasive species pose the single greatest threat to Hawaii’s health, environment, economy and people.” His comments, to kick off the recent Hawaii Invasive Species Awareness week, are a reminder that we must work together to slow or stop the spread of some of the worst offenders
Plant Pono is an organization that is dedicated to keeping invasive plant species under control. Its website lists 30 of the most invasive plants that grow in Hawaii. More than 1,500 plants have been reviewed through the Hawaii Pacific Weed Risk Assessment. Its trained botanists review information on a plant’s biology, ecology and weedy potential in other parts of the world prior to their rating. The current list of Hawaii Pacific Weed Risk Assessment (HPWRA) designated plants is available at plantpono.org under “risk assessments for plants.” The list shows 621 plants that were determined to be at high risk of becoming weedy in Hawaii and 676 that pose a low risk. The rest need further evaluation.
Plant Pono and the Big Island Invasive Species Council have recently started a program to offer an endorsement to nurseries that don’t sell invasive plants or harbor newly spreading invasive species.
Nurseries that receive Plant Pono Endorsement must agree to codes of conduct which include reviewing invasive potential prior to marketing new plant species, maintaining communication with HPWRA, Big Island Invasive Species Council (BIISC) and University of Hawaii College of Tropical Agriculture and Human Resources to stay current on invasive species and offer alternatives to these plants. Nurseries also must follow laws related to importing and shipping plant material and use best management practices to control little fire ants and coquis on their site.
Once surveys are complete, the list of endorsed nurseries will be posted on the Plant Pono and BIISC websites. For more information about the program or to have a nursery reviewed, contact Jimmy Parker at the Big Island Invasive Species Committee at 333-0263 or jameslp@hawaii.edu.
When planting, you may often find native Hawaiian plants that can substitute for invasives. Hawaiian hapuu fern, the native thornless raspberry, Rubus hawaiiiensis, the fragrant white hibiscus or the koaia can be used in place of the Australian tree fern, mysore raspberry, night blooming jasmine or Formosan koa. The native alternatives carry many similar qualities. Native Hawaiian pritchardia palms can be used where the palmate shaped fronds of the Mexican fan palm are desired. Ilima papa makes an attractive ground cover as an alternative to wedelia.
To encourage butterflies to visit, consider planting red pentas, passion vine, citrus trees or mamaki. The milkweed species Asclepias curassavica attracts monarchs and is low on the weedy list. If you do plant it, plan to collect the seeds to keep it from spreading. These are preferable to the invasive butterfly bush.
Medinella is a large invasive shrub. The showy pink flowers of a double pink hibiscus offer the same flower color as well as a similar growth habit.
Attractive and fragrant flowers such as kahili ginger and night blooming jasmine are crowding out some endangered species in our native forests. Consider planting a dwarf poinciana, Caesalpinia pulcherrima, for its orange and yellow flowers, similar in color to kahili, or install a bird of paradise, heleconia or blue ginger if you are looking for tropical exotics.
Diana Duff is a plant adviser, educator and consultant with an organic farm in Captain Cook.
Tropical gardening helpline
Karen asks: I want to plant a ground cover on the sunny slope behind my house. Any suggestions?
Answer: If you want low-growing magenta flowers, consider hearts and flowers. Ilima papa and perennial peanut both produce yellow flowers and rank low on the invasive scale.
Many low-growing plants produce long stems that can root and take over a space quickly. If they also produce flowers and seeds, they are probably on the invasive species list. If you keep them well-contained and remove flowers before seeds form, you can grow some ground covers that rate at 5 points or below. Those are at low risk of becoming invasive.
Plants rated at No. 1 to 14, or higher, on the Plant Pono website have invasive potential. Those that rate less than 1 have very little potential risk of becoming invasive in Hawaii.
Choose carefully to avoid causing problems in your garden, on your neighbor’s landscape or farm or in native forests or upland ranch lands.
Email plant questions to konamg@ctahr.hawaii.edu for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.