Why don't we just pass a law to keep invasive plants out of Hawai‘i? There are significant legal hurdles that prevent Hawai‘i from making more restrictive plant import laws. In short, the U.S. Constitution's Commerce Clause (Art I., Sec. 8, Clause 3) and Supremacy Clause (Art VI, Clause 2) gives Congress the authority to regulate commerce with other nations and between the states (in other words, federal law is the supreme law of the land). Further, the federal Plant Protection Act affirms that the US Secretary of Agriculture has the ultimate authority over the movement of plant species in interstate and foreign commerce, and specifically prevents states from being more restrictive on the movement of plants.
Therefore, Hawaii uses relatively short lists of noxious seeds and weeds (HRS 141 HAR4-67 and 4-68A), to manage about 80 plants, most of which are already present in Hawai‘i. Until federal preemption is addressed, or some other creative mechanism is found for restricting the importation and/or sale of invasive plants, we have to rely on public education and personal commitment to planting pono.
Hawai‘i is not alone in its efforts to assess and predict the invasive potential of plants before they spread and cause harm. In 1994, a series of questions on a plant's biology and ecology called a Weed Risk Assessment was developed and tested in Australia. Because it was so accurate and successful, it was modified and tested in New Zealand in 1995, and it is currently used in both countries as a condition of importation for plants (for more information, see History). In 1998, Dr. Curtis Daehler of the University of Hawai‘i at Manoa modified and tested the system for use in Hawai'i, and presented the system and results in 2001 to agency and green industry representatives at the Kaulunani Urban and Community Forestry Program workshop on urban plantings and invasive species. This collaborative working group recommended an integrated course of action to reduce the negative impacts of invasive species on the native ecosystems, with the modified Hawai‘i-Pacific Weed Risk Assessment system (HPWRA) being one of the key tools. Kaulunani funded the next phase of testing for the HPWRA, a research collaboration between Dr. Daehler and Dr. Julie Denslow of the U.S. Forest Service, with more than 600 plants screened by 2004 and a finding that the screening system correctly identified 95% of major pests and correctly identified 85% of non-pests.
In 2005, the HPWRA transitioned from being a UH research project to one of the Hawai‘i Invasive Species Council's key prevention projects, where it remains today. Although the HPWRA is the best tool available, the program still relies on voluntary participation, and results are provided for information purposes only.
Please note that the HPWRA results should not be used as primary justification for removing or unconditionally excluding trees or other plants from landscapes, nor should a plant that is deemed high risk for being invasive be interpreted to mean, "do not plant under any circumstances." For example, the HPWRA rates seashore paspalum grass as "high risk," due largely to its ability to spread vegetatively, and the fact that unmanaged plantings in marsh areas have caused harm to native species. However, seashore paspalum is also one of the few turf grasses that can thrive in areas with high salt spray or brackish water irrigation. Thus, the HPWRA provides a picture of relative risk, which should be weighed against community needs and benefits of a species. However, when you have a choice, we urge you to choose a non-invasive.