By Jan TenBruggencate
Advertiser Columnist

How can you tell whether a plant is likely to become an invasive pest?

Researchers in New Zealand and Australia have developed a test that ranks potential problem plants, and a Hawai'i team last week announced it had modified it to meet local conditions. The Weed Risk Assessment uses a range of criteria to help determine whether a particular species is likely to become a widespread weed in the Hawaiian ecosystems.

One of those factors is how well it reproduces. It might be that it produces lots of viable seed, like the forest pest miconia, or that its vegetation propagates itself very quickly, like Lake Wilson's aquatic fern Salvinia molesta. Not all plants are so virulent. Plumeria occasionally produces seed, but you can pretty much count on your tree staying put and not reproducing unless you break off a chunk and stick it in the ground.

Another simple test is whether a plant grows well in the Islands, which can host many kinds of plants, since we have many climate conditions, from hot and dry to dark and wet, from salty sand dunes to deep fertile soils, and from dense forest to sparsely vegetated alpine heights.

University of Hawai'i botany professor Curt Daehler, who helped create the Hawai'i Weed Risk Assessment system, said that "an important indication of whether or not a plant will become a pest here is if it is a pest elsewhere in similar environments."

Miconia has taken over vast stretches of former native forest on Tahiti, where the climate is similar to Hawai'i. Salvinia is a problem weed in waterways across many of the Southern states and elsewhere around the world. Plumeria, on the other hand, isn't much of a problem anywhere.

Daehler said that when the weed assessment team put the three plants cited here through their tests, they ranked as you might expect. Salvinia ranked as a superweed, with a score of 29. Miconia got a severe weed rating of 14. Plumeria came in at a safe negative 5.

A plant that scores under 0 is generally not invasive. The higher it scores, the bigger the potential problem.

The researchers hope the new ranking system will be applied to the many plants that might be imported into Hawai'i, so that the dangerous ones can be flagged and turned back at the border.

"If we were to ask these simple questions about a plant before inviting it into our state, we could save ourselves so much trouble," said Christy Martin, of the statewide Coordinating Group on Alien Pest Species.

Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Reach him at (808) 245-3074 or jant@honoluluadvertiser.com.