Australian tree ferns: Plant possible invasive concern
by Carolyn Lucas
West Hawaii Today
clucas@westhawaiitoday.com
Wednesday, March 18, 2009 9:07 AM HST
Though prized for its distinctive looks and rapid growth, the Australian tree fern is becoming an increasingly common and alarming fixture in Kona neighborhoods, according to a recent islandwide survey by the Big Island Invasive Species Committee's early detection crew.

On the underside of its triangular fronds with lacy blades are millions of spores, which breezes -- as well as feral ungulates -- carry far and wide. This tree fern (Spahaeropteris or Cyathea cooperi) is already displacing native forests on Kauai, Maui and Oahu. Watersheds are also being choked. Such propagation is just beginning on the Big Island, said Page Else, spokeswoman for the Big Island Invasive Species Committee.

"The tree fern crowds out our native hapuu, the mother of the forest. The soft fibers of the hapuu nurture young native seedlings of native plants like ohia and olapa," she said. "The Australian tree fern has a harder, scaled trunk which doesn't make a good home for seedlings. Consequently, the foreign plant can take over in our forests, changing the forest structure, lessening the number of native plants and decreasing the ability to capture water and provide good water quality in watersheds."

It's no wonder that the Australian tree fern appears on a list of the most invasive plants in Hawaii, prepared by the Hawaii State Alien Species Coordinator and the Department of Land and Natural Resources' Division of Forestry and Wildlife. Officials urged people to not cultivate any of the listed species anywhere in Hawaii.

The Big Island Invasive Species Committee recently issued an Australian tree fern alert, spreading awareness about the weed and hoping that residents will help prevent its spread. People should report tree ferns by calling the state pest hotline at 643-7378.

Those who have purchased the popular ornamental but suffered buyer's remorse and now want to remove it can either snip the bud or cut down the tree fern and treat it with herbicide.

"They should be aware that their tax dollars will be put to work combating the rapid growth of this plant in public watersheds as seeds spread out of their yard into other locations," Else said.

Two years ago, the Kauai Watershed Alliance contracted the Nature Conservancy to coordinate a response to the invasive tree fern. Crews on foot are sent into valley uplands to remove plants at a cost of $365 per acre. Helicopters with herbicides spray in more remote, dangerous areas. The aerial spraying costs $560 per acre, Else said.

James Leary, invasive weed specialist at University of Hawaii at Manoa College of Tropical College Agriculture and Human Resources, has been experimenting with an innovative way to apply herbicide selectively to Australian tree ferns by using paintball guns with a special pesticide formulation.

While people may be fond of the tree ferns, a wide variety of noninvasive alternatives, including the native hapuu, can be used as ornamentals. Because each yard is different, residents can visit a nursery and talk to an expert to find the best fit.

Many nurseries -- including 15 on the Big Island -- agreed not to sell the tree fern after learning its potentially detrimental impact.

For more information, call the Big Island Invasive Species Committee at 933-3345.

The future forests of Kauai and probably all the islands will have no shortage of these ferns. Sadly, they can form near monotypic stands where the massive amounds of leaf litter from fallen fronds smother most understory life and create much debris (and far more than the Hapu'u).

Perhaps if they cut most, if not all, the tree ferns down in peoples Kona yards, they could buy a few decades. I have seen them popping up on the Hilo side even in remote areas. Once I noticed one while in the Hapu'u forests.

You can kill them by dicing up the growing top. Make sure you chop down until you get in the core.

I disagree with feral ungulates carrying spores (The spores are everywhere in the air) but do notice that open dirt, as found in pig diggings, under houses, and in yards raked to the dirt, tend to have more seedlings coming in. They are extremely- and I repeat- extremely shade tolerant. I have them growing under my house among other places.

They are easily distinguished from Hapu'u once you get the eye for it.