Introduced Silk Oak Runs Rampant

posted in: September 2000 | 0

Silk oak, also called silver oak or he oak, is not native to Hawai`i – and it’s not an oak at all. More formally known as Grevillea robusta, it is an import from Australia. But you’d never know that to look at Pu`uwa`awa`a. There, as with several other areas in the state, the tree has made itself right at home and has formed dense groves that are spreading across the landscape like the wildfire
The tree is attractive, with its lacy silver leaves and flame-like yellow-orange flowers. But Hawai`i conservationists are little taken by its charm. In every other respect but the aesthetic, the tree has been a disaster.

The silk oak was deliberately introduced to the Hawaiian islands as a source of commercial timber. It has a fine grain that makes beautiful hardwood flooring or cabinets. But the timber is not harvested and, with no natural enemies or commercial market, the trees continue to spread across the landscape.

What went wrong? As it turns out, silk oak is a kind of arboreal poison ivy. When people who are sensitive to its toxicity inhale the sawdust, they can have a strong allergic reaction.

But no one thought of this when, between 1919 and 1959, some 2.2 million silver oak trees were planted on all major islands except Kaho`olawe. Now, as plentiful as the trees are, and as much as the state wants to promote the timber industry, “we can’t give it away,” says Jon Giffin of the Division of Forestry and Wildlife on the Big Island. “It’s a product nobody wants.”

When the state began managing the wildlife sanctuary at Pu`uwa`awa`a, it was full of silk oak. Most of that is gone now. According to Giffin, the tree is that it is “an easy weed to control.” “We had a crew with hand-held power drills and drilled holes in the trunk of each tree. Then we gave it a shot of herbicide. That kills them in a month or two.”

— Patricia Tummons

Volume 11, Number 3 September 2000

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