The greatest technological invention of the ancient Hawaiians—the voyaging canoe—was made possible by the koa tree, a true native of the Hawaiian forest. When you consider that these canoes were hewn from single trunks, and that some of the old canoes were 150 feet long (or half the length of a football field), you can begin to imagine the god-like scale of the primeval Hawaiian forest.

You won’t find koa trees of that stature anymore, for the great forests that once blanketed the Islands have receded to pockets and high-mountain preserves. Nor will you find men of that stature anymore, men who could fell one of these giants using stone adzes (an ax-like tool with a curved blade at right angles to the handle)…men who could then haul the thing thousands of feet down the mountain, finish the job with adzes and coral scrapers, and finally sail the finished product thousands of miles across the trackless Pacific.

Reforestation has been attempted many times using eucalyptus, Christmas berry, ironwood, paperbark, silk oak, firetree, and numerous other foreign species. But none of these efforts has succeeded in creating anything like the original habitat of the koa.

The leeward slopes of Haleakala, for example, used to be covered with forest—a light, open forest full of diverse species of plants, lichens, insects, and birds. Today nearly all of those native creatures have vanished. This is now rocky grassland, patrolled by cattle that would gobble a koa seedling lickety-split. The air here is often misty, but rainfall is scarce.

What’s missing here is the primary virtue of the koa—its generosity.

Perhaps it sounds irrational to call a plant "generous." But the word seems apt in the light of a botanical anecdote related by Bob Hobdy, the retired state forester whose lifetime of observations "in the field" have made him an icon among the natural scientists of Hawai‘i. Once, Hobdy was dropped by helicopter into the remnant koa forest high on Haleakala, far from roads and trails and chewing cattle. He pitched his tent under a tree. Then it started to rain. Spik spak spok. He stepped out into the open air and there was no rain. Mist, ground-hugging cloud, but no precipitation. And yet, under the koas there it was—spik spak spok.

Koa trees create their own weather.

They do this with their leaves, which are hard, slick blades streaked with parallel grooves. Any mist that condenses on such a blade will slide into the groove, stick to other mist-bits, and form a ball of water that rolls like a marble down the length of the groove to the leaf tip. Spok.

These leaves are falcate—that is, gracefully curved like sickles. This circularity gives koa a distinctive loopy look, as though its gray limbs have been strung with thousands of hoop earrings.

It’s an odd leaf to find on an acacia. Yes, koa is a native Hawaiian variation on the acacia, a very successful genus—1,200 species—in the legume family (Faceaea). Many acacias are drought-tolerant features of the urban landscape. The typical acacia leaf is small and toothbrushy, nothing like koa.

In fact, the juvenile leaves of the koa, which can be seen on sapling twigs and sucker growth, resemble most acacias-pinnately divided into flimsy round leaflets. Koa’s falcate hoop is actually the leaf stem, which sheds its leaflets, flattens and swells into a blade, then starts trolling the air for rain.

Koa trees grow open and loose, leaving plenty of room for companions, which they water with their artificial rain. And they are legumes. Most people know that the pea-and-bean family "fixes" nitrogen in the soil, making it more fertile. Authoritative sources assert that koas also lift water and minerals from the depths of the soil and deposit this nourishment within reach of all forest residents. In other words, they never steal the ground, strangle competitors, or choke the forest into a darkened monoculture the way so many other trees do. They make room and they make life. That’s why koas may accurately be labeled "generous."

The hard wood of koa can be polished to an astonishing red-brown luster, a marble effect that no ancient Hawaiian with his adze ever achieved. This is a prized material of Island woodworkers. Koa floors, walls, cabinets, frames, artifacts—ukuleles, for example—adorn the best houses (including ‘Iolani Palace) as the forests themselves continue to recede.

So it’s pleasant to see so many koas planted these days as landscaping features. They grow fast and easy. If you want to plant them, here’s a tip: Growing in isolation, koas tend to branch out early and get bushy. If you want a tall trunk, do what the old-timers do—overcrowd them at first to force them vertical. Then thin out the weakest and let your giants tower. Give them a couple hundred years, and you’ll be able to cut your own canoe.

When you do, bring an umbrella.