The Harold L. Lyon Arboretum celebrates its 100th anniversary on Saturday — a momentous occasion for the 193.5-acre oasis deep in Manoa Valley that some kamaaina don’t even know exists.

It’s hard to imagine that the lush green land that the arboretum now occupies was nearly denuded a century ago by farming, free-ranging cattle and unchecked harvesting of koa, sandalwood and ohia lehua. At the time, without a healthy forest to hold it in place, rainwater was flowing to the ocean rather than replenishing the groundwater table, Oahu’s primary source of potable water.

The Hawaiian Sugar Planters’ Association was concerned as it was exploring the possibility of growing sugar, a thirsty crop, there, and it hired Harold Lyon, a professor of botany at the University of Minnesota, to assess the situation. He concluded that the damaged forest could not recover on its own, but if it were replanted, the watershed could be restored.

In 1918, the association reached a verbal agreement with the landowner to purchase 124 acres to test sugar cane along with other species that could be used for reforestation. Planting began that year, and the deed was officially signed in 1919.

Known as Manoa Arboretum, the experiment station was supervised by Lyon, its director, and Edward Caum, also a botanist, who oversaw day-to-day operations as the superintendent. By the late 1940s, the association had completed its research and decided not to grow sugar cane on the site. Lyon envisioned a botanical garden that would preserve the area’s beauty, continue fieldwork and ensure protection of the watershed.

Thanks largely to his efforts, the University of Hawaii acquired the parcel from the association for $1 in 1953, with the stipulation that it would “… use, maintain and preserve the granted premises as an arboretum and botanical garden only.”

Lyon used his own money to cover operating expenses and set aside part of his estate to help fund the arboretum in perpetuity. After Lyon died in 1957, the university’s Board of Regents renamed the arboretum for him.

For two decades, the property was maintained as a research center, closed to the public. Monthly tours began in 1973 and the community-based support group Friends of Lyon Arboretum was established a year later.

In 1999, the university received 69.5 acres for Lyon Arboretum from the City and County of Honolulu in exchange for a parcel near Dole Street and East-West Road. Today, the arboretum is renowned as a cultural, educational and scientific resource with more than 7 miles of hiking trails and 5,000-plus species of tropical and subtropical plants from around the world.

Annual rainfall averages close to 14 feet, keeping its 10 themed gardens vibrant year-round. Flourishing in addition to familiar species such as ti, bamboo, ginger and heliconia are dozens of lesser-known plants with interesting histories, physical characteristics and traditional uses.

For example: The arboretum’s bo (fig) tree is a direct descendant of the tree under which Buddha sat to attain enlightenment. The talipot palm lives 30 to 80 years before producing the largest inflorescence of any plant (up to 26 feet tall, comprising several million flowers), then dies. The hapuu fern produces pulu (a soft, woolly material at the base of the fronds) that ancient Hawaiians used as bandages, to stuff pillows and mattresses and to embalm the dead.

Nature lovers could spend hours admiring the botanical treasures, even the smallest fern and flower so precious when you consider Hawaii has 1,400 native plant species, nearly 90 percent of which are endemic (found nowhere else). Hawaii also has the dubious distinction of being the “Endangered Species Capital of the World,” with 366 plant species listed by the U.S. Fish & Wildlife Service as threatened or endangered (808ne.ws/2Lv25E7).

That’s a sobering fact. As Lyon Arboretum enters its second century, its mission — “to inspire and cultivate the conservation of tropical plant biodiversity and connect it to Hawaii’s culture through education and research” — is even more important and meaningful.

Cheryl Chee Tsutsumi is a Honolulu-based freelance writer whose travel features for the Star-Advertiser have won several Society of American Travel Writers awards.