It all started when a 25-pound rock landed on the lanai.

Roberta and Arthur Sprague had lived in their Manoa home since the mid-1960s, perched high on a hill with a sweeping view of the valley. Their backyard was an overgrown hillside of tangled vines that hid a series of crumbling rock walls.

That rock, the size of a basketball, had tumbled down from the slope, breaking a tree branch and pot as it landed in a rectangular reflecting pool on their back lanai about three years ago.

“We didn’t know what had done it until we looked in the pond,” said Roberta Sprague. “Anyways, we decided this is not acceptable. So we called some landscape contractors.”

To prevent further erosion and rockfalls, the Spragues decided it was time to get the steep hillside landscaped. The first task was to clear away a thick mat of invasive trees, including haole koa, octopus trees and fiddlewood. Anthony Ortiz, owner of Greenspace Hawaii, was one of the contractors they called. His business specializes in native Hawaiian plants and maintains the garden at Manoa Heritage Center.

As the hillside was cleared, Ortiz saw a blank canvas that he and his team filled with native plants of various textures and shades of green. Now the quarter-acre hillside is covered in kokio kea (white hibiscus), hapuu ferns, palapalai ferns, uki uki (a native lily), sugar cane, pili grass and coastal naupaka.

Two poinciana trees that Arthur Sprague, 85, grew from seeds anchor the setting, along with three kukui trees and native koa that were planted as saplings.

Roberta Sprague, 84, a former volunteer docent at Lyon Arboretum, no longer climbs up the hillside, but enjoys surveying their yard every morning.

“I love the kokio,” she said. “I think that’s why we have so many of them.”

THEIR HOME was designed by architect Hart Wood in the late 1920s to maximize the view of the valley and take advantage of the cooling tradewinds. It was a fixer-upper when the couple bought it 50 years ago as a place to raise their two children and to allow for Arthur Sprague, then an anesthesiologist, to be closer to the Queen’s Medical Center. They added a covered, wraparound lanai that affords a view of Diamond Head.

For their first 11 years on the property, there was no driveway, and they had to climb 81 steps to get to the house. Roberta Sprague said it was a hike they enjoyed when they were younger. Today visitors drive up a steep driveway flanked by two lion sculptures. And while they also used to climb the steep hillside in the back, which ascends higher than the rooftop and abuts a natural forest reserve, they now are content to observe the garden from a window.

Surrounding their lanai is a bed of colorful anthuriums, popcorn orchids cradled in a plumeria tree and hapuu ferns. Papaya trees, herbs and tomatoes grow in containers. Situated in two generous-sized pots are native moa, a sprawling fern named after the forest chicken because the fronds look like cockscombs.

“I wanted different colors and textures,” said Ortiz, “and the kokio are touching the mao hau hele (yellow hibiscus) touching the iliee so that there’s not a blank spot.”

The goal was to completely carpet the hillside to avoid any open spots that could encourage erosion. The project was labor-intensive, he said, and took about two years to complete.

Native shrubs, including iliee, a low, sprawling plant with light-green foliage and clusters of small, white flowers, help control erosion. The plant grows in thatches, which works well for catching loose rocks, according to Ortiz. Another is akia, with small, oval-shaped leaves. Both are hardy plants requiring little water.

“I wanted to leave something that’s going to be here long past us, and the status of native plants is really bleak, so I wanted to make sure it’s always going to be here,” Ortiz said.

Reach Greenspace Hawaii via email at greenspacehawaii@gmail.com.

“Garden Party” spotlights Hawaii’s unique and exceptional gardens. Call 529-4808 or email features@staradvertiser.com.