Probably few of the drivers zipping along Highway 101 just south of San Francisco pay much attention to a large, hilly expanse west of the freeway, although it’s notable for what it doesn’t contain — buildings, cars and concrete.

Since urbanization has destroyed most of the “Franciscan bioregion” (the natural environment between SFO and the Golden Gate), the 3,400-acre San Bruno Mountain remains that region’s largest, richest open space, and it’s an indigenous habitat for many endangered species.

“It’s hidden in full view,” says San Francisco biologist Joe Cannon. “Most people don’t realize that there are trails, canyons and a lot going on there.”

One of the most important activities on the mountain — much of it a state and county park — is a program to protect and restore the area’s biodiversity, led by Cannon and scores of passionate volunteers involved with the environmental nonprofit San Bruno Mountain Watch.

Buy native plants

The centerpiece of this decadelong effort is Mission Blue Nursery in Brisbane. Named for one of the mountain’s three endangered butterfly species, it just might be the best place in the Bay Area to buy native plants — not the widely available generic California natives, but flora that is “locally adapted,” Cannon says. “A lot of people don’t realize there’s a lot of variation in ecotypes.” The ones on the mountain thrive in the fog or heat or other elements of its microclimates, where some other plants might not.

According to Cannon, the plants available at Mission Blue Nursery include many “hard to kill” natives. Seeds for these plants are gathered on the mountain and propagated by brigades of volunteers, who also remove weeds to give the natives breathing room.

Since the mountain is surrounded by urban areas, non-native weeds are a continuing problem, Cannon says. Volunteers dig out invaders such as pampas grass, fennel, aster, mustard, the broom family and Himalayan blackberry.

His crew aims for “long-term sustainability,” he says. “The goal is to get the first few invasives, and keep coming back and get a few each year. If you wait 10 years, you’ve lost the site.”

Threatened species

The stewardship effort depends upon people who care about protecting the mountain’s 13 rare and endangered plant species, as well as the endangered butterflies that feed on them.

The threatened-plant list includes the fuzzy-leaved, sweet-scented coast rock cress, which bears pretty, purple flowers; the rare San Francisco wallflower, whose linear leaves send up stems with clusters of cream-colored flowers; and the hyper-local shrub Montara manzanita, with its deep red stems and gorgeous, seasonal cone-shaped clusters of pink and white flowers.

“In some of these areas where we’ve gotten rid of the invasives, in spring (the view of plants in bloom) can be really breathtaking,” says Chuck Heimstadt, a South San Francisco resident who, with wife Loretta Brooks, has been removing weeds from the mountain for several years.

Though the official volunteer schedule calls for weeding twice monthly, Heimstadt and Brooks trudge onto the mountain with their weeding tools daily. “We figure we’re getting more exercise than when we were jogging, because the mountain is so steep,” Heimstadt says with a chuckle.

Armed with their favorite weeder, the hand mattock, he and Brooks have cleaned up acres of land on the mountain. According to Heimstadt, “If you weed around a little poppy plant (so) it doesn’t have competition on all sides, it will grow to 2 feet in diameter — a giant plant. But if it’s surrounded with grasses, it’ll stay the size of a softball.”

Work is never done

Although their work is never done, the couple finds satisfaction in the effort. “It keeps me going,” Heimstadt says. “I remember what some of these areas looked like before we started. One thing I wonder is if there will be people picking up when we leave off.”

Enlisting long-term volunteers is an objective of San Bruno Mountain Watch, which uses proceeds from plant sales to raise much-needed funds aimed at protecting the land from encroaching development.

Cannon says “politics” have steadily weakened the Endangered Species Act of 1973, and developers now have various avenues to destroy wildlife habitat legally.

How this has played out on San Bruno Mountain can be seen in a housing development on the hill overlooking the nursery, which was built on butterfly habitat. There, the only sign of these colorful creatures is the streets named after the endangered butterfly species whose space they appropriated.

San Bruno Mountain Watch will continue to fight against such incursions in the future.