Bay Nature magazineSpring 2004


Serpentine Splendor

April 1, 2004

“Different hues of yellow, orange, blue, purple, pink, even white—who can count?!—and that’s just color. The variety of form—so many ways to be a flower—even one species of flower. Right here before me are—how many?—two dozen species? It’s literally stunning, like a peaceful concussion that stops me in my tracks, immobilized, in a stupor…”

Perhaps my spring notebook entry says as much about me as about the location, Edgewood Park and Natural Preserve. But I’m not alone in falling head-over-heels in love with Edgewood. Many devoted others saved it, steadfastly preserve it, and labor to restore it. You could say that Edgewood is being loved back to life.

A treasure any time of year, Edgewood dazzles in springtime. Profusions of blooming goldfields indeed transform green fields to gold. Tidytips carpet the hills a paler yellow. Patches of contrasting flowers create a mosaic effect. Over the months, colors shift in slow waves and by June, clarkias, or farewell-to-spring, tinge the browning fields pink.

Tucked up against Interstate 280 and flanked by suburban Redwood City and San Carlos, Edgewood is small, just under three-quarters of a square mile. But this modest sanctuary in San Mateo County shelters two kinds of abundance, both striking. Abundances of certain species produce the famous wildflower sheets. Another, more fragile abundance contrasts with those carpets—remarkable species diversity. On a single well-timed walk, you might see as many as 100 species of blooming plants. Docents lead hikes every spring weekend to help point them out. The great majority of Edgewood’s species are native, and among these are nearly a dozen rare or endangered plants. A threatened butterfly, the Bay checkerspot, made its home here too, until recently, and perhaps will again soon.

Why do so many rare species hang on at Edgewood? What fuels the deep, contagious devotion among Edgewood’s volunteer stewards? Some answers lie in strange soils, botanical diversity, and a preciousness born of scarcity.

Edgewood’s diversity begins with its geology—not just the lay of the land, but the very composition of it. The process of plate tectonics deserves much of the credit for the lovely landscape and vegetation. Indeed, you can see the rift of that great geologic mixer, the San Andreas Fault, just west of the preserve at the base of the Santa Cruz Mountains. The colliding North American and Pacific Plates created the jumble of rocks known as the Franciscan complex, which dominates Edgewood. Different rock types strewn about contribute to Edgewood’s diversity, but serpentine is the celebrity. (Technically, serpentine is the name for a mineral group, and serpentinite is the rock. But serpentine is often used for both.)

Serpentine is a mottled, greenish-gray, waxy-feeling rock named for a resemblance to snakeskin. It originates below the earth’s crust, from the upper mantle. Wet oceanic sediments diving beneath a neighboring land mass create the conditions for the chemical addition of water to heavy mantle rock. Softer, more slippery, and more buoyant, the resulting rock squeezes like toothpaste upward along fault lines. Subsequent slicing and mixing raise it to the surface. Appropriately, it’s the state rock of seismically active California, even though serpentine covers only about 1 percent of the state. Edgewood, however, is nearly 40 percent serpentine.

Edgewood rises to the west, from around 300 feet of elevation at its eastern edge near the main entrance at the Day Camp, to a rolling serpentine plateau at roughly 600 to 750 feet. Even the serpentine soils vary. Above the serpentine, the greenstone (metamorphosed basalt) of the preserve’s central ridge rises to 872 feet. Trails are generally well graded, and wide enough to be easily navigable, yet narrow enough that you feel immersed in your surroundings.

Tidytips, with their white-tipped petals, form thick carpets in April and May. Photo by Fran Cox.

The land accommodates four general vegetation types, which subdivide further into nine plant communities, some of them as unique as the soils they inhabit. Edgewood is nearly half grassland, and most of that is serpentine grassland with its characteristic rich native diversity. Woodlands cover a little over a third of the preserve, mostly on north- and east-facing slopes and in the canyons. Shrublands—chaparral, coastal scrub, and mixed serpentine chaparral—occupy about 15 percent of the area, mostly on south- and west-facing slopes. Finally, the small remainder is wetland.

The park’s famous diversity stems from the quirks of serpentine. During the earth’s geological evolution, many chemical elements sweated out of the mantle into the crust and atmosphere. Mantle rock is thus enriched in iron and magnesium, but lacks many elements now common in the crust. Serpentine inherits this unusual composition, and soil derived from it challenges plants with a chemical double whammy. It’s extremely low in elements that plants need lots of—calcium, nitrogen, phosphorus, and potassium. Plus, it’s much too high in magnesium, of which plants need only a little. It may contain other heavy metals, too, such as nickel, chromium, and cobalt, which can be toxic. Serpentine soils also tend to be crumbly and shallow, so they dry out quickly, essentially shortening the growing season. Nonnative plants (and many California natives too) just can’t hack it on serpentine soil.

But over evolutionary time, some natives have adapted to the inhospitable soil. The first problem is surviving with few nutrients. It’s not that they do a lot with very little. Rather, they do the minimum possible with very little—they succeed by staying small. Natives may also be better at gathering nutrients. Plants require much more calcium than magnesium, and normal soils usually provide that. But serpentine soils drastically reverse the ratio. Within plant tissues, excess magnesium can substitute for calcium, taking its place, but not doing the right jobs. There isn’t a lot of research on this, but two studies, one on the common goldfield, suggest plants compensate by absorbing calcium more efficiently. Plants employ several strategies for handling excess metals. In some, root tissues exclude unwanted metals in the first place. Other plants accumulate, taking up nickel for instance, and sending it to outer leaf layers where it deters insects.

Because of such adaptations, serpentine soils favor natives and diversity. This serpentine time machine preserves islands of native life, offering glimpses of what California might have looked like before European grasses took over.

If you take the southern arm of the Sylvan Trail, you gain good eastward vistas almost as soon as you climb up out of the woodlands. You can see Bair Island near the Port of Redwood City, the Bay itself and the San Mateo and Dumbarton Bridges, then the East Bay Hills and Mount Diablo. Soon you’ll reach the Serpentine Loop Trail, which circles the ridge, traversing both greenstone and serpentine soils. It passes some of the best wildflower areas, demonstrating both kinds of abundance. It’s no coincidence that these are the places where the trail meanders well onto serpentine soil.

At the northwestern end of the loop, between the junctions with the Edgewood and Franciscan Trails, tidytips often form thick carpets in April and May. Goldfields peak about a month earlier. Owl’s clover, blue dicks, bicolor lupine, and poppies provide contrast. Short side trips along the Franciscan or Edgewood Trails could add sun cups, cream sacks, and other flowers to your list.

But if you’re seeking diversity, head for the southeastern end of the Loop, near the two junctions with the Clarkia Trail. The vegetation here is sparser, but more diverse, and that’s probably not a coincidence either. You’ll still see goldfields, tidytips, owl’s clover, and poppies—and much more.

Larkspurs’ tall dark blue spikes usually make them easy to spot. (Royal blooms earlier than western, but they overlap, so you can compare.) Cream cups appear in good numbers, March to April. Small-flowered linanthus (Linanthus parviflorus) whitens large patches on both sides of the trail from April to June. Look for tiny blue grassland gilia beside the trail from March to June. Dwarf brodiaea always amazes me, a delicate blue-violet lily sitting on the ground above a barely visible plant.

A few plants, called serpentine endemics, grow only on serpentine. Edgewood has seven, and the most accessible occur at this end of the preserve. In May and June, follow the Serpentine Loop just past where it cuts back on itself heading to the northeast and look for state and federally threatened dwarf flax (Hesperolinon congestum). Some rare serpentine linanthus (Linanthus ambiguus) blooms March to June near the lower Serpentine/Clarkia junction. Leather oaks are easy to spot near the upper Clarkia junction. You won’t see the last naturally occurring population of the endangered San Mateo thornmint (Acanthomintha duttonii) because it’s nowhere near a trail—probably the best protection.

Some plants, called serpentine indicators, are mostly, but not strictly, confined to serpentine. Purple mouse-ears is an example. Its proud little form often appears on what looks like bare rock. It blooms from February to April, but only one flower at a time, each lasting only a day or two. Some natives can thrive on or off serpentine, such as clay mariposa lily and purple sanicle. But on serpentine, they often appear dwarfed, with smaller, reddish leaves.

Unique plant communities often provide havens for unique animals, and that’s true at Edgewood. Or was in the particular case of the Bay checkerspot butterfly. (Two rare spider species seem to be holding on.) Several checkerspot populations thrived here until recently, but all have died out. The butterfly was last seen in 2001 at the west edge of the preserve. Traffic on nearby I-280 killed them, indirectly, in a disastrous collision of smog and diet.

Bay checkerspot butterflies
Bay checkerspot butterflies, like this one near Mount Hamilton, were once plentiful at Edgewood. Photo by Stuart Weiss.

The butterflies sip nectar from a handful of native plants, mainly tidytips, goldfields, lomatium, linanthus, and muilla. The caterpillars munch California plantain (Plantago erecta), a humble plant less than 6 inches high with silverish papery flowers. Purple owl’s clover comes in a distant second. They’re very picky eaters, according to consulting ecologist and Bay checkerspot expert Stuart B. Weiss, who has studied the butterfly for over 20 years.

The plantain had been holding its own on the serpentine soils at Edgewood. But in recent years, Italian ryegrass (Lolium multiflorum) has invaded and crowded out the plantain, and with it the Bay checkerspot. How? Weiss has shown that vehicle exhaust essentially fertilizes the soil with nitrogen, usually low in serpentine soils, destroying the native advantage.

The butterfly is gone here, but Weiss hasn’t given up, and his habitat restoration experiments suggest ways to fight back, with help from the remaining healthy checkerspot populations surviving in Santa Clara County. Well-timed mowing can tip the balance back in favor of the plantain. Mowing can occur after the natives’ seeds ripen, but before the nonnatives’ do, preventing reproduction. It’s relatively easy and inexpensive, and necessary only every few years. If all goes as hoped, Weiss may reintroduce the butterfly in 2006.

Threads of similar vision and devotion run through the tapestry of Edgewood’s recent history and the efforts to save it. The hills and serpentine soils here were ill-suited for agriculture, so the land escaped development, although people did live here, near the current Day Camp. During the mid- to late-1960s, I-280 was built, slicing prime serpentine habitat in two. In 1967 the state bought the parcel for a state college, but that project was soon dropped, and the years of neglect, abuse, and struggle began.

Poorly maintained signage and fencing led to dumping, woodcutting, and off-road vehicle (ORV) abuse. Thirty-year-old ORV scars still linger. Various proposals for recreation complexes also targeted the area, some with multiple golf courses. Golf course proponents persisted, even after San Mateo County acquired the property in 1980.

Fortunately, during the late 1960s, a concerned citizen, Susan Sommers, recognized something extraordinarily special here. She began documenting Edgewood’s rare species and diversity, and she started the fight to protect it. The California Native Plant Society eventually became involved, and in the mid-1980s it took legal action on behalf of the endangered species. More than 40 organizations formed a coalition to preserve Edgewood. Finally, in 1993, the San Mateo County Board of Supervisors declared Edgewood County Park a Natural Preserve Area. The land was safe, and the Friends of Edgewood was born that same year to help keep it that way. Now the healing could begin.

If you visit Edgewood any Friday morning, or on a Wednesday evening during daylight-saving time, you might see small groups of people crouching off-trail, combing through the vegetation. Part of what’s special about Edge-wood is what you don’t see, and you can thank these people, the “weed warriors.” Some of them have tended Edgewood for over a decade. Under expert guidance, and with a permit issued to CNPS, they remove invasive plants that threaten Edgewood’s habitats. (Normally, no one is allowed to go off-trail or pick or disturb anything at Edgewood—it’s a preserve. You’re welcome to join guided weeding parties, but do not try to “help” by pulling plants on your own.)

Students pull Italian thistle
Though serpentine soils keep many exotics at bay, dedicatedvolunteers play an essential role in keeping others out. Here, highschool students from Belmont pull Italian thistle in May 2002. Photo byKathy Korbholz.

This specialized weeding balances a complicated equation that considers the weeds’ invasiveness, preciousness of habitat encroached upon, time of year or life cycle, available equipment, and number of volunteers. Edgewood’s program targets more than two dozen pests, and some are now considered under control. You won’t see mature French broom, teasel, or bristly oxtongue, but they once ran rampant. That’s partly why the weeders keep at it—they know they’re making a difference.

Edgewood’s weeders have averaged about 2,000 volunteer hours a year for several years. Other Friends of Edgewood projects generally double that tally. Something about Edgewood just fosters contagious dedication. Botanist Toni Corelli worked on Flowering Plants of Edgewood Natural Preserve part-time for a decade because she thought Edgewood deserved its own book. Now her book, and the many knowledgeable devotees, help make Edgewood a great place to learn about native plants.

The next big dream is an interpretive center, a project well into the planning stages. Inside, visitors will encounter the seeming paradoxes of a small, suburban preserve harboring an abundance of rarity—a place where harsh soils favor magnificent wildflower displays and stunning diversity. Here, the land itself, as well as devoted people, thwarts invading plants. It’s a remnant island shaped by geotectonic and social forces—including a disrespectfully placed highway that both threatens the preserve and makes it easy to visit. And as more visitors come to appreciate Edgewood, I’m certain that many more will also fall in love.

Getting there:

By car: Interstate 280 to Edgewood Road exit. Turn east on Edgewood Road (toward San Carlos and Redwood City). Proceed approximately 1.5 miles. The main park entrance is on the south (right) side of the road.

By bus: SamTrans Bus #261, which runs weekdays only, can get you from Caltrain in San Carlos to within walking distance of the preserve. Get off at La Mesa Drive, and walk southward a half mile down Crestview Drive to Edgewood Road. The park entrance is just to the east. Check schedules with San Mateo County Transit, (800)660-4287;

About the Author

Science writer Carolyn J. Strange has written for local and national publications, including a couple of years as a weekly columnist.

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