Bringing back the wildflowers

By Kathleen Wong, UC Natural Reserve System

The road linking the northern tip of Lake Berryessa to the southern shores of Clear Lake is particularly arresting in spring. Climbing through the crinkled hills of the inner Coast Range, it passes bucolic meadows, stands of chaparral, and copses of slender, long-needled gray pines.

But in April and May, one stretch in particular looks markedly different from the rest. At the height of spring, it erupts into a kaleidoscope of color. Wildflowers in jewel tones of purple, pink, and gold form a carpet unparalleled virtually anywhere else in Northern California.

Made up almost entirely of native California wildflowers, these meadows are part of the UC Natural Reserve System’s McLaughlin Natural Reserve. The stunning wildflower displays on its nearly 7,000 acres are the fruits of 15 years of restoration work by reserve restoration manager Paul Aigner and reserve director Cathy Koehler. Their efforts have turned back the calendar on these lands by centuries, to a time before European annual grasses came to dominate California’s open fields.

To accomplish this feat, Aigner and Koehler have pioneered new ways to manage noxious invasive weeds in state grasslands. Their intensive management program combines controlled burns, hand weeding, and the judicious application of narrowly specific herbicides. The work advances the science of restoration while improving habitat for native species and reducing the risk of catastrophic wildfire to boot.

A hardy foe

Aigner and Koehler embarked on their restoration odyssey soon after arriving at the reserve in 2002. The faculty director of the reserve, Susan Harrison, had noticed nonnative barb goatgrass starting to engulf reserve roads and meadows since the mid-1990s.

In a state where most grasslands are now dominated by non-native annual grasses, the success of Aegilops triuncialis isn’t remarkable. But this Eurasian annual stood out for the territory it conquered: the reserve’s outcroppings of serpentine soils.

Formed at high heat and pressure beneath the ocean floor, serpentine soils are toxic to many plants. High levels of heavy metals and magnesium, combined with too little nitrogen and calcium, keep expanses of this green-black rock largely bare.

The presence of serpentinite “has a dramatic, unique effect on the plant community,” Aigner says. Most non-native species haven’t evolved a tolerance to the soil’s unusual chemistry. For this reason, “serpentine is where all the wildflowers are, and serves as a refuge for native species diversity.”

Peculiar habits

Barb goat grass is an exception to this rule. The plant was introduced to California before 1920, possibly as a contaminant in livestock feed. Unpalatable to grazers, and with seeds that stick to the hides of livestock, it was able to establish a toehold in the Central Valley. It has since spread into serpentine-rich areas of the Sierra foothills and Coast Ranges.

On serpentine soils, goatgrass leaves space for native plants to persist. But where soils contain more clay, “it can reach cover areas of 95 percent; it rapidly displaces a lot of the natives,” Aigner says.

The serpentine meadow southeast of the reserve entrance offers a glaring example of of goatgrass growth on different soils. The meadow itself bursts with mountain dandelions, checker mallows, mariposa lilies, and goldfields. But a distinct line at its edge marks the beginning of a field swamped by tall annual grasses. This is the seam of an earthquake fault, which scraped a blob of serpentinite up against more ordinary sedimentary soils.

Sputtered flame

To halt the grass’s march across the reserve, the reserve directors asked CalFire to burn one goatgrass-afflicted meadow in 2004. They were following advice from other UC researchers, who found that two consecutive years of burning were effective at eradicating the weed from the seedbank.

The controlled burn at McLaughlin knocked back the population the following year, just as anticipated. But when Koehler and Aigner attempted to re-burn the next year, the fires refused to catch. Vegetation on the serpentine outcrops was too sparse to carry a flame.

“Without any follow up treatment, we knew the grass would be pretty much at the same density as we started,” Aigner says. “That stymied us for a while.”

A discovery in the herbicide aisle

One day, while shopping for garden supplies, Koehler came across an herbicide that claimed to kill only grass. She bought a bottle, sprayed a small area, and waited. As promised, it killed every annual invasive grass it contacted while leaving native wildflowers, which are broad-leaved forbs, unscathed. It seemed just the holy grail reserve staff were looking for.

That initial euphoria faded when they realized they’d be flying blind. “To that point, there were no descriptions of it being used in wildland management before. There was no scientific literature to base our approach on,” Aigner says.

Advancing restoration science

Undaunted, reserve staff did the necessary research themselves. They set up experimental plots on reserve grasslands to compare four different treatments: grass-specific herbicides (fluazifop and clethodim); glyphosate (often marketed as Roundup); mowing before the goatgrass seeds matured; and hand-pulling weeds.

After two years, the results were nearly as ideal as they could have hoped. Glyphosate killed every plant it touched, including native plants. Mowing helped but was extremely labor intensive. By contrast, the grass-specific herbicides “produced a dramatic resurgence in native species abundance and diversity. Forbs were not affected at all by the herbicide,” Aigner says. Even though the chemicals did hurt native perennial grasses, the plants bounced back fast.

Reassured, Aigner and Koehler drastically expanded their restoration efforts. They bought a truck outfitted with a tank and boom spray, and hired crews of interns to spray, mow, hand weed, and map the extent of the invasion. Aigner, who took on most of the responsibility for the restoration, settled into a spring routine: burn, apply herbicide, weed, and repeat applications every year until no invaders were left.

A long-term commitment

When Aigner realized the burn-spray routine worked, he set his sights sky high. He initially treated not just reserve lands but also neighboring tracts in an attempt to eradicate goatgrass from the watershed. Then he came to the uncomfortable realization that treated areas needed maintenance every year.

Aigner and colleagues have found that goatgrass persists in the seed bank far longer than previously reported. Though perhaps 95 percent of goatgrass seeds germinate within two years, the rest can remain viable for much longer.

 “The amount of time it takes to manage a piece of land doesn’t go down over time from totally infested to almost none left. You spend all of your time killing it at first, and spend all of your time finding it in the end,” Aigner says.

“Now when we’re trying to clean up areas we’ve managed for over a decade, we find most goatgrass comes up where gophers have brought up seeds dormant for years,” Aigner says.

Adaptive management

Aigner now treats a more feasible 200 acres or so of reserve lands annually. “Once we have an area restored, we’re able to maintain that within a larger area infested with nonnative grasses,” Aigner says.

Rather than attempting to eradicate one invasive species across the reserve, he’s now aiming to achieve as close to complete native cover as possible in areas with serpentine soils.

The change in focus accommodates the management of Italian ryegrass, another invasive annual. “We started to worry that if we were not getting rid of ryegrass at the same time, we were just making space for rye to take over new areas.” Thankfully, Lolium multiflorum responds to Aigner’s burning and spraying regime much like barb goatgrass.

Meadow perfection, however, has a cost. “The closer we get to that, the more intensive the work becomes,” Aigner says. “I think of it now almost as a botanic garden given the amount of maintenance it needs.”

For this reason, different sections of the reserve are managed to different standards. In some areas, only goat grass goes; in other places, all annual grasses; and in serpentine meadows, all non-native plants, including yellow star-thistle and hairy vetch get the axe.

Weighing whether to spray

As conscientious stewards of the reserve, Aigner and Koehler have investigated whether herbicides might have unintended consequences on the ecosystem. Wondering how the chemicals affect insects, they compared pollinator populations in treated and untreated plots. They found the only factor that affected native bee diversity and abundance was wildflower availability. To that end, Aigner says, herbicide treatment “offers huge benefits to the bees. It’s creating food resources and nesting habitats.”

Meanwhile, other research has demonstrated that goatgrass has a large effect on California grasslands. An annual that dies after one season, it alters the decomposition and nutrient cycling rates in meadows that were once dominated by perennial plants.

Keeping invasive grasses in check may also offer a hedge against wildfires. A recent study has shown that Western wildlands invaded by non-native annual grasses are up to three times more susceptible to burning than non-invaded areas. This likely holds true in California’s Central Valley, where open meadow landscapes are thought to have remained green year-round before Europeans arrived. And the specter of wildfire at McLaughlin is no joke. In 2015, the Jerusalem and Rocky fires incinerated 100,000 acres in Lake County, including large stretches of McLaughlin reserve.

On balance, Aigner says, “we don’t see any reasons to stop using these herbicides, which have been such powerful tools for us,”

His primary worry now is surviving plants might evolve resistance to the chemicals. Herbicide resistance has emerged before in Italian ryegrass sprayed in farm fields. Aigner and his interns counter this possibility by hand-pulling or mowing any surviving weeds.

The personal toll

Keeping reserve grasslands native is physically grueling. From May into June, Aigner spends his daylight hours driving the spraying rig near treatment areas, then hauling the hose reel up and down the reserve’s steep and rocky terrain. These are the months when goatgrass and ryegrass are flowering and easy to identify. To maximize his spraying time, Aigner seldom travels far from the reserve for the duration.

To stay motivated despite the long hours and backaches triggered by spraying, all Aigner needs to do is look at the acres of wildflowers thriving across the reserve.

Space for native species

“One of our objectives has been to maximize the habitats of rare plants, so populations will occur in more places, and have more places to occupy in a new climate regime as well,” he says.

In fact, “most of our rare plants have been increasing since we’ve been here. We keep finding more and more in areas we’ve been managing.”

These include a number of rare, endemic plants that are serpentine soil specialists. These include pink cream sacs (Castilleja rubicundula ssp. rubicundula) as well as Jepson’s milk vetch (Astragalus rattanii var. jepsonianus), which has been spreading its clumps of violet-tipped flowers along weed-managed portions of the reserve.

The fact that these and other native wildflowers are now rebounding, says Aigner, is a constant reminder of his mission. “I’m trying to increase their abundance and distribution to give them the best chance going into the future.”

Related research

Aigner, P.A. and Woerly, R.J. 2011. Herbicides and mowing to control barb goatgrass (Aegilops triuncialis) and restore native plants in serpentine grasslands. Invasive Plant Science and Management 4(4):448–457.

Case, E.J., Harrison, S., and Cornell, H.V. 2016. After an invasion: understanding variation in grassland community recovery following removal of a high-impact invader. Biological Invasions 18:371–380.