Plants Facilitate Transition to Higher Water

This winter, crews with the nonprofit Save The Bay weeded more than two dozen acres of land near Bedwell Bayfront Park in Menlo Park. The workers were maintaining a “habitat transition zone,” sometimes called an “ecotone” or a “horizontal levee.” It’s an area where ecosystems meet and meld. In Menlo Park, it’s where rehabilitated tidal marshes merge with the land. 

These transition zones, built from packed dirt and rocks and vegetated by organizations like Save The Bay, provide a natural buffer against erosion, as well as habitat for local wildlife.

“The ideal situation is that we get the right amount of native plant material out onto the slope during the right time of year, and we only have to do like three to five years of weeding and maintenance,” says Save The Bay’s Habitat Restoration Director Jessie Olson.

A vegetating project’s location, size, and function dictates which species are planted at a site and how. For smaller lots, volunteers and staff will handplant specimens grown in nurseries around the Bay, says Olsen. But for the larger transition zones, this approach can be too expensive and time intensive. In Menlo Park, Save The Bay hired a local farmer to mechanically churn plant material into the soil. Native seed mixes, made up of species like tarweeds and fiddlenecks, are also scattered all over the site. 

This year, Save The Bay will be vegetating a 16-acre habitat transition zone that separates a tidal marsh and a landfill in Mountain View. 

But no matter the technique or the location, the primary goal is the same: to increase the likelihood that perennials — plants whose life cycles last multiple years — settle on the slopes. Even better is when those perennials are rhizomatous, meaning they propagate through their underground stem system. Local examples include some species of pickleweed and western ragweed. 

“They are able to tolerate a lot more drought and tough conditions, possible erosion,” making them particularly useful in protecting infrastructure like trails, says Olson. “They really hold these slopes together and take up space and provide habitat to native annual and insect species year-round.”

Their adaptation ability also means they’re better at withstanding sea level rise, as they can grow up the slope, Olson says.

But perennials can take a few years to establish themselves,  so annuals — plants that last a single year — are also added to the slopes to combat the spread of invasives in the short term. Still, the slopes need to be maintained and weeded of persistent non-native species, such as mustard and Russian thistle, for the first few initial years to give the native species a chance to take hold. 

And though most of the species planted thrive in the Bay’s brackish water, projects like Palo Alto’s wastewater treatment horizontal levee introduce water that’s significantly less salty to the slope, so freshwater plants are needed. The result is species native to the tidal marsh growing alongside plants from riparian wetlands like sedges and willows, according to Olson.

To give native plants their best shot at flourishing across any project, they’re typically spread on slopes in the fall, when they’re “naturally dormant” and there’s less rain, Olson explains. Then the wet season comes, and they start putting out new shoots.

“Once they get healthy root systems, they’re set,” Olson says. “They can take off.” 

Top Photo: Save The Bay staffer Danielle Perez Garcia lays out containers of native species for planting on a habitat transition zone in the Ravenswood pond complex by Bedwell Bayfront Park in Menlo Park. Courtesy Save The Bay.