Rain brings back moss? Actually, it never left
Dormant plants are springing to life across Northern California
As the winter rain drips down, a bright green furry plant is springing to life all around the Bay Area and Northern California.
Moss is appearing seemingly everywhere — on roads and sidewalks, in gardens, on the sides of trees and the tops of roofs — spurred on by the abundance of damp weather and sunless skies.
Some in the Bay Area have referred on social media to what seems to be the sudden and widespread appearance of moss as an “invasion” or a “takeover,” but biologists and plant experts point out that much of the moss people are seeing has been dormant or otherwise unseen, and some isn’t moss at all.
“This is the time of year when people see them,” said Jim Shevock, a biologist at the California Academy of Sciences in San Francisco who researches mosses. “During the summer, they dry out and turn brown. People can walk by them and not even see them. They’re just waiting for that first rain. They can green up in a matter of seconds.”
While it may seem like a recent intruder, moss is a native California plant, has been around almost forever and is anything but an invasive species, experts say.
Moss is a bryophyte, an ancient type of nonflowering plant that has existed for 450 million years and endured a number of climate changes, according to biologists and botanists. They lack roots, instead using rhizoids to attach themselves to rocks, soil and bark. About 650 varieties of moss exist in California, Shevock said, including about 100 in San Francisco.
Like other plants, mosses produce their own food through photosynthesis, using the minerals and nutrients from water for nourishment. People often mistake — or even name — other plants including algae and lichens as moss. Spanish moss, for example, is a lichen, not a moss.
Mosses help protect soil in areas like recently excavated or bulldozed land or forests that have been ravaged by wildfire, Shevock said. Mosses cover the soil and help prevent erosion.
“Mosses are some of the first things to pop up and turn green in a fire area,” he said. “They’re like sponges. They soak up water. They’re really important in forests.”
Moss may seem like an uninvited guest to a garden party, particularly when it appears on roads, sidewalks, driveways or roofs.
But it does no harm to those surfaces since it has no roots and is simply sitting on top of its host, not digging into it.
“It’s not hazardous,” said Steven Swain, an environmental horticulture adviser for the University of California Cooperative Extension in Marin County. In fact, if a lawn or garden is overtaken by moss, it could be doing a gardener a favor by pointing out a problem.
“If moss is taking over your lawn,” he said, “you’ve got drainage problems. The problem is not the moss, it’s your drainage.”
The only real damage it can inflict, Shevock said, is aesthetic.
“If you have blacktop,” he said, “you don’t want it green.”
Also, moss is often accompanied by algae and lichen and can be slippery, making it a potential safety hazard for people walking on sidewalks or trails.
While moss may have detractors, it also has fans. Moss gardens have become popular among some gardeners, and it’s often used in terrariums or aquariums. Some landscapers use it to decorate rocks or areas in a garden where shade keeps other green plants from growing. Some decorators even create moss murals or walls.
For those who want to get a closer look at moss — and Shevock advises getting just inches away to see the different types — now is the time.
“You’ll find moss everywhere here in the Bay Area, on any trails, walking in any of the woodlands, on Mount Tam,” he said. “Even in city parks, you’ll see mosses everywhere you go — if you start looking.”
“(Mosses are) like sponges. They soak up water. They’re really important in forests.” Jim Shevock, biologist, California Academy of Sciences in San Francisco