In May 1941, next to news of the Nazi savagely bombing London, The Los Angeles Times published a memorial profile of “California’s Mother of Gardens” — a hopeful antidote to the undoing of the human world, celebrating the woman who covered Southern California with the loveliest trees and flowers, having made a life at the crossing point of nature’s capacity for beauty and human nature’s capacity for delight.

After becoming the first woman to earn a degree in science from Berkeley in 1881, Kate Sessions (November 8, 1857–March 24, 1940) took a job teaching mathematics at a San Diego public school. It was a shock to leave the redwoods for what appeared to her a desert landscape — plants had always been her great love: She had spent her childhood climbing trees and filling her herbarium with wildflowers; as a teenager, she had made an art of elaborate flower arrangements; as a young scientist, she had reveled in the dazzling molecular structure of trees under her microscope. Restless to do something about the dearth of greenery in her new home, she launched her own nursery and flower shop.

But then she dreamt bigger.

Just before her thirty-fifth birthday, Kate persuaded the city of San Diego to let her lease a 30-acre piece of barren public land to use as growing grounds. In exchange, she grew 100 new trees in it each year and gave another 300 to be planted along the city’s streets, around its plazas, in its public school yards. Soon, San Diego was covered in pine, oak, elm, cypress, eucalyptus, and pepper trees.

People rushed to her nursery to populate their own gardens with beauty. Vivacious and warmhearted, clad in her practical working clothes, Kate greeted visitors with a hug and led them through garden — across the miniature meadow of mesembryanthemums the colors of the rainbow, under the fragrant Australian eucalyptus trees never previously seen in California, past the rare heathers from South Africa — excited to show them “the latest plant pet,” as one friend recalled.

She traveled up and down the California coast, searching for beautiful overlooked plants that could thrive in a drought-prone climate, which she cultivated in her nursery and shared with the community. Soon, gardeners all across Southern California were walking through their backyard wonderlands, lovingly touching their proudest plants and saying, “Kate Sessions gave me that.”

In the interlude between the two World Wars, Kate set out to learn about the world’s plants. She traveled to Hawaii and the Alps, drank in the “warm though bracing air and the wonderful blue of the Mediterranean” and admired the cypress-covered mountains of Italy “silvering with lichens on the rugged rocks,” visited the majestic gardens of Versailles and a tiny nursery on the shore of Lake Geneva, a stone’s throw from where Mary Shelley dreamt up Frankenstein.

Kate returned from her expedition with 140 new species, among them the purple-blooming tree now so iconic of Southern California — the beloved Jacaranda mimosifolia, also known as fern tree, which for forty million springs has been gracing sub-tropical South America with its ravishing blossoms.

Throughout her life, Kate corresponded tirelessly with other horticulturalists, hosted tree-planting parties, and wrote hundreds of magazine articles and papers — notes from her foreign expeditions, reports from flower shows, meticulous growing tips for particular plants, from wisteria to Chilean jasmine.

She never married. Her life was animated by her relationship with the Canadian horticulturalist Alice Eastwood, rooted in their shared love of flowers: “our children,” Kate told Alice, “which I am growing and you are naming.”

When San Diego declared September 15 Kate Sessions Day, celebrating her botanical makeover of the city, she responded simply:

I didn’t do it. It was the plants that did it.

Complement her story with that of the Victorian visionary Marianne North, who traveled the world at the risk of her life to revolutionize art and science with her botanical paintings, then revisit this centuries-wide meditation on flowers and the meaning of life and this posy of poems celebrating the delights of gardening.