Location, location, location. Alcatraz Island, situated in the middle of the entrance to the San Francisco Bay, has always been prime real estate. In 1846, the military recognized a fortress strategically placed on the island would protect the bay against the threats of the impending Civil War.
On the naturally bare island with only a few sparse grasses, soil was needed to hold the cannons in place ringing the steep cliffs. In 1869, soil was imported from nearby Angel Island and with the soil came seeds of native plants: Baccharis pilularis (coyote bush), Eschscholzia californica (California poppy), Rhamnus californica (coffeeberry), and spores from several types of ferns.
Recent winter rains have caused the island to erupt into a lush green oasis after the dry summer months. Ferns and moss drape over walls, staircases and down hillsides.
There are four species of ferns that grow well on the island: Polypodium scouleri (leathery polypody), Polypodium californicum (California polypody), Polystichum munitum (western sword fern), and Pentagramma triangularis (goldback fern).
All of these ferns thrive in moist sea air. The leathery polypody and the western sword fern are evergreen, while the California polypody and goldback fern are only appreciated during the rainy winter months.