Olea europaea on the island in Regan Hall Circle.
Olive trees (Olea europaea) grow all around Davis. Most people are familiar with the green or black pickled fruit, and often don't recognize olives out of the can. In Davis, olive trees can be found growing along Olive Drive and Russell Blvd, where they stain the ground black in the Autumn. There's also a big olive tree right in front of City Hall.
Although olive trees are not native to this region, they have been grown here since 1842. The Wolfskill family is credited with introducing the trees; they brought the first cuttings up from their Los Angeles ranch to start an orchard at Rancho Rio de los Putos. Part of this orchard was eventually donated to UC Davis, and olive trees are still grown there to this day under the name of Wolfskill Experimental Orchard. UC Davis actually harvests a lot of the olives that drop from these trees, bottles it up, and sells high quality olive oil! You can also buy quite good, reasonably local, olive oil at the Farmers Market.
The fruit needs a long period of heat to ripen, making Davis a perfect habitat. Olives are bitter in their natural state, and are only edible after a long treatment process (often involving lye) that removes the natural alkaloids. The University of California's Division of Agriculture & Natural Resources publication 8267 "Olives: Safe Methods for Home Pickling" details the process. The trees can reach up to 50 feet in height. Olive trees' foliage is narrow, pointy, and grayish-green.
For info on other local plants see Town Flora.