California Buckeye

California Buckeye © TCorelli

California Horse Chestnut
Aesculus californica

Description (Jepson,

    • Eudicotyledon
      • Eudicots are a major lineage of flowering plants; see family for general characteristics
    • Soapberry Family (Sapindaceae)
    • Summer-deciduous tree or large shrub
    • Often multi-trunked, with a rounded, broad crown
    • Smooth, silver gray bark
    • Leaves
      • Opposite (2 leaves at each junction with stem)
      • Palmately compound (separate leaflets radiating from a single point) with 5 leaflets
      • Margins finely-toothed
      • Soft fine hairs on leaf surface
    • Flowers
      • Inflorescence (flower arrangement) is an erect, showy, 4-8 in. long column, “candelabra-like”
      • Fragrant, white to pale rose, 4-petaled flowers
      • Unisexual and bisexual flowers on the same inflorescence
        • Most flowers in each cluster are male
          • Look for the exserted (extending beyond petals), orange-colored anthers (pollen-producing part of the stamen/male structure)
        • Only a few flowers at the tip of the panicle are fertile and will produce fruit (Raiche 2019)
          • Look for the exserted (extending beyond petals) central pistil (female flower parts)
      • Ovary superior (above the attachment of other flower parts)
    • Fruit is a fig-shaped, leathery capsule (a dry, multi-chambered fruit that splits open at maturity)
      • Contains 1 large (≤ 2 in.), shiny brown seed
    • Height to 13-40 ft.
    • Lives 50-150 years


      • Native to California
        • Grows on slopes and in canyons in foothill woodlands, mixed evergreen forests, coastal sage scrub, and riparian areas
        • See Calflora for statewide observations of this plant
      • Outside California, grows in southwestern Oregon
      • Grows at elevations to 5,600 ft.

      Uses (San Mateo County Parks prohibits removal of any natural material)

        • Not extensively browsed by wildlife
          • Only wildlife known to eat the fruit are ground squirrels, which are not found at Edgewood (Anderson and Rodrick 2006)
          • Toxic to livestock (Howard 1992)
        • Source of nectar and pollen for hummingbirds and numerous insects, including bees and many butterfly species, especially migrant butterflies in late spring (Hinsta 1992 and Ridgeway 2020)
          • Toxic to non-native honey bees, causing “buckeyed bees” (Taylor 2013 and Ridgeway 2020)
            • Adult bees develop paralysis and die
            • Pollen consumed by the queen may produce deformed offspring and decrease or stop egg production, ultimately destroying the colony
        • Native people had many uses for California buckeye
          • Bark used for toothaches and as a poultice to treat snake bites
          • Fruit applied as a salve for hemorrhoids
          • Fruit used for food when acorns not available
            • Requires extensive leaching of toxins and tannins before grinding and making into a mush
          • Mashed fruit used to catch fish in still water
            • Soap-like, toxic saponins interfere with the functioning of gills, killing or incapacitating the fish (Eeckhaut 2015)
          • Wood made into bowls and bows, and into drill sticks for making fire
        • CAUTION – All parts are toxic (bark, leaves, stems, fruits, seeds, etc.) from glycosidal compounds; ingestion of large quantities of the seeds can be fatal
        Flower Stalk (L), Male Flower (LM), Female Flower (RM), Fruits (R) © DSchiel (L, R), FPeale (LM, RM)

        Name Derivation

          • Aesculus (ES-kew-lus) – from the Latin for an oak with edible acorns, applied to this genus by Carl Linnaeus in 1753
          • Buckeye – the brown seed peers through the split leathery covering, resembling a buck’s eye


            • Largest seed of any native California plant (Eaton 2008) and of any non-tropical plant species (Lukes 2019)
              • Matures on the tree through the summer and early fall, when the capsule withers to release it
              • Dispersed by gravity (they’ll roll down hills) and water (they’re carried down streams), rarely by animals
              • Packed with food and water, as well as toxins, to give the seedling a quick, safe start
                • Watch for sprouting buckeye seeds on the woodland floor after the rains begin in the fall
              • Germination must occur within a few weeks of the fruit’s fall
                • Seed will not survive desiccation
              • Destroyed by fire, but the tree will re-sprout from the root crown when the top is consumed
            • Buckeyes are a very old lineage of trees
              • Paleocene fossils indicate they were present in the American West just after the time of the dinosaurs (Eaton 2008)
              • Their range has constricted, but they have survived by adapting to a drier, hotter climate
            • Leaves turn brown and fall beginning in late spring/early summer (summer deciduous) to conserve water
              • California buckeye is one of the first trees to put out new leaves when the winter rains return
            • Deep taproot is another adaptation to dry summers and drought
            • The largest recorded buckeye grows in Swanton Pacific Ranch in Santa Cruz County: 46 ft. tall, with a trunk circumference of 14.5 ft. and a crown spread of 60 ft. (registered as a California Big Tree)
            • Is the buckeye a kind of chestnut?
              • True chestnuts are in the genus Castanea, in the Oak family, and have edible fruit
              • Buckeye is in the same genus and family as the ornamental horse chestnut (Aesculus hippocastanum), and their fruit is poisonous
            • Host to the fungus-like microorganism Phytophthora ramorum, which causes Sudden Oak Death (SOD)
              • At Edgewood, the 2 species known to be highly susceptible to SOD are coast live oak (Quercus agrifolia var. agrifolia) and Pacific madrone saplings (Arbutus menziesii)
              • For a complete list of known hosts and host associates see USDA Risk Analysis for Phytophthora ramorum, pp.6-9

            ID Tips

              • No other tree at Edgewood has palmately compound leaves
              • Check out this short Jepson video for more ID tips

              At Edgewood

                • Found in woodlands
                • Flowers April – June

                See General References

                Specific References

                  Anderson, K. 2006. Plant Guide: California Buckeye. U.S. Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS), National Plant Data Center.

                    Eaton, J. 2008, Oct. 1. Fall of the Buckeye Ball. Bay Nature.

                      Eeckhaut, I., et al. 2015. Effects of Holothuroid Ichtyotoxic Saponins on the Gills of Free-Living Fishes and Symbiotic Pearlfishes. University of Chicago. Biological Bulletin, vol. 228:3.

                        Garvey, K.K. 2011, May 18. Lovely to Look at But…. Bug Squad Agriculture and Natural Resources, University of California, Berkeley, California.

                          Hinsta, K. 1993. Watching Butterflies on Mount Diablo. Mountain News, Spring-Summer, Mount Diablo Interpretive Association, Walnut Creek, California.

                            Howard, J.L. 1992. Aesculus californica. Fire Effects Information System. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

                              Lukes, L. 2019, Mar. 22. Finding an Ecological Niche:  A Three-Part Series on Selected Foothill Woodland and Chaparral Species, Part 3 of 3: The Buckeye. The Real Dirt Blog.

                                Raiche, R. 2009. Pacific Horticultural Society. California Buckeye: A Tree for All Seasons.

                                  Ridgeway, S. 2019. Aesculus californica-California Buckeye. University of California Master Gardener Program of Sonoma County.

                                    Taylor, C. 2013. California buckeyes know what to do in summer dry spell – hibernate. Bay Nature.