Aesculus californica (California Buckeye or California Horse-chestnut) is a species of buckeye endemic to California, and the only buckeye native to the state.
It is a large shrub or small tree growing to 4-12 m tall, with gray bark often coated with lichens or mosses. It typically is multi-trunked with a crown as broad as it is high. The leaves are dark green, palmately compound with five (rarely seven) leaflets, each leaflet 6-17 cm long, with a finely toothed margin and (particularly in spring) downy surfaces. The leaves are tender and prone to damage from both spring freezing or snow and summer heat and desiccation.
The flowers are sweet-scented, white to pale pink, produced in erect panicles 15-20 cm long and 5-8 cm broad. The fruit is a fig-shaped capsule 5-8 cm long, containing a large (2-5 cm), round, orange-brown seed; the seeds are poisonous. The California Buckeye has adapted to its native Mediterranean climate by growing during the wet winter and spring months and entering dormancy during dry summer and fall months; it begins the year's growth in early spring and begins dropping leaves by mid-summer.
This buckeye is found growing in a wide range of conditions from crowded, moist, semi-shaded canyon bottoms to dry south-facing slopes and hilltops. It is also widely distributed in the state, growing along the central coast, in the foothills and lower elevations of the Sierra Nevada, up to 1700 m altitude and in the Cascade range.
In the coastal ranges north of Big Sur it is found growing alone on slopes or intermingled with Valley Oak, Oregon Oak, Coast Live Oak and California Bay Laurel. In the foothills of the Sierra Nevada it can be found standing alone in grassland at the lowest elevations, intermingled in Blue Oak woodlands at intermediate elevations, and in mixed evergreen forests of Black Oak, Digger Pine, Ponderosa Pine and Interior Live Oak as it nears the limit of its range.
The tree acts as a soil binder, which prevents erosion in hilly regions. It is sometimes used as an ornamental. Local native American tribes, including the Pomo, Yokut, and Luiseņo, used the poisonous nuts to stupefy schools of fish in small streams to make them easier to catch. The bark, leaves, and fruits contain the neurotoxic glycoside aesculin, which causes hemolysis of red blood cells.
Native groups occasionally used the nuts as a food supply when the acorn supply was sparse; after boiling and leaching the toxin out of the nut meats for several days, they could be ground into a meal similar to that made from acorns. The nectar of the flowers is also toxic, and it can kill honeybees and other insects. When the shoots are small and leaves are new they are lower in toxins and are grazed by livestock and wildlife.