Found nowhere else in the world,
Aesculus Californica is a true California Native.

In late winter and early spring, the California Buckeye blooms with long strands of sweet smelling flowers. This early blooming season is a unique adaptation of the plant to its environment.
Another unique adaptation is when it goes dormant, during the arid summer months.
In Nevada County, (Calif.), Buckeye bushes are found at lower elevations, in grasslands, and growing near oak trees and Ponderosa Pines.
Some beekeepers transport their bees to the valley when the Buckeye blooms because the flower pollen and nectar contain alkaloids. Alkaloids are toxic to honey bees. It’s not only the flowers that contain toxins, but the fruit, leaves, and shoots too.
Nisenan Tribal Member Processing Buckeye Nuts
Native American Uses for Buckeye
Native Americans used the plant’s poison to their advantage. They ground nuts or hulls into a powder, throwing it into pools in the creek. Fish in those pools, stunned by the water additive, floated to the surface where they could be scooped out. (Toxins didn’t transfer to the person eating the fish.)
In seasons where acorns were not plentiful, Native Californians ate Buckeye nuts. Processed like acorns, bitterness and harmful substances are leached out before eating.
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Resources:
Calflora – California buckeye, California horse chestnut
CalPhotos – Aesculus californica, California Buckeye
Pacific Horticulture – California Buckeye: A Tree for All Seasons (excellent photos)
Primitiveways.com – Fishing with Poisons
SFGATE – Not all plants are safe for bees – honey bee & California Buckeye
UC Master Gardener Program – California Buckeye
USDA – Natural Resources Conservation Service – California Buckeye PDF
Wikipedia – Aesculus californica