According to observations of naturalists and beekeepers.
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Toxic for bees
SOURCE FOR HONEY BEES:
California buckeye is endemic to California. It occurs in the Klamath and Coast Ranges from Siskiyou County south to Los Angeles County. In the Cascade Range and the foothills of the Sierra Nevada, it occurs from Shasta County south to Kern County. California buckeye is occasionally found in the Central Valley in Yolo, Colusa, and Stanislaus Counties.
California buckeye is a large shrub or tree up to 23 feet (7 m) tall. The 2-to 6-inch-long (5-15 cm) leaves are deciduous and palmately compound. Flowers are borne on a terminal panicle 4 to 8 inches (10-20 cm) long. The pear-shaped, light brown fruit contains one to six glossy brown seeds 0.8 to 1.2 inches (2-3 cm) in diameter.
The bark, leaves, and fruits contain the neurotoxic glycoside aesculin, which causes hemolysis of red blood cells.
California buckeye flowers from April to September. The nectar of the flowers is toxic to the Asian/European honeybee, so the trees should not be planted near apiaries.
New leaves emerge from March to June while soil moisture is abundant. Fruits ripen from September to October and are dropped from November to December.