California Buckeye :

California Horse Chestnut
Aesculus californica
NATIVE

Description (Jepson, PlantID.net)

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 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)
  - At the tip of the inflorescence, a few bisexual flowers produce pistils (female flower parts), in addition to stamens (male flower parts)
    - Look for flowers with an exserted (extending beyond petals) central pistil
    - Only 1-3 fruits are produced per inflorescence
  - 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

Distribution

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)

Wildlife
- 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
- 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
- Fruits were used for fishing
  - Crushed fruits were thrown in slow-moving creeks or pools to stun fish, aiding in their capture (Anderson and Roderick 2006)
  - Soap-like saponins in the seeds enter through the gills and interfere with oxygen absorption and transport (Rosenthal 2014)
  - Fruits of common snowberry (Symphoricarpos albus var. laevigatus), the roots of California manroot (Marah fabacea), and the bulbs of soap plant (Chlorogalum pomeridianum var. pomeridianum) were similarly used for fishing
- 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

Adaptations

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 (rolling down slopes), water (carried down streams), and squirrels
- 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 as the seed does not survive desiccation
Summer deciduous
- Leaves turn brown and fall beginning in late spring/early summer 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
Destroyed by fire, but the tree will re-sprout from the root crown when the top is consumed

Notes

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
Is the California buckeye a kind of chestnut? Yes, but it’s not a “true chestnut”
- True chestnuts are classified in the genus Castanea, in the Oak family (Fabaceae)
  - Genus includes the European sweet chestnut (Castanea sativa) and the American chestnut (Castanea dentata)
  - Fruit is edible
- California buckeye is classified in the horse chestnut genus (Aesculus) in the Soapberry family (Sapindaceae)
  - Genus includes the ornamental horse chestnut (Aesculus hippocastanum) and the Ohio buckeye (Aesculus glabra)
  - Fruit is poisonous
The largest recorded buckeye grows in Swanton Pacific Ranch in Santa Cruz County: 46 ft. tall, with a trunk circumference over 14.5 ft., and a 60 ft. canopy
- Registered as a California Big Tree
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 native tree at Edgewood has palmately compound leaves
Check out this short video (Jepson 2019) for more ID tips

At Edgewood

Found in woodlands
- See iNaturalist for observations of this plant
Flowers April – June

See General References

Specific References

Anderson, K. and W. Roderick. 2006. California buckeye Aesculus californica (Spach) Nutt. Plant Guide. United States Department of Agriculture, Natural Resources Conservation Service, National Plant Data Center c/o Plant Science Department, University of California, Davis, California.

Eaton, J. 2008, Oct. 1. Fall of the buckeye ball. Bay Nature.

Garvey, K.K. 2011, May 18. Lovely to look at but…. Bug Squad. University of California Agriculture and Natural Resources (UC ANR).

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. United States Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

Jepson Herbarium. 2019, Nov. 19. Aesculus californica (California buckeye) [Video]. The Jepson Videos: Visual Guide to the Plants of California. The Regents of the University of California. YouTube.

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. UC Master Gardeners of Butte County, University of California Agriculture and Natural Resources (UC ANR).

Raiche, R. 2009. California buckeye: A tree for all seasons. Pacific Horticulture. Pacific Horticultural Society.

Ridgeway, S. 2019. Aesculus californica – California buckeye. UC Master Gardener Program of Sonoma County. University of California Agriculture and Natural Resources (UC ANR).

Rosenthal, S. 2014, Jan. 13. The many uses of spring-blooming soaproot. Bay Nature.

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