hippocastanum – Latin name for horse-chestnuts (from the Greek hippos, horse + kastana, chestnut)

Native range: S.E. Europe

Leaves:

• opposite
• palmately compound
• no stipules
• very large, long petioles
• large, sessile leaflets
• leaflets obovate
• serrate veins are impressed in leaflets, making them look corrugated (ridged)
• stomata only on the abaxial (underside) leaf surface

Flowers:

• inflorescence – terminal upright panicles
• white with yellow spots (yellow changes to pink after pollination)
• male flowers at top of panicle, with hermaphrodite flowers below
• in hermaphrodite flowers, stigma mature before anthers

Fruit:

• 3-valved capsules
• large (1”)
• has spines
• seed inside is shiny

Other characteristics:

• deciduous tree
• 50-75 ft.
• large winter buds
• large leaf scars – horseshoe with nails
• lenticels on branches
• branches sweep down, then up

Relevant info:

• poisonous seeds and leaves
• introduced in Western Europe (to England ~400 yrs ago), Western Russia, and Eastern U.S. and Pacific NW
• common street and shade tree in towns and urban areas (e.g., Ravenna Blvd)
• tolerant of heavy metals, radioactive elements, and air pollution, which enhances its appeal in urban areas
• planted populations are vulnerable to invasive leaf-miners (larvae of leaf- miner moths)
• naturalizes in PNW
• can be an invasive species in forests
• nuts (seeds) are known as “conkers” in Great Britain and are used for a game: attached to strings, two conkers are attached to a cord and competitors flick their conker at the other until one breaks; world championship is held each October in England
• common name reflects misconception that tree is closely related to American chestnut and other spiny-cased seed-producing members of Castanea (Fagaceae, beech family), which produced edible nuts

Ecology & Adaptations:

• native to the broadleaf and mixed (broadleaf and conifer) mountain forests of the Balkans (SE Europe between the Adriatic (Mediterranean) and Black Seas), Turkey, and Turkmenistan
• drought tolerance:
  • juvenile trees are drought-sensitive due to minimally developed root systems
  • older trees are moderately drought tolerant, with deep tap roots that can reach low water tables
  • resin on the “sticky buds” is thought to help increase resistance to drought
  • bark has a small proportion of lenticels containing embedded waxes, which may reduce water loss
  • in native range, seed germination may occur in the autumn coinciding with autumn rain, which may give seedlings an advantage in allowing establishment and growth before summer drought the following year
  • proteins in seed cells may help prevent dehydration over winter, and may also protect against cold stress
• pollination:
  • large bees (e.g., Bombus spp.), honeybees, hoverflies, and solitary bees are attracted to sucrose-rich nectar
  • insects tend to work their way upwards on a panicle from female, to hermaphrodite, to male flowers, helping to reduce self‐pollination
  • stamen become erect when shedding pollen, presumably as a mechanism for aiding pollen removal by pollinators
  • anthers secrete lipid droplets that may act as food bodies
  • pollinators are attracted to flowers by yellow floral guide spots on the petals, which turn to red, and nectar and scent productions are greatly reduced or stopped after flower is pollinated
  • large amount of pollen makes wind pollination viable
• seed production – large quantities every 2 years in native region
• seed dispersal:
  • via rodents, including squirrels that carry them to caches (storage piles)
  • gravity and moisture result in consistent occurrence of horse‐chestnut along mountain streams and rivulets in native Greek populations
• vegetative reproduction:
  • re-sprouts from damaged stumps
  • new shoots occasionally grow from adventitious buds on the roots of established trees
  • branches that touch the ground may root and produce new shoots, especially on rocky and steep slopes in its native region
• herbivory defense:
  • multiple compounds (glycoside aesculin, saponin aescin, possibly alkaloids) serve as defense against herbivory (e.g., insects) by eliciting physiological responses (e.g., paralysis, vomiting)
  • spiny capsule and tough seed coat provide physical protection of seeds
  • chemical defenses also protect against fungal and bacterial infection
  • highest concentrations are in seeds
• shade tolerance – juvenile trees are somewhat shade tolerant, with young stems containing chlorophyll and being capable of photosynthesizing
• succession – dense canopy of mature tree castes deep shade late into the growing season and gives this species a competitive advantage in its native region and open woodlands it invades outside its native region
• mycorrhizal fungi on root tips aid nutrient and water uptake