Aesculus hippocastanum

Pladias, the Czech flora database, gives May as the flowering month and classes the tree as insect pollinated with white flowers.

The Bavarian forestry institute LWF rates horse chestnut as flowering in April and May with good nectar and pollen supply.

The species is native to the mountains of the Balkans in south east Europe; the Bavarian LWF describes this natural range as small and strongly disjunct, a relict of its former distribution.

The pollinators are honey bees and bumblebees; solitary bees such as Osmia and Andrena and hoverflies also visit the flowers.

Yellow guide spots on the petals attract pollinators; after pollination the spots turn red, which insects find unattractive.

In Poland a flower secretes on average 2.64 mg of nectar, with a comparatively low sugar content of about 25 to 32 percent, of which about 90 percent is sucrose.

An inflorescence produces a total of about 1 mg of pollen, at a rate of 0.5 mg per day.

The Bavarian LWG rates horse chestnut 3 of 4 for nectar and 3 of 4 for pollen, calling it a good April and May pollen source with highly concentrated nectar.

LWG notes that the buds supply bees with much propolis resin and that horse chestnuts are bumblebee pastures.

LWF finds no concrete evidence that horse chestnut is an invasive problem neophyte, although seedlings establish near plantings.

Horse chestnut is a neophyte in Central Europe: Clusius received the first seeds from Constantinople in Vienna in 1576, and the tree has been cultivated here since the end of the 16th century.

The horse chestnut leaf miner Cameraria ohridella is one of the main pests and diseases affecting the tree in Europe; seeds of infested trees are 40 to 50 percent smaller.

The EU forest atlas reports the species as near threatened at the European scale and vulnerable in Greece and Bulgaria; unprocessed seeds are poisonous.