Aesculus

A large, stately deciduous shade tree from the Balkan mountains of southeastern Europe, long planted across cool-temperate parks, avenues, and large lawns for its dramatic spring bloom and dense summer shade. In May it covers itself in upright, candle-like panicles of white flowers blotched yellow then pink, carried above big, coarse, palmately compound leaves of five to seven leaflets. By autumn it drops spiny green husks that split to release glossy mahogany-brown seeds — the 'conkers' of British schoolyard tradition. It is grand but high-maintenance and strictly ornamental: all parts, and especially those tempting shiny seeds, are toxic to people and livestock, and the tree is plagued by leaf blotch and the horse-chestnut leaf miner that brown the foliage by late summer. Do not confuse it with the unrelated, edible sweet chestnut (Castanea sativa) — that name overlap is a genuine and dangerous source of poisonings.

A garden hybrid (Aesculus hippocastanum x A. pavia) grown for its showy, upright panicles of rose-red to pink flowers in mid-spring, set against bold, dark-green, palmately compound leaves. It makes a tidy 30-40 foot medium shade or specimen tree with a dense, pyramidal-to-rounded crown. Its main selling point over the common horse chestnut is health: red horse chestnut is far less prone to the leaf blotch and leaf scorch that brown out and disfigure A. hippocastanum by late summer, so the canopy holds up better through the season. The flowers draw bees and, with their narrow rose-red tubular florets, hummingbirds. One load-bearing caution applies: like all Aesculus, the smooth brown seeds (conkers) and other parts are toxic if eaten, so site it thoughtfully where there are children or pets. It is a low-seed-set hybrid, not weedy or invasive.