The European native oak Quercus petraea is an oak of many common names -- Durmast, Irish, Welsh, Cornish and sessile. While related to the English oak (Quercus robur), the Sessile oak has long-stalked leaves and almost stalkless acorns. Both species are members of the white oak group and produce acorns annually.

The genus Quercus includes more than 600 species of the oak tree, of which 90 are native to North America; the Chicago Botanic Garden's collection contains more than 60 varieties and over 1,000 individual trees. Twenty oak species are native to Illinois.

Oaks are slow growing, long lived, hard wood trees that produce fruit we all know as acorns. Within the white oak group, acorns mature annually; in the red oak group, acorns take two years to mature (biennial). Oaks are often imposing shade trees at maturity and provide habitat and food for a variety of wildlife.