The leaves of Bur oak are green, sinuate, up to 15 cm (5.9 in) long and up to 13 cm (5.1 in) broad. The leaf margin is smooth.
The flowers form greenish - yellow drooping catkins. Flowering and leafing sprout together.
Big acorns, up to 5 cm (2 in) in diameter. The fruit cups wrapped half the acorn.
The branches are brown, rough, thickened at the end. The buds are small and brown.