Flower:

Male and female flowers are borne on the same branch, the male flowers on 2 to 4 inch, green, string-like clusters (called catkins) from bud clusters at the tip of the previous season's growth. 1 to 5 female flowers, with reddish styles and a short, stubby, green stalk, sit in the leaf axils of new growth.

Leaves and bark:

Leaves are simple and alternate, the blades highly variable, often nearly as wide as long (oval in outline), 3½ to 6½ inches long and 3 to 6¼ inches wide, typically with 5 (or 7) primary lobes. For leaves with very deep sinuses, the primary lobes can have three or more secondary lobes, all lobes sharply pointed. Leaves with shallow sinuses tend to have more blocky angles. Mature leaves are dark green and shiny on the upper surface, the lower surface paler with loose, scruffy hairs across the surface and tufts of hairs in the vein axils.

The trunk can be over 2 feet in diameter at breast height (dbh). In closed canopy forests it is tall and straight with a high canopy of branches. In more open areas, the crown is rounder and closer to the ground. The bark is gray to nearly black in mature trees, medium to coarse textured with blocky, vertical ridges deeply furrowed between. Twigs are reddish-brown or greenish-gray, smooth and stout, though new growth can have fine hairs.

Fruit:

Fruit is a nearly round nut (acorn) ½ to ¾ inch long, set in a deep cup covering 50-75% of its length, with flat plate-like scales, loosely fringed at the tips and dense fine hairs lining the inner surface

Notes:

Black oak is an eastern/southern species that barely ranges into Minnesota's SE border counties. Extremely drought tolerant, it persists in our driftless region on droughty slopes and ridges where other tree species cannot compete well. It's highly shade intolerant and in moister locations it is a successional species dependent on fire for regeneration. It may be very difficult to distinguish from Northern Pin Oak (Quercus ellipsoidalis) without mature acorns. Black Oak has fine, dense hairs on the inner surface of the cap, and the outer scales of the cap are loose at the tips, often fold back around the rim. Black Oak also has short fine hairs on the leaf underside that easily rub off. Like with most of the oaks, hybridization can occur with similar oak species.