Quercus rubra var. ambigua

Native

Common

Moderate moisture. Hardwood forests. Moderately shade tolerant when young.

Early May to late May

Green

60′ to 70′

This is a fast growing, deciduous tree rising on a single trunk from a deep, spreading root system. When it is on deep soils it develops a taproot. It is moderately long-lived, often surviving 300 or more years. In Minnesota mature trees are usually 60′ to 70′ tall and 36″ to 42″ in diameter, though individuals can reach over 110′ in height.

In open areas the trunk is short and massive and the crown is extensive, broad, round, and symmetrical. Where it has competition the trunk is long and straight and the crown is small, and round. The branches are stout and widely spreading.

The bark on young trees is gray to reddish brown, smooth, and somewhat shiny. On mature trees the bark is thick and dark gray or grayish-brown. It is broken into long, smooth, flat-topped, pale gray ridges separated by shallow, dark furrows. The long, pale gray ridges have the appearance of ski trails.

Young twigs are slender, bright green, and shiny. As they age they become moderately stout and reddish-brown.

Terminal buds are reddish-brown, shiny, egg-shaped, pointed, round in cross section, and ¼″ to 5 ⁄16″ long. The scales near the tip may have a few brownish, silky hairs at the tip but the buds are otherwise hairless. They are surrounded by a cluster of lateral buds.

The leaves are alternate, egg-shaped, inversely egg-shaped, or elliptic in outline, 4″ to 7″ long, and 3″ to 5½″ wide. They are on hairless, ¾″ to 2″ long leaf stalks that are yellowish with a tinge of red, often mostly red with yellow at the base. The leaf blade is broadly tapered or nearly squared off at the base. There are 2 to 4 large primary lobes separated by U-shaped sinuses and 5 to 18 smaller, bristle-tipped, secondary lobes per side. Most of the sinuses extend less than half way to the midrib. The deepest sinuses extend 45% to 70% of the way to the midrib. The upper surface is dark green, dull, and hairless. The lower surface is yellowish-green or grayish, hairless except for minute tufts of hair in the vein axils, and sometimes covered with a whitish, waxy bloom (glaucous). Young leaves are toothed, not lobed, and are pink and downy when unfurling. In autumn the leaves turn scarlet or deep red, then brown. The name of this tree refers to the color of the autumn foliage.

Male and female flowers are borne on the same branch. Male flowers are in slender, greenish, 1½″ to 3½″ long catkins that hang downward from buds on branchlets of the previous year. Female flowers are bright green and appear singly or in pairs on a short stalk rising from leaf axils on branchlets of the current year. The flowers appear after the leaves in early May to late May.

The fruit is a broadly egg-shaped, ⅝″ to 1″ long, ½″ to ⅞″ wide acorn. They occur singly or in pairs on a short, stout stalk. A shallow, scaly, saucer-shaped cup encloses ⅓ of the lower part of the nut. The cup resembles a beret that barely covers the bottom of the nut. The scales on the cup are flat and the tips of the scales are tightly appressed. The inside of the acorn cup is hairless or has a few scattered hairs. The acorns stay on the tree for two growing seasons. The nuts ripen in mid-August to mid-September of the second year. It tastes very bitter. This is the largest acorn of the oaks found in Minnesota.

Black oak (Quercus velutina) leaves have 2 or 3 primary lobes per side. The acorn is smaller, no more than ¾″ long. The tips of the acorn cup scales are free, not tightly appressed. In Minnesota it is found only in the southeast and in Hennepin County.

Northern pin oak (Quercus ellipsoidalis) has mature bark broken into short, rough, slightly blocky ridges. Terminal buds much smaller and are 5-angled in cross section, not round. The leaf blades are more deeply lobed. Most of the sinuses extend more than half way to the midrib. The deepest sinuses extend 65% to 90% of the way to the midrib. The upper leaf surface is dull. The acorn is smaller, no more than ⅝″ long, and encloses ⅓ to ½ of the lower part of the nut. The kernel is yellow.

Northern red oak (Quercus rubra var. rubra) has acorns with shallower cups that cover only 1 ⁄5 to ¼ of the lower part of the nut.

No information available.