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Northern Red Oak vs Hill's Oak

These two species are occasionally confused where their ranges overlap, but they can be distinguished with skill at any time of year. Northern red oak prefers richer sites and is much more shade tolerant. The two species can hybridize and form intergrades, so it may not be possible to identify all individuals.

Northern Red Oak (Quercus rubra)

Hill's Oak (Quercus ellipsoidalis)

A large tree and one of the major components of the forests of eastern North America, the northern red oak has a wider range than its name may suggest, extending quite far into the "deep south", but also covering a large portion of the northeastern U.S. and into Canada.
A cold-hardy, drought-tolerant oak mostly found in a small region of the upper midwestern U.S. Also called Northern Pin Oak, but more closely related to other species than pin oak.
Leaves have 7-11 lobes. Leaves, especially those growing in lower light, have shallow sinuses between lobes, their depth usually less than halfway the distance to the midrib.
Photo © Aaron Boers, CC BY 4.0.
Leaves have 5-7 lobes. Sinuses between lobes are consistently deeper, on mature trees usually extending at least halfway to the midrib.
Photo © Rob Foster, CC BY 4.0.
Acorn caps usually cover closer to 1/4 of the nut, occasionally as much as 1/3. Acorns usually more stout.
Photo © Chris Kratzer, CC BY-SA 4.0.
Longer, more narrow acorns having caps that cover 1/3rd to 1/2 of the nut.
Photo © Doug Goldman, CC BY 4.0.
Buds less conspicuously angular. More sharply pointed. Terminal buds average longer (4-7 mm) and less pubescent at the tip. More bud scales visible.
Photo © Leila Dasher, CC BY 4.0.
Buds more conspicuously 5-angled in cross-section. More ovoid (blunted or rounded) in shape. Terminal buds average shorter (3-5 mm) and more pubescent at the tip. Fewer bud scales visible.
Photo © Doug Goldman, CC BY 4.0.
Denser, more full crown, reflecting greater shade tolerance.
Photo © Charlie Hohn, CC BY 4.0.
Sparser, more open crown, reflecting lesser shade tolerance and higher light needs.
Photo © Rob Foster, CC BY 4.0.