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)
Northern Red Oak (Quercus rubra) A large tree, one of the dominant components of the forests of eastern North America, native to a wide range of the northeastern U.S. and Canada, extending well into the south.	Hill's Oak (Quercus ellipsoidalis) 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.
Northern Red Oak (Quercus rubra) 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.	Hill's Oak (Quercus ellipsoidalis) 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.
Northern Red Oak (Quercus rubra) 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.	Hill's Oak (Quercus ellipsoidalis) Longer, more narrow acorns having caps that cover 1/3rd to 1/2 of the nut. Photo © Doug Goldman, CC BY 4.0.
Northern Red Oak (Quercus rubra) 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.	Hill's Oak (Quercus ellipsoidalis) 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.
Northern Red Oak (Quercus rubra) Denser, more full crown, reflecting greater shade tolerance. Photo © Charlie Hohn, CC BY 4.0.	Hill's Oak (Quercus ellipsoidalis) Sparser, more open crown, reflecting lesser shade tolerance and higher light needs. Photo © Rob Foster, CC BY 4.0.

References & External Resources

These short lists show only links helpful for ID. For a complete list of references and resources also covering other aspects of ecology, visit the links section of the full article on each plant, which is the first entry here.