Trees, rarely shrubs, 3-52 m. Bark gray or brownish, smooth with fissures in younger trees, becoming ridged and sometimes deeply furrowed or exfoliating with small platelike scales or long strips or broad plates. Twigs greenish, orangish, reddish, or rusty brown, or bronze, terete, slender or stout, pubescent and scaly or glabrous; leaf scars shield-shaped or 3-lobed, large; pith solid and homogeneous. Bud scales valvate or imbricate, glabrous or variously pubescent; axillary buds protected by pair of valvate bracteoles (i.e., prophylls) or bracteoles fused into hood. Leaves odd-pinnate; petiole pubescent and/or scaly or glabrous. Leaflets 3-17(-21), petiolulate, distal leaflets largest, 2-26 × 1-14 cm; surfaces abaxially with nonglandular hairs (unicellular common to all species, fasciculate with 2-8 rays in 1 rank, multiradiate with 8-17 rays in 2 ranks) and glandular scales (capitate-glandular and large peltate scales common to all species; small peltate scales round, irregular, or 2- or 4-lobed), adaxially with scattered hairs and scattered to abundant scales in spring or concentrated along midrib and secondary veins to essentially glabrous in the fall. Staminate catkins in fascicles of 3 (except sect. Rhamphocarya of se Asia) from 1st-, sometimes 2d-year twigs, sessile or pedunculate; stamens 3-10(-15) per flower, with or without hairs. Pistillate flowers in terminal few-flowered spikes. Fruits nuts enclosed in husks, compressed or not compressed, husks completely or partially dehiscing, sutures smooth or winged; nuts brown, reddish brown, or tan, sometimes mottled with black or tan, compressed or not compressed, angled or not angled, smooth, rugulose, or verrucose; shells thin or thick. Seeds sweet or bitter. x = 16.
e North America, Mexico, e Asia.
Species 18 (11 in the flora).
Carya was widespread during the Tertiary; fossils have been reported from the states of Colorado and Washington, and from China, Japan, Europe, and western Siberia. Today two sections of the genus occur in southeastern Asia (sect. Rhamphocarya and sect. Sinocarya) and two in North America (sect. Apocarya, the so-called pecan hickories, and sect. Carya hickories). Both 2n = 32 and 2n = 64 chromosome numbers are known for the genus; tetraploidy, however, is confined to sect. Carya.
The commercial use of Carya is substantial. The cultivated pecan, C. illinoinensis, is the most important nut tree native to North America, and the wood o hickories is unequaled for its use in tool handles because of the combined strength and shock resistance. Hickory nuts are also an important, high-quality food source for wildlife because they are high in proteins and fats. Carya cordiformis, C. glabra, and C. ovata are grown extensively in central Europe for timber.
Characters of the buds and bark are taxonomically important in Carya, but shoots with terminal buds and information about bark characteristics are frequently absent on herbarium specimens. Phenotypic variation from tree to tree is often considerable and difficult to quantify. Most of this variation undoubtedly results from adaptation to local and regional conditions; hybridization has probably played a subtle role as well. Sympatry of two or more species is common, and artificial pollinations suggest that even diploid × tetraploid crosses produce viable seed.
|1||Scales of terminal buds valvate; axillary buds protected by pair of valvate bracteoles or by bracteoles fused into hood; leaflets (5–)7–13(–17), symmetric or falcate; staminate catkins at base of leafy shoots on new wood, and commonly on reduced shoots from old wood; husk sutures winged; shells thin or thick; seeds sweet or bitter (sect. Apocarya).||> 2|
|1||Scales of terminal buds imbricate; axillary buds protected by bracteoles fused into hood; leaflets 3–9, symmetric; staminate catkins at base of leafy shoots on new wood, rarely on reduced shoots from old wood (C. texana); husk sutures usually without wings, infrequently with narrow wings (C. floridana, C. glabra, C. texana); shells thick; seeds sweet (sect. Carya).||> 5|
|2||Bark fissured or exfoliating, separating freely into long strips or broad plates; terminal buds ovoid; leaflets (5–)7–9, abaxially with dense coating of peltate scales throughout spring and fall, bronze color; shells thick; seeds sweet.||Carya myristiciformis|
|2||Bark ridged or exfoliating in long strips or platelike scales; terminal buds oblong; leaflets (5–)7–13(–17), abaxially with light to dense coating of peltate scales in spring, becoming moderate to light in fall, not bronze color; shells thin; seeds bitter or sweet.||> 3|
|3||Terminal buds sulfur yellow to brown; axillary buds protected by pair of valvate bracteoles; leaflets (5–)7–9(–11), symmetric, abaxially large peltate scales retained, concentrated near margins of base and apex on fall specimens; fruits not compressed or only slightly so; husks dehiscing to middle or slightly below; nuts not compressed or only slightly so, not angled, rugulose; seeds bitter.||Carya cordiformis|
|3||Terminal buds yellowish brown to reddish brown or black; axillary buds protected by bracteoles fused into hood; leaflets (5–)7–13(–17), symmetric or falcate, abaxially large peltate scales mainly lost by fall or at least not concentrated near margins of base and apex; fruits compressed or not compressed; husks dehiscing to base; nuts compressed or not compressed, angled or not angled, smooth or verrucose; seeds bitter or sweet.||> 4|
|4||Bark exfoliating in long strips or platelike scales; leaflets (5–)9–11(–13), margins serrate to entire; lateral petiolules 0–2 mm; midribs adaxially villous near base; fascicles of male catkins pedunculate; nuts compressed, angled, verrucose; seeds bitter.||Carya aquatica|
|4||Bark ridged or with appressed scales or exfoliating with small platelike scales; leaflets (7–)9–13(–17), margins serrate; lateral petiolules 0–7 mm; midribs mostly adaxially glabrous, rarely hirsute near base; fascicles of male catkins sessile or pedunculate; nuts not compressed, not angled, smooth; seeds sweet.||Carya illinoinensis|
|5||Leaflets (3–)5(–7), serrations with hairs tufted below apex, and at least some hairs persisting into fall; bark exfoliating in long strips or broad plates; fruits spheric or nearly so; husks thick, dehiscing to base.||Carya ovata|
|5||Leaflets 3–9, serrations sometimes lightly ciliate, subapical tuft absent; bark ridged, often deeply furrowed or exfoliating in long strips or broad plates; fruits spheric to obovoid; husks thin to thick, dehiscing partially or completely to base.||> 6|
|6||Twigs stout; terminal buds 8–20 mm; leaflets (5–)7–9(–11), abaxially hirsute, with abundant unicellular, fasciculate, and multiradiate hairs; husks 4–13 mm thick; nuts strongly angled toward stylar end.||> 7|
|6||Twigs slender; terminal buds 3–15 mm; leaflets 3–7(–9), abaxially glabrous except near midrib, occasionally hirsute with unicellular and fasciculate hairs, never with multiradiate hairs; husks 2–5 mm thick; nuts not strongly angled toward stylar end.||> 8|
|7||Bark exfoliating in long strips or broad plates; petiole and rachis lightly pubescent; leaflets apically acuminate, tapering, abaxially hirsute with abundant unicellular and fasciculate hairs, occasional multiradiate hairs; husks minutely hirsute.||Carya laciniosa|
|7||Bark ridged; petiole and rachis densely hirsute; leaflets apically acute, abaxially hirsute with low density of unicellular hairs and high density of fasciculate and multiradiate hairs; husks rough, glabrous.||Carya tomentosa|
|8||Terminal buds 4–10 mm; leaflets (5–)7(–9), abaxially with dense covering of small 4-lobed, irregular, and round peltate scales in spring.||> 9|
|8||Terminal buds 3–15 mm; leaflets 3–7(–9), abaxially with sparse to dense covering of small irregular and round peltate scales in spring, 4-lobed scales uncommon.||> 10|
|9||Petiole and rachis hirsute with scattered fasciculate hairs and hairs concentrated adaxially near leaflet insertions; leaflets abaxially with inconspicuous scales, large silvery tan peltate scales dense; mainly e of Mississippi River.||Carya pallida|
|9||Petiole and rachis with few hairs, hairs not concentrated adaxially near leaflet insertions; leaflets abaxially with conspicuous small rusty brown scales, large silvery tan peltate scales infrequent; mainly w of Mississippi River.||Carya texana|
|10||Terminal buds 3–9 mm, densely scaly, golden brown to rusty; leaflets 3–7, margins coarsely serrate, surfaces abaxially without small round, dark brown peltate scales; fruits obovoid to oblong, bronze to dark brown; husks dehiscing to base; c Florida.||Carya floridana|
|10||Terminal buds 5–15 mm, sparsely scaly, reddish brown to tan; leaflets (3–)5–7(–9), margins finely to coarsely serrate, surfaces abaxially with small round, dark brown peltate scales; fruits obovoid to spheric, tan to reddish brown; husks dehiscing to base or only partially dehiscent; throughout e United States.||Carya glabra|