Key to the Pines of the Southeastern United States

Source: Hardin, J. W. 1971. Studies of the southeastern United States flora. II. The gymnosperms. Journal of the Elisha Mitchell Scientific Society 87(2):43–50.

1.	Needles 5 per fascicle, sheath deciduous.	P. strobus
1.	Needles 2-4 per fascicle, sheath persistent; cone scales distally thick and often armed with a prickly umbo.	2.
2.	Annual growth uninodal, with 1 whorl; seed cones subterminal at end of shoot; cone scales unarmed.	P. resinosa
2.	Annual growth usually multinodal; seed cones lateral; cone scales usually armed with a prickly umbo.	3.
3.	Sheath longer than 13 mm; needles 3 per fascicle, 20-50 cm long; foliar buds 15-30 mm long with white fimbriate scales; seed cones (12)16-28 cm long.	P. palustris
3.	Sheath shorter than 15 mm; needles 2-4 per fascicle, 5-25 cm long; foliar buds less than 20 mm long, brownish or gray; cones less than 16 cm long.	4.
4.	Needles 3, very rarely 2 or 4, per fascicle.	5.
4.	Needles 2, or 2 and 3, per fascicle.	7.
5.	Older twigs brown; needles flexible; foliar buds not resinous; seed cone 7 cm or longer, sessile and seldom reflexed, umbos with a stout prickle; seeds usually more than 2 mm thick.	P. taeda
5.	Older twigs orange-brown; needles stiff; foliar buds resinous; seed cone less than 7 cm long, pedunculate and usually reflexed, umbos with small prickle or flat; seeds seldom more than 2 mm thick.	6.
6.	Needles 15 cm or longer; seed cones serotinous, broad-ovoid to globose, usually more than 3.7 cm wide, with a small prickle.	P. serotina
6.	Needles less than 15 cm long; seed cones opening, narrowly to broadly ovoid, usually less than 3.7 cm wide, umbo slightly raised and with a small usually persistent prickle.	P. rigida
7.	Needles over 13 cm long; seed cones 6-16 cm long, not persisting.	8.
7.	Needles less than 13 cm long; seed cones less than 10 cm long, persisting for several years.	9.
8.	Seedlings with erect, slender, pencil-like stems and scattered buds; needles in fascicles of 2-3 on the same tree; opened seed cones usually flattened at base.	P. elliottii var. elliottii
8.	Seedlings with grass stage; needles mainly 2, infrequently 3 per fascicle; opened seed cones usually rounded at base.	P. elliottii var. densa
9.	Seed cones light brown and lustrous, scales armed with a very broad, stout, upwardly-curved prickle.	P. pungens
9.	Seed cones dull brown or gray, scales armed with a slender, weak or stout prickle.	10.
10.	Bark on small branches loose, blackish, flaking off and showing orange-tinged inner bark; few 3-needle fascicles.	P. echinata
10.	Bark on small branches smooth, gray; only 2-needle fascicles present.	11.
11.	Needles to 8 cm long; cones about 5 cm long, with weak or deciduous prickles.	P. glabra
11.	Needles 8 cm or less; cones about 8 cm long, wih persistent slender prickles.	12.
12.	Needles twisted, cones opening at maturity, seed wings about 7 × 3 mm.	P. virginiana
12.	Needles not twisted, cone opening or serotinous, seed wings to 20 × 6 mm.	P. clausa