Jack pine is a small- to medium-sized pine tree with a scrubby, irregular growth habit.
Leaves are needles, ¾–1½ inches long, in bundles of two; stiff, twisted and dull dark green.
Bark is thin, reddish-brown to dark gray, breaking into scaly plates.
Twigs are orangish-brown to reddish-brown, often with a white waxy coating; turning gray to reddish brown with age.
Conifers do not technically "flower"; pollen is shed March–May.
Fruits are cones, 1¼–2 inches long; tan, light brown or yellow-brown; narrowly ovoid; curved or arched; scales not shiny, mostly without spines but sometimes with a small, curved spine near the tip. Cones usually remain closed on the tree for many years.
Similar species: Missouri has only one native pine species, the shortleaf pine (Pinus echinata). The other five pine species included in our flora (including jack pine) are non-natives that are commonly planted in timber plantations, for wildlife habitat, for erosion control, or as ornamentals: Austrian pine (P. nigra), eastern white pine (P. strobus), loblolly pine (P. taeda), and scrub pine (P. virginiana). These species frequently produce cones and reproduce themselves within their populations, thus they can become naturalized locally and are counted as part of our state's flora. Other pines are grown only as ornamentals or on Christmas tree farms and do not reproduce on their own, so they are not considered part of our flora; these include ponderosa pine (P. ponderosa), red pine (P. resinosa), and Scotch pine (P. sylvestris). The bottom line is, unless you are at an old home site or at a place where the non-native pines have been cultivated and might persist on a local scale, the only type of pine you will encounter in the wild in Missouri is almost always the shortleaf pine.