Quercus arizonica

Quercus arizonica is the heavy, pale-barked white oak of the Madrean Sky Islands, the ranges that lift out of the desert in southeastern Arizona, southwestern New Mexico, west Texas, and northern Mexico. Growth is slow and the frame is broad: stout limbs, an irregularly spreading crown, and in time a trunk three feet through on a tree capable of sixty feet where the site allows, though garden specimens usually settle nearer thirty. The leaves give the species away, thick and stiff as harness leather, blue-green above and conspicuously pale and felted beneath, holding through most winters and dropping only as new growth pushes in spring. Old bark goes ashy white and deeply furrowed, which is where the common name comes from. Arizonica is straightforward, though the range runs well past Arizona, through New Mexico and the Trans-Pecos into Chihuahua, Coahuila, Durango, and Sonora, mostly between five and seven thousand feet on rocky slopes and canyon sides. Quercus is the classical Latin name, and this species sits in the white oak group, the lineage whose acorns are comparatively sweet and ripen in a single season rather than two. Hybridizing runs rampant among the Madrean oaks, and Arizona white oak crosses freely with several neighbors, which is a good part of why field identification in that country remains a running argument among botanists. Acorns from the white oak group carry far less tannin than those of the red oaks, and the peoples of the Southwest ground them into meal after leaching, though where there was a choice the acorns of several neighboring oaks were preferred. The wood is dense and burns hot, and Arizona white oak has been cut for fuel and fence posts for as long as anyone has kept records. In the Sky Island woodlands the species is a keystone: acorns feed black bear, javelina, deer, wild turkey, and the Mexican jay, and the broad crowns hold up the canopy of an ecosystem that exists nowhere else on earth. In cultivation this is a tree for gardeners who want something nobody else on the street has and who can offer a hot, open, sharply drained site. Full sun, lean soil, and restraint with the hose once established; the roots resent the steady moisture that suits an eastern oak perfectly well. Hardy through zone 7 and untroubled by heat, drought, or alkaline ground. Expect slow growth and low branching unless you prune for clearance early. One practical note: leaf retention shifts with the winter, so in mild years the foliage holds right through and in hard ones the leaves brown and drop, which is worth knowing before siting the tree where you want dependable winter screening.