Bur Oak

Size, growth and what to expect

Bur oak grows slowly, about a foot a year in our growth estimator. Shelterbelt trees in the northern Great Plains grew about as much. Expect 60 to 80 feet with an equal spread. On the best bottomland soils it grows 80 to 100 feet tall, while on dry western bluffs it may stay a shrub. Many trees live 300 to 400 years.

It flowers from April in the South to mid-June in the North as the leaves open, and the acorns ripen the same year, dropping from August to November. Trees seldom bear before about 35 years, then give good crops every 2 to 3 years and go on bearing as late as 400. Each acorn is 1 to 2 inches long, biggest in the South, with the cup covering half or more of it. Red squirrels, deer, black bears, wood ducks, cottontails and mice eat them. The Cheyenne ate them too, in a mush with buffalo fat.

It grows wild from New Brunswick and Maine west to southern Manitoba and eastern Montana, south through the Great Plains to Texas, and east to Alabama, Tennessee and Maryland.

How to grow Bur Oak

Bur oak wants full sun or light shade and grows in almost any soil, from sand and gravel to heavy clay. It takes lime, drought and city air better than most oaks. Bottomland trees take a brief spring flood, but standing water kills it. Set trees 60 feet apart, and keep each one 40 feet from a wall and more than 50 feet from overhead wires.

Saplings ship bare-root or potted. Its taproot runs deep from the first summer, so plant it young. Plant from leaf fall until bud swell, in a hole wider than the roots and no deeper. Set the root flare at the soil line, put the native soil back in, and keep the mulch clear of the trunk. Bur oak acorns die if they dry out, and those from northern trees wait for a winter's cold before they sprout. Sow them on arrival in fall, 1 to 2 inches deep, under hardware cloth. For spring, hold them damp in the refrigerator for 30 to 60 days first.

Water every week or two through the first summer, then only in a long drought. Deer, rabbits and livestock browse young bur oaks, so guard each sapling with a tube or cage for its first three years. Keep one leader while the tree is young, and prune only in the dormant season.

The flowers are wind pollinated, and an acorn crop wants another oak nearby. A tree sheds its pollen before its own female flowers are ready, so it seldom if ever pollinates itself. As a white oak, it ripens its acorns in one season. Oak wilt troubles it less than the red oaks, though the disease can pass through joined roots and has killed whole groves, so prune in the dormant season. In one Illinois study, insects had infested 43 to 100 percent of the acorns gathered from bur oaks. The oak lace bug can strip shelterbelt trees in dry weather, and bur oak blight has been found in a few places in Illinois.