The Post Oak’s “Roll” in the Cross Timbers Region

By Melinda Adams The post oak got its name from early settlers who commonly used the decay resistant wood for fence posts. Post oak takes several forms though out its range but the tree we are most familiar with is an upright majestic shade tree. Its twisted and knarled branches make it one of the most prominent deciduous trees in the winter landscape. Post oak leaves are five lobed and somewhat resemble a crucifix. They are among the last trees to leaf out in the spring and one of the earliest to stop growing, which contribute to their slow growth rate. The average post oak takes more than ten years to grow 2” in trunk diameter. However, it becomes a dominant species on poor sites due to its drought resistance. The cross timbers consisted of upland forests of drought resistant, slow growing trees, that cut through sections of the prairie from Central Texas, Oklahoma and into southern Kansas. Fort Worth is flanked with the Eastern and the Western Cross Timbers. The drought resistant post oak is a dominant species in both, but in the more arid sandier Western Cross Timbers they are closer together, smaller and slower growing. Reportedly, the post oak will not begin to bear acorns until it is about 25 years old. Though acorn production is sparser than other oaks, they are more important to wildlife because they are higher in fatty acids. The University of Arkansas Tree-Ring Laboratory has been conducting research in the Cross Timbers for over thirty years. They have found thousands of post oaks between 200 and 400 years old in all three states. The oldest are usually found on sandy soils or on steep rocky terrain. The height of the post oak is rarely over 100’, or greater than 48” in diameter and the largest recorded post oak is 117’ tall and 52” in diameter. However, in its extreme southern range it often takes the form of a small tree seldom taller than 40 feet. The oldest post oak ever documented in the cross timbers is over 400 years old, but is only 20 feet tall.
A large post oak tree

Post Oak

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Post oak trees in a park

Post oaks provide shade and a view of the past

You are fortunate indeed if you have a post oak in your yard. Because of their slow growth rate and their difficulty surviving transplanting you will never find one at a nursery. Your tree successfully took hold in the arid grassland or cross timbers and grew slowly and steadily. It was one of the lucky few that survived construction and it provides an abundance of shade for you and food for urban wildlife. If you are lucky enough to have a post oak under your care, like a cactus, it will be healthier if you simply ignore it. Don’t over prune it or feed it, and avoid putting flowerbeds or driveways under the canopy where you will disturb its roots. Just enjoy its beauty and its shade. When your neighbor’s exotic imported trees are wilting in the hot Texas sun, you’ll be as happy as your post oak with your reduced water and electric bill.