Clay Soils & Burr Oak Trees
American woody plant expert Dr. Michael Dirr of the University of Georgia says that the burr oak (Quercus macrocarpa) is adaptable to many types of soil that do not flood. Rarely grown as an ornamental shade tree because of its slow growth rate and eventually massive size, this oak drops lots of twig, leaf and acorn debris. Burr oak is suitable for sunny, spacious landscapes across U.S. Department of Agriculture hardiness zones 3 through 9a.
Clay Soil Characteristics
Campbell's Nursery of Lincoln, Nebraska, says there are both advantages and disadvantages in growing plants in clay soil. Clay soil particles are small and reduce the rate of water soaking and drainage, as well as penetration of needed air molecules for root gas exchange. Clay soil is nutrient-rich (fertile), and retains moisture well to diminish the effect of drought. However, once clay soil dries it become hard and difficult to penetrate. Clay soils in central North America tend to be neutral to alkaline in pH.
Ideal Growing Conditions
The U.S. Forest Service and the Society of Ontario Nut Growers state that the ideal soil conditions for culture of a burr oak is well-drained sand- or clay-loam with a minimum topsoil depth of 3 feet. However, both comment that within its natural range across North America, burr oak often dominates drier, calcareous (alkaline) soils comprised of heavy clay and gravel.
Oak trees are notoriously difficult to transplant, as they initially develop long taproots when saplings, up to 3 feet in the first growing season after germination. Disturbance of this primary root can kill or severely reduce growth and health of oaks. Likewise, the burr oak is best planted from seed and allowed to grow undisturbed in its permanent landscape location. The Society of Ontario Nut Growers states that 1-year-old burr oak seedlings can be successfully transplanted to a permanent location. All young trees benefit from an evenly moist soil environment free from aggressive root competition from grass, weeds and other woody plants.
The natural characteristics of clay soils may limit growth and performance of the burr oak, even though this species is known to tolerate and even prosper in it. Dirr and the U.S. Forest Service note that burr oaks adapt to urban growing conditions well. The Forest Service also mentions that growth of burr oak is diminished when clay soils are drier and when loam and clay soils are trampled around the tree base. Compacted clay soil lacks aeration and is physically difficult for roots to grow through, especially since lateral burr oak roots are shallow but wide-spreading.
For success with burr oaks in clay soil environments, cultivate the soil prior to planting and avoid compaction of the soil particles with equipment or excessive foot traffic. Repeated application of organic mulch over the root zone of burr oaks improves clay soil surface texture and quality, lessens soil compaction, retains soil moisture and overall benefits the plant. Avoid planting burr oak where the roots are confined by concrete or other permanent barriers.
- U.S. Forest Service: Bur Oak
- "Dirr's Hardy Trees and Shrubs"; Michael A. Dirr; 1997
- Society of Ontario Nut Growers: Species Profile of the Bur Oak
- Campbell's Nursery: Clay Soil