Oaks, members of the genus Quercus, are a large group of trees that bear male flowers in drooping clusters called catkins in spring and small single female flowers that develop into a nut called an acorn. An acorn typically sends down a taproot as it sprouts, but since some species of oak grow in wet and some in dry areas, the developing root system may be deep or shallow.
Since roots supply water and nutrients to the tree, their health is critical. Many oaks are native to low-rainfall areas and are able to use water in the deeper layers of soil. The root system also acts to stabilize the spreading canopy of the tree, allowing the tree to remain upright in strong winds.
The pin oak, Quercus palustris, also known as swamp oak and water oak, grows wild in swamps, flood plains and on river edges. With no lack of water, the roots maximize their intake of oxygen, essential for their growth, by being shallow and fibrous.
White oak, Quercus alba, grows well in almost any well-drained soil and has roots to 4 feet. In spite of the early development of a taproot from the acorn, the root system eventually develops major side branches that end in fine hair-like roots, especially near the surface.
In general, oaks follow the white oak pattern unless specifically adapted to wet soil.
In loose, deep soil the roots may go down 3 or 4 feet, but in shallow soils they may be able to penetrate only 1 or 2 feet. As the tree matures, the roots spread out beyond the dripline, the circle of the branches, to as much as 90 feet. The farthest and shallowest roots are small, fibrous and have the greatest capacity for collecting water and nutrients.
Many people think that the roots of a large tree are 6 to 10 feet beneath the surface. In reality, the surface of the soil around a tree may be dense with fine roots that need careful treatment to avoid damaging the tree.
Gardening Under Oaks
The fine roots that oaks develop near the surface can compete with perennials and shrubs for both water and nutrients, making the establishment of an elaborate landscape difficult. In addition, many oaks dislike summer watering and only drought-tolerant native or Mediterranean plants should be used. Never mound soil over the roots.