Describing the knobby vertical protrusions of the roots of the bald cypress tree (Taxodium distichum), "cypress knees" is a clever, colloquial term. Bald cypress trees are popular landscape trees for use around lakes and streams as singular specimens or in grove clusters. They are appropriate to grow across U.S. Department of Agriculture winter hardiness zones 5 through 11 or Sunset climate zones 2 though 10 and 12 through 24.
Bald cypress trees are native to the coastal plain and lower Mississippi River delta in the southeastern United States. According to the U.S. Forest Service, these trees inhabit flat topography, often seasonal and intermittently inundated with floodwater. Ninety percent of the natural cypress tree woodlands occur at an elevation of 100 feet or less from Delaware to Florida and the southern tip of Illinois to southeastern Texas.
Bald cypresses are deciduous conifers. They lose their feathery, soft and green needles each autumn, turning bright rusty orange before dropping from the twigs. The bald cypress also doesn't flower but instead reproduces by producing male and female cones. Only the female cones ripen to become dry, brown cones that persist into winter. The tree's bark is attractively reddish brown and exfoliating and in wet to shallowly submersed soils, the roots of bald cypress form upward, bony-looking "knees" that resemble dwarf stalagmites.
For decades, scientists believed cypress knees occurred on trees in wet, oxygen-depleted soils as a means for the roots to exchange oxygen with the atmosphere. Dr. Gerald Klingaman, retired University of Arkansas Extension horticulturist, states that research was conducted on the trees that tend to diminish this old hypothesis. Klingaman mentions that the knees where cut off from trees growing in saturated ground and there were no ill effects on the plants. The knees do seem to improve anchorage of bald cypress trees in soft, wet soils. If a bald cypress tree is grown in average, well-drained soils away from the swamps or edge of water, these knees never develop.
A mature bald cypress typically grows 50 to 70 feet tall, although rare trees can grow as tall as 100 feet according to Dr. Michael Dirr of the University of Georgia. It attains an upright, narrow to broad pyramid-like shape, sometime becoming more oval with great age or when crowded by other nearby trees. Its mature width is between 20 and 30 feet. Knees can range in height above the soil or water line anywhere from 6- to 36-inches or more.
Plant bald cypress trees on the edge of freshwater bodies or in parts of the landscape that are low and tend to slowly drain after rains. They are remarkably adaptable, and can be grown in upland landscapes in average garden soils and even demonstrate a sound tolerance to seasonal droughts. Site these trees in sunny locations for best branching structure and overall habit. Avoid alkaline soils as it leads to a nutrient deficiency, causing foliage to be unattractively yellow-green. Dr. Michael Dirr shares that this tree develops a strong taproot and is best transplanted when small and young as either container-grown or balled and burlapped (B&B) plants.