Bald cypress trees (Taxodium distichum), also known as swamp cypress, are impressive deciduous conifers native to the wetlands, floodplains, and river bottoms of the southeastern United States. These trees can reach heights of 100 to 150 feet or more, with broad, buttressed bases that taper upward and feathery, needle-like leaves that turn a striking bronze or copper color in autumn before dropping—earning the species its “bald” name.
Bald cypress trees are highly adapted to environments where soils are permanently or seasonally flooded. Their main trunks grow straight and tall, often flaring at the base for stability, while their roots spread widely through soft, waterlogged soil. In such oxygen-poor conditions, most trees would struggle to survive, but cypress trees thrive. The knees are part of this specialized root system and usually appear only when the trees grow in standing water or saturated ground; cypress trees growing on dry land often have few or no knees.
What Are Cypress Knees?
Cypress knees are part of the tree’s root system. They are not separate organisms but specialized root growths called pneumatophores, a term derived from Greek meaning “air bearer.” They arise from the tree’s horizontal, shallow-growing roots, which spread widely to anchor the tree in the soft, waterlogged soil of its habitat. Unlike the main trunk, knees have a porous, spongy tissue and grow in irregular, often twisted shapes, resembling stalagmites or, as their name suggests, human knees poking above the surface.
Cypress knees vary widely in shape and size: they can be conical, irregular, or multi-tipped, ranging from just a few inches tall to exceptionally rare specimens over 14 feet high. They are most prominent in swampy or flooded habitats, where the trees grow in soft, mucky substrates, and they tend to be absent or minimal when the trees are on drier ground or in very deep water.
The function of cypress knees has puzzled botanists, naturalists, and scientists for nearly two centuries, earning them the label of an “enduring enigma.” No single explanation has achieved universal consensus, and multiple hypotheses have been proposed over time, each supported by some observations but challenged by others.The most enduring and widely discussed theory is that cypress knees serve as aeration structures, allowing the tree to transport atmospheric oxygen to its submerged roots in oxygen-poor (anoxic) swamp environments. Similar to pneumatophores in mangroves, the knees protrude above the water or soil surface, potentially permitting gas exchange through their bark or lenticels.
A notable study by researchers at the University of Kansas measured oxygen levels in the attached underground roots and found significantly higher oxygen concentrations (about three times higher) when the knees remained exposed above water, compared to when they were fully submerged. This provided strong evidence that the knees facilitate oxygen diffusion to the roots, suggesting aeration played a major role in their evolutionary development.
Another prominent hypothesis focuses on structural support and anchorage. In the soft, unstable, waterlogged soils where bald cypresses grow, the trees face risks from flooding, currents, and high winds. The knees may act as anchors or buttresses, stiffening the horizontal roots and providing mechanical resistance to uprooting.
Some researchers describe them as elastic, sometimes hollow structures that grip the substrate like flukes on an anchor, helping the tree remain stable during storms or floods. This idea gains support from the observation that buttressed bases and knees are more common in flood-prone lowlands than on higher, firmer ground.