If you’ve recently visited the Arboretum, you may have noticed what appears, at first glance, to be a flash-mob of squirrels convening around Blueberry Pond. Thanks to some winter rejuvenation pruning around the pond, it’s now easy to see this otherworldly spectacle of stalagmite-like structures, or “tree knees”, sticking up above the waterline and in the wet grassy areas nearby Taxodium distichum, commonly known as bald cypress.

Bald cypress is an iconic tree of the Deep South that thrives in steamy swamps and along riverbanks. Fossil evidence dates its survival as a species in North America to more than 100 million years.  With some nearing 2000 years old, it is believed to be the oldest tree species in eastern North America.

Bald cypress “knees” grow upward from the tree’s shallow horizontal roots. The height of the knees seems to have a direct relationship to average water depth and the age of the tree. But despite many theories and much research, there appears to be no widespread agreement in the scientific community as to what stimulates the growth of tree knees, what physiological function they serve, or whether they are even essential to the tree’s survival.  It’s commonly believed that the knees may exist to provide stability needed for a tree growing in wet and mucky soil, or the knees may supply oxygen to submerged roots. There is little scientific evidence currently to fully support either of these theories or any alternative proposals.

Although native to the southeastern US and Gulf Coastal Plain, bald cypress is cold hardy and adaptable to dryer soils, which explains why you can see these trees here at the Arboretum in well-drained average soil as well as on the water’s edge. Bald cypress trees planted in dryer landscapes, however, tend not to produce knees.

My personal experience suggests that tree knees exist solely to trip gardeners who are trying to prune their close friend, the swamp rose, but this hypothesis may not stand up to scientific scrutiny. Following a thorough review of the existing literature, Dr. Christopher Briand of Salisbury University described the cypress knee as an “enduring enigma”, concluding: “The truth may be that cypress knees evolved in response to past environmental pressures that no longer exist, in which case their function may be lost in the depths of time.” (Arnoldia, Vol. 60, p. 24).