Cypress (Taxodium) Knee Seasonal Growth Is Stimulated by Flood Water Levels and Constrained by the Tree Dormant Season: A 14-Year Study.

Cypress knees are woody root structures of cypress trees (Taxodium spp.) that curiously protrude upwards out of the ground. To better understand knee growth characteristics, 30 knees in a southwestern Florida depressional cypress swamp were measured monthly for 14 years (2012-2025) for tip growth. Substantial knee growth, when it occurred, was during the months of June through September, but only when the month coincided with water levels above ground. Regardless of water levels or the canopy's leafiness, knee growth rates consistently declined in October, and knee growth was rarely detectable from November through March. In other words, seasonal growth of cypress knees was triggered by above-surface water levels occurring during the tree's typical growing season. These conclusions are further supported by extreme weather years when knees failed to grow during flood-less summers (2012, 2023), and knees remained dormant during unusual January/February wet conditions (2016, 2024). Growing seasons with higher water levels and longer hydroperiods were positively correlated with greater cypress knee tip growth, as well as a higher percentage of knees growing that season. Considering that 90% of knee tips were above mean high water at the onset of the study, the advantage of expanding a knee's thin-barked growing tip during high water conditions may be, in addition to gaining height, to augment gas exchange with the inner tissues of the knee where aerobic respiration aids in starch storage processes and phloem's distribution of oxygenated resources. For this sample of knees, the mean seasonal growth rate was 0.39 cm/year, and a single-season individual maximum was 3.6 cm/year.