Pycnanthemum verticillatum var. verticillatum, a rare wildflower, needs full sun or dappled shade to thrive1

The conservation of a rare species requires an understanding of its habitat requirements, including its response to different light levels. Accordingly, we conducted a study of the light requirements of Pycnanthemum verticillatum var. verticillatum, a New York State S1S2-Endangered perennial forb found on Staten Island, Richmond County, NY. Much of its present habitat in this urban county is along disturbed roadsides and is undergoing increased shading due to woody encroachment. We grew plants in full sunlight (no shade cloth), medium sunlight (30% shade cloth), and low sunlight (60% shade cloth), and measured their growth, reproductive, and morphological responses. Plants were largest and most likely to produce flowers in the full and medium sunlight treatments. Plants exhibited typical shade-avoidant responses such as stem elongation and increased secondary ramet production in the low sunlight treatment. We conclude that Pycnanthemum verticillatum var. verticillatum may survive in deep shade, but partial and full sun result in more vigorous growth and reproduction. Canopy thinning to maintain no more than 30–50% shade is necessary for the plants to thrive. Managing for sunny habitats, such as meadows and open-canopy forests, would have broad conservation benefits for this species and other native plants that require high-light environments in the region.