Lythrum salicaria (purple loosestrife) is an invasive wetland perennial in North America native to Eurasia. Because light environment may limit the species’ distribution, information on the relationship of forest canopy coverage to relative height and seed set in its native environment could give insight into its control in North America. This study examined the effect of various light environments related to forest canopy structure on seed set and stem height in L. salicaria in three biogeographical regions of Turkey spanning latitudes from 36 to 39° N. In Turkey, Lythrum salicaria grows in discrete patches in the canopy gaps of riverine forests, which often are dominated by Populus alba. The mean number of seeds per individual and stem height increased with decreasing canopy coverage (10437±284 to 20652±664 seeds individual⁻¹ and 116±2 to 173±2 cm, respectively). A number of insect seed herbivores were noted on these Turkish populations, most notably a seed predator (Nanophyes marmoratus), which destroyed all seeds in infested capsules. Our study suggested that the light characteristics associated with canopy gaps influenced the seed set and height of L. salicaria. Other factors likely played a role, such as insect herbivores, soil fertility, and competition with other species, which may also be affected by the light levels provided at various levels of canopy coverage.