A new variety of Alnus maritima subsp. oklahomensis plant named `September Sun` that is characterized by fast vertical growth of .apprxeq.89 cm (35 inches) per year over the first two years after establishment in the landscape and fast canopy volume growth of .apprxeq.4.8 m3 (170 cubic feet) during the second year after establishment. The strobili (infructescences) of `September Sun` are small (17 mm long and 10.2 mm wide) and slender (length:width ratio=1.7) compared to the average for oklahomensis. In combination, these traits set `September Sun` apart from all other existing varieties of Alnus maritima subsp. oklahomensis known to the inventors.