Alnus maritima subsp. oklahomensis 'September Sun'
Common Name: seaside alder 
Type: Tree
Family: Betulaceae
Zone: 4 to 7
Height: 20.00 to 25.00 feet
Spread: 16.00 to 20.00 feet
Bloom Time: August to September
Bloom Description: Yellow
Sun: Full sun to part shade
Water: Medium to wet
Maintenance: Medium
Suggested Use: Hedge, Water Plant, Rain Garden
Flower: Showy
Leaf: Good Fall
Tolerate: Drought, Erosion, Clay Soil, Wet Soil


Best grown in medium to wet soils in full sun to part shade. Best in wet soils in full sun. Thrives in moist areas along streams and rivers and in swampy areas. Grows in standing water. Tolerates flooding. At the other end of the spectrum, this shrub/tree also has some tolerance for dry, infertile soils.

Noteworthy Characteristics

Alnus maritima, commonly called seaside alder, is a deciduous, multi-trunked, upright-rounded, fall-blooming large shrub or small tree that typically grows to 20-30' tall. It is usually found in wet soils at low elevations along ponds, streams and rivers, including some dense pure stands in swamp-like areas with standing water. It is an uncommon to rare species that is native to the U. S. where it occurs in three small disjunct populations, with each population now being classified as a separate subspecies: (1) Alnus maritima subsp. maritima (coastal plain of Delaware and Maryland - Delmarva Peninsula); (2) Alnus maritima subsp. georgiensis (Bartow county in NW Georgia); and (3) Alnus maritima subsp. oklahomensis (Johnson and Pontotoc Counties in south central Oklahoma near the upper Blue River). Notwithstanding the remote locations of these three subspecies, fossil records seem to suggest that this Alnus maritima once enjoyed a much larger geographic distribution many years ago.

Subsp. oklahomensis is noted for its (a) dark green foliage, (b) pendulous fall-blooming yellow staminate catkins and (c) vigorous appearance under growing conditions difficult for most types of woody plants. Narrow elliptic to oblong to obovate leaves (2-4" long) have sharp but widely spaced marginal teeth. Leaves are glossy dark green above and pale green with yellowish veins beneath. Leaves turn mottled tones of yellow, orange and brown in fall. Flowers are monoecious (separate male and female flowers on the same tree), appearing in catkins which first begin to develop in July. Narrow, pendulous male catkins (each to 3" long) appear in clusters (2-6 per cluster) at the branch ends. Tiny, barrel-shaped, female catkins appear solitary in leaf axils near the male catkins. Catkin flowers do not open until mid-August to mid-September (unlike all other North American species of alder). After bloom, female catkins form small spreading or hanging fruiting cones (strobiles) composed of winged seeds. Fruiting cones do not mature until summer of the following year, and remain on the tree for almost one additional year after maturity. Reddish brown bark ages to gray.

Genus name is the Latin name for alder.

Specific epithet comes from Latin meaning growing by the sea.

'September Sun' is distinguished from subsp. oklahomensis by its more rapid growth, more symmetrical canopy, denser foliage and smaller strobili. It grows to 8' tall and 6' wide over the first three years, eventually rising to 25' tall over time. 'September Sun' was selected from a trial planting of subsp. oklahomensis seeds in 2000 at Iowa State University, Ames, Iowa. Seed for the trial came from an open-pollinated parent growing on the banks of the Blue River near Tishomingo, Oklahoma. Cultivar name of 'September Sun' comes from the yellow flowers which bloom in September. U. S. Plant Patent PP18,101 was issued on October 2, 2007.


No known serious insect or disease problems.


Good selection for difficult sites such as moist to wet low spots. Effective as a windbreak. Landscape specimen.