Alnus maritima (seaside alder) is a rare woody species that occurs naturally as three subspecies from widely disjunct provenances in the United States. Alnus maritima subsp. oklahomensis is found naturally in only two counties in south-central Oklahoma, A. maritima subsp. georgiensis is isolated to one county in northwestern Georgia, and A. maritima subsp. maritima is found only on the Delmarva Peninsula in four counties in Maryland and two counties in Delaware.
Seaside alder is a multi-trunked large shrub or small tree (20-30 ft. at maturity) that blooms with vivid yellow pendulous catkins in the autumn when few other species are in flower and produces brown infructescent cones that persist through winter and provide ornamental character even after leaves have fallen. Like the other members of the genus Alnus, seaside alder fixes nitrogen through symbioses with soil organisms called Frankia, and therefore, is suitable for planting in nearly any type of soil with little or no need for fertilizer. While seaside alder is a riparian species in nature and can thrive in saturated soils indefinitely, it lives quite happily on well-drained sites. Seaside alder grows best in full sun, but can also grow well in partial shade in managed landscapes. Plants of all three subspecies grow well in USDA Hardiness Zones 4 through 7. Of the three subspecies of seaside alder, the one from Oklahoma (Alnus maritima subsp. oklahomensis or “Oklahoma alder”) is considered superior for ornamental use.
ʻSeptember Sunʼ -Seaside Alder
In 2004, William Graves and James Schrader, researchers in the Department of Horticulture at Iowa State University, registered the first cultivar of seaside alder named ʻSeptember Sun,ʼ a selection made from subspecies oklahomensis. ʻSeptember Sunʼ is known best for its fast growth, dense foliage, and symmetrical canopy shape compared to other genotypes. Schrader and Graves have begun a plant-breeding program with ʻSeptember Sunʼ and Himalayan alder, Alnus nitida, hoping to capitalize on the ornamental character of these two autumn-blooming species.
How to get plants:
– Sunshine Nursery & Arboretum of Clinton, Oklahoma offers Seaside Alder (Alnus maritima subsp. oklahomensis) for sale on their website: http://www.sunshinenursery.com
– ʻSeptember Sunʼ will soon be available wholesale through Greenleaf Nursery Company (http://www.greenleafnursery.com)
– Seaside alder can be propagated easily from seed that matures in late October. James Schrader of ISU Horticulture may have small quantities of seed available. If you are interested in trying to grow plants from seed, you may contact him by email: firstname.lastname@example.org
Viewing Seaside AlderSeaside alder can be seen growing in the landscape at the
Iowa Arboretum, 1875 Peach Ave., Madrid, IA 50156,
where many of the pictures on this page were taken.