Alnus maritima (Marsh.) Nutt.

Seaside Alder, Alnus maritima is a deciduous, spreading, autumn-flowering shrub or small shrubby tree to 10 m tall that grows in multi-stemmed clumps. It is restricted to three disjunct regions of the United States and in each area grows in wet soils by ponds, streams and rivers at low elevations. Fossil records show that A. maritima is an ancient species and was once more widespread. Its current distribution represents remnants of Pleistocene and post-Pleistocene distributions, following climatic change. It is believed an ancestor migrated from southern Asia, across Beringia and into North America (Furlow 1979; Schrader & Graves 2004). We follow Schrader & Graves (2002; 2004) in treating the three disjunct populations as subspecies. Populations today are separated by over 900 km. Subsp. maritima occurs on the Delmarva Peninsula, Delaware and Maryland; subsp. oklahomensis is restricted to four populations in Oklahoma; while subsp. georgiensis occurs along opposite sides of a single pond in Bartow County, Georgia (Jones 2013). Seaside Alder is classed as Critically Endangered in the wild (Shaw et al. 2014) due to its limited distribution and pressures from climate change, grazing, land draining, pollution, siltation, and low genetic diversity. It occupies a limited climatic and ecological niche and is therefore susceptible to possible extinction.

Alnus maritima is most easily identified by virtue of its autumnal flowering, with female inflorescences 1(–2) in leaf axils along the main stems with cones 12–28 × 12–22 mm, peduncles 5–10 mm, and leaf venation often much branched near the margin (semicraspedodromous) or infrequently ending in the teeth (craspedodromous). It is a shrub, unlike all other autumn-flowering alders, and its pistillate inflorescences and cones are almost always solitary in leaf axils rather than in paniculate or racemose clusters. It could be mistaken for the spring-flowering A. serrulata, but this has craspedodromous veins, smaller fruiting cones (10–22 × 6–12 mm) in clusters of 3–5 with very short peduncles (1–5 mm).

The taxonomic position of Alnus maritima has received much attention. It has traditionally been placed in Subgenus Clethropsis alongside A. nepalensis, A. nitida and A. formosana, all of which are autumn-flowering species which have sometimes been lumped within Seaside Alder (e.g. as A. maritima var. formosana Burkill). However, while molecular studies suggest that this Subgenus is not a natural, monophyletic grouping (see genus introduction), no consistent picture of its relationships has yet emerged (Chen & Li 2004; Ren, Xiang & Chen 2010).

First introduced into cultivation in the US in the early 1870s, and to the UK from seed sent by Sargent to Kew in 1878 (Bean 1976), Seaside Alder has always remained a rare plant in cultivation. Today it is only found in a few specialist collections including the Holden Arboretum, The Scott Arboretum of Swarthmore College, Denver Botanic Garden, the Plant Heritage National Plant Collection of Alnus at St. Aubins, Jersey, Channel Islands, at Kew and White House Farm, Kent. The largest in British cultivation (8 m tall) grows at the Plant Heritage National Plant Collection of Alnus at St. Aubins, Jersey, Channel Islands (The Tree Register 2023). Due to its rarity in the wild, ex situ plants of this species are significant. Seaside Alder is insensitive to shade and prefers more or less permanently wet soils, although it can grow well in dry, infertile soils. Its compact habit and attractive autumn catkins make it a useful shrub for difficult and waterlogged soils and it merits wider cultivation. It is quick to grow from seed and will flower after about 2 years, but take 10 years to reach full maturity (Schrader & Graves 2002).