Photo: Robert H. Mohlenbrock, Public domain · Wikimedia Commons

Seaside Alder

Alnus maritima

Also called brook alder, beach alder.

The one native alder that blooms in fall, hanging gold catkins in August and September on a rare shrub that stands in water and feeds its own soil.

RareShowy flowersTolerates wet soilNitrogen fixingHedge or screenWildlife value

Type: Shrub
Mature height: 12–30 ft
USDA zones: 3–9
Growth rate: Medium
Sun: Full, Part
Soil moisture: Average, Wet
Grows wild in: Stream banks, swamps and wet flats
Native range: Delaware, Georgia, Maryland and Oklahoma See what is native in your state

Hardiness zones Find your zone by ZIP code

Seaside alder is a many-stemmed shrub or small tree that survives in the wild in three stands hundreds of miles apart. One lines the rivers of the Delmarva Peninsula, one follows two creeks in south-central Oklahoma, and one grows around a single pond in northwest Georgia. Each is now its own subspecies, and genetic work says the gaps come from a range that shrank long ago, not from planting. NatureServe ranks it vulnerable, and every state it grows in treats it as rare. Plant it to screen a pond shore or a rain garden, where its catkins light up September.

Size, growth and what to expect

Seaside alder grows at a medium pace, about a foot and a half a year in our growth estimator. In gardens it reaches 20 to 30 feet tall and 16 to 30 feet wide. Wild Delmarva shrubs run nearer 12 to 20 feet. It flowers about 2 years from seed.

Its catkins open in August and September, after every other native alder has finished. The small woody cones take about a year to ripen, and the seed falls from December to March (Graves and Schrader). The dense leaves give birds cover for nests.

It grows at the edges of ponds and small streams, often standing in water. All three wild stands sit where winters are mild, yet all 450 plants in field trials survived winters as cold as zone 3a (Schrader and Graves 2003).

How to grow Seaside Alder

Seaside alder wants full sun or light shade and a soil that stays moist to wet. It will stand in water, and it takes flooding, clay and alkaline ground. Once rooted it rides out a short drought, but it is not a plant for a dry bed. Set plants 10 feet apart for a screen, and keep them 15 feet from a wall and 15 feet from overhead wires.

Saplings ship bare-root or potted. Plant them dormant, from leaf fall until bud swell, in a hole wider than the roots and no deeper. Set the crown at the soil line, put the native soil back in, and skip fertilizer, since the roots fix their own nitrogen. Seed needs six to eight weeks of cold, damp storage in the refrigerator. Sow it in spring barely covered, about a fifth of an inch deep, and keep the mix wet.

Water weekly through the first two summers unless the ground stays wet. In a trial on dry Utah soils, plants left unwatered died within a month of leafing out. Deer and rabbits chew young alders, so cage them for two winters where they visit. It needs little pruning. Cut out dead stems in late winter.

Seaside alder carries male and female flowers on the same shrub and the wind pollinates it, so one shrub sets seed alone. Seed from the Delmarva stand germinates far less often than seed from Oklahoma or Georgia. No serious pest or disease troubles it, though cankers, alder aphids and leaf miners can show up.

Heights and zones are typical ranges for the species in cultivation; local conditions and seed source shift them.

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