Seaside Alder Plant Care

Seaside Alder (Alnus maritima) belongs to the Betulaceae family within the Alnus genus. Seaside Alder is native to the United States. Seaside Alder is also known as River Alder.

Seaside Alder displays a fast-growing deciduous tree growth habit, reaching up to 50 ft tall with a spread of approximately 30 ft. Seaside Alder produces inconspicuous catkins.

Leaf size: approximately 4 inches.

Water a young or newly planted Seaside Alder every 7–10 days, soaking the whole root zone, until it is established — usually one to two growing seasons. Established trees mainly need water during long dry spells. In a container, water whenever the top few centimetres of soil are dry.

Seaside Alder thrives in full sun to partial shade.

Alnus maritima tolerates temperatures of 0–100°F. Seaside Alder prefers humidity levels of 40–70%.

Moist, well-draining soil with high organic content. Good drainage is essential for Seaside Alder to prevent waterlogging, which leads to root rot and fungal issues.

Feed Seaside Alder every 8 months during the growing season with a balanced, slow-release fertilizer at the rate on the label. Established trees in good soil often need little extra feeding, so let weak growth or pale foliage guide you.

Stop feeding by late summer so new growth can harden before winter.

The PlantoScan app can remind you when to water and when to fertilize each plant.

Choose a spot with full sun to partial shade and moist, well-draining soil with high organic content. Avoid low spots where water collects after rain.

Plant container-grown or balled-and-burlapped trees in spring or early autumn, setting the root flare at soil level, and water well after planting.

Seaside Alder can spread to about 30 ft (9.1 m) and reach about 50 ft (15 m) tall. Space plants about 30 ft (9.1 m) apart so mature crowns don't crowd each other, and plant at least half that distance from buildings, fences and paths.

Planning a row or hedge? Try the tree spacing calculator.

Prune Seaside Alder while it is dormant in late winter: first remove dead, damaged and crossing branches, then thin lightly for shape.

If it flowers in spring, prune right after flowering instead so you don't remove next year's buds. Never take off more than about a quarter of the tree in one year.

Water deeply in late autumn before the ground freezes, and spread a 5–8 cm (2–3 in) layer of mulch over the root zone, keeping it away from the trunk.

Young and container-grown trees need extra protection: roots in a pot freeze much more easily than roots in the ground.

Seaside Alder can be propagated through seeds, hardwood cuttings. Similar propagation techniques also apply to andean alder plant care, which shares the same genus.

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Seaside Alder is susceptible to alder aphid, birch leafminer, tent caterpillars. Inspect the foliage regularly, especially along leaf undersides and stems. Treat infestations promptly with insecticidal soap or neem oil to prevent spread.

Yellow leaves, brown tips or drooping? See common plant problems and how to fix them.