Smooth Alder (Swamp Alder)

Size, growth and what to expect

Smooth alder feeds itself. Root nodules fix nitrogen, which makes it a pioneer for reclaiming bare wet ground. It grows fast, nearly three feet a year in its youth on an average site in our growth estimator's model. It makes a multi-stemmed, suckering shrub or small tree 10 to 20 feet tall and 8 to 15 feet wide that spreads into a thicket.

Male catkins 1½ to 2½ inches long and half-inch red female catkins open in March and April. The female catkins ripen into inch-long cones that turn dark brown in fall and hang on through winter, and birds feed on the seed. Deer browse the foliage, birds and small mammals eat the pollen-rich male catkins in spring, and it hosts the caterpillars of the harvester butterfly.

It grows wild from Nova Scotia west to Illinois and Missouri and south to east Texas and north Florida. Its home is boggy ground along streams, lakes and rivers, wet meadows, ditches and swamp borders.

How to grow Smooth Alder

Smooth alder wants its feet wet: a stream bank, a pond shore or a ditch, in full sun or light shade. Do not put it in a dry bed. Clumps spread by suckers, so keep it 10 feet from a building and 6 feet between plants along a shore.

Plant dormant saplings, bare-root or potted, between leaf drop and bud swell, in a hole wider than the roots and no deeper, crown at the soil line, native mud back in. It fixes its own nitrogen, so skip fertilizer. Seed is tiny: chill it damp for a month, then sow on the surface of wet mix in spring, uncovered, and keep it wet under a screen.

Water only if the site dries the first summer. Deer, rabbits and beaver chew alder, so cage it for two years where they visit. It needs no pruning; a clump cut to the ground in late winter resprouts.

Smooth alder carries male and female flowers on the same shrub, but alders set little seed with their own pollen. Plant two or more from different parents so the wind can carry pollen between them. Expect a thicket, not a specimen; smooth alder suckers into a bank-holding colony. Catkins open in late winter before the leaves. Flea beetles chew leaves in summer and woolly aphids cluster on twigs; both are harmless.