Photo courtesy of Missouri Botanical Garden
Here’s a family of deciduous shrubs that deserves to be better known and widely grown. One or another of the species is native from North Africa to Korea, from Zone 6 to Zone 2, and there are several native to North America. Flowers appear when few other shrubs bloom--in autumn or very early spring. The catkins are dangling chains of male flowers, and the female flowers are small globes that become cones. The cones hang on the bare branches in winter like Christmas decorations.
Alders have a deal with Frankia, bacteria that take nitrogen (the gas) from the air, turn it into ammonia (the form that roots absorb), and share it. The bacteria live in nodules made for them by the roots, which share in return their sugars and minerals. When you grow an alder you’re also enriching the soil. Almost all live soils--soils with organic matter and good drainage--have Frankia, and alders will thrive in them. Alders thrive in full sun and most soils, but they also tolerate lingering moisture and wet soil.
I’m lucky with alders. There are mature specimens at Iowa Arboretum, and research at Iowa State Univerity is showing that a rare North American species, Alnus maritima (the seaside alder), is a great plant. It flowers in autumn, it grows in almost any soil, needs little or no fertilizer, and tolerates soaked soil indefinitely, making it ideal for pond sides and wet spots.
Here is a mail-order source that has other species, but not the most common garden species in commerce, Alnus glutinosa, the black alder, which is native to Europe. From it came the selection, ‘Aurea,’ which is gold when the leaves are young and another selection, ‘Imperialis,’ which looks like it has bed hair (because the leaves have narrow lobes).
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