Common Alder

Common Alder (Alnus glutinosa)

Full to part sun, wet to moderately moist, tolerates seasonal flooding and heavy clay, pH 5.5–7.5.

40–60 feet tall by 20–30 feet wide; blooms late winter to early spring (February–March) with pendulous male catkins 51-102 mm long and small red female catkins; fruit is a persistent woody cone-like strobilus, 1.3–1.9 cm, ripening in fall and persisting through winter.

Native region: Not native to Tennessee; ornamental introduction from Europe, western Asia, and North Africa.

Alnus glutinosa is widely planted along Tennessee stream banks and retention pond margins for its exceptional tolerance of saturated soils and capacity to fix atmospheric nitrogen via root symbiosis with Frankia bacteria — improving soil fertility without fertilizer inputs. In Middle Tennessee's clay-heavy soils it establishes rapidly in low-lying areas where most other canopy trees fail. However, UT Extension disease surveys document susceptibility to Phytophthora root rot when drainage is inadequate even for this moisture-tolerant species; planting on slightly elevated berms in flat sites reduces this risk. Not considered invasive in Tennessee but has escaped cultivation in some Mid-Atlantic states — monitor for self-seeding near natural waterways. Catkins provide early-season pollen for mining bees and other native bees before spring bloom. The corky, nitrogen-enriched leaf litter accelerates riparian soil development.