Alnus Mill.
Trees or shrubs. Buds either stalked with 2-3 scales (subgen. Alnus Endl.), or less often sessile with 3-6 imbricate scales (subgen. Alnaster Endl.). Lvs usually serrate or dentate. ♂ catkins: bracts peltate with 4 bracteoles to each; stamens usually 4, beneath bract. ♀ catkins < ♂, with 2 fls to each bract and bracteoles. Fruiting catkin cone-like, ovoid to ellipsoid, long- persistent. Scales 5-lobed or truncate, woody. Fr. a nutlet, usually winged, 2 in the axis of each scale.
Key
c. 35 spp., N. temperate, S. to Indochina, Andes. Naturalised spp. 2.
Most Alnus spp. have been introduced to N.Z., but the only ones common in cultivation in addition to the 2 described here are A. incana (L.) Moench, grey alder, with hairy shoots, and lvs ± glaucous below in the ordinary form (a yellow-leaved cv. is also grown), A. cordata Desf., Italian alder, with serrulate lvs, not plicate in bud, ± cordate at the base and 1-3 ♀ catkins in each cluster, and A. rubra Bong., red alder, with lobulate or crenate-serrate lvs, slightly glaucescent below, with revolute margins and orange or reddish veins, and ♀ catkins 6-8 in each cluster. Red alder sometimes regenerates profusely in the vicinity of parent trees. Flowering is usually in spring although a few spp. flower in autumn.
