Alnus × spaethii Callier

A hybrid between A. japonica and A. subcordata which originated at the Späeth nurseries of Berlin in 1894 and entered the trade in 1908 (Bean 1976). The name Alnus × spaethii refers to any hybrid of this parentage, but Späeth selected a single clone for release from the original batch of seedlings which is generally listed as a cultivar, ‘Spaeth’, to avoid conflict with other occurences of the same hybrid (Jablonski 2018; Hatch 2021–2022).

Späeth’s original clone forms a distinctive, hardy tree with a uniformly upright oval crown to 20 m or more with elliptic-ovate leaves which are violet-purple when young and purple-red in autumn; it is also notable for its large and remarkable catkins which give long-lasting winter interest. It is most easily distinguished by the foliage: the leaves are similar in outline to those of Alnus japonica which are elliptic-ovate with acute to acuminate tips, but much larger than those of most cultivated A. japonica, similar in this regard to A. subcordata (De Langhe 2022). As noted in Grootendorst (1972) this hybrid was highly rated in trials held at Boskoop, the Netherlands, and remains one of the best alders for general planting. It is a fast-growing tree capable of attaining about 15 m in fifteen years in optimal conditions, and has been extensively planted as an amenity tree including as a street tree (Clarke 1988). Although at its best in wetter conditions A. × spaethii can tolerate periodic drought; it can be propagated by summer cuttings. The largest tree on record measured 25 m × 1.12 m dbh in 2019, growing at Ryston Hall Park in Norfolk, UK (monumentaltrees.com).