Pinus strobiformis Engelm.

Pinus strobiformis is little known in cultivation but in the wild forms a substantial tree, with a relatively narrow crown in its youth, although this becomes more rounded with age, with far-reaching branches. It is related to P. ayacahuite and resembles it in many characters, but its leaves are shorter and the average sizes of its handsome cones are smaller. The seeds of P. strobiformis are functionally wingless (Perry 1991). Northern populations (United States, northernmost Mexico) are intermediate between P. strobiformis as described by Engelmann, from southern Chihuahua, and P. flexilis, with smaller cones (12–22 cm) and needles with only a few apical serrations, and have been distinguished as P. flexilis var. reflexa Engelm. or P. reflexa (Engelm.) Engelm. (Farjon & Styles 1997, Farjon 2005a). These northern trees are commonly cultivated in the United States and seem to have a wide tolerance, growing even in the Chicago area (for example, at the Morton Arboretum). In Europe P. strobiformis is much less familiar, although there are a few dwarf cultivars in circulation, and ‘Vanderwolf’s Pyramid’ is widely available (usually sold as P. flexilis). TROBI records a few individuals scattered through the United Kingdom, the recognised champion being an 11 m specimen at the Manor, Walton-in-Gordano, Somerset, measured by Owen Johnson in 2006. Two trees of 11 m and 10.8 m grow in the Hillier Gardens, where they were received in 1985 as P. ayacahuite (Priest 107), and it is possible that other specimens from this collection exist elsewhere (A. Coombes, pers. comm. 2008). It is also present in some collections in continental Europe, but could be grown much more widely. It is, however, very susceptible to white pine blister rust; there has been extensive mortality in New Mexico recently, since the disease invaded the state in 1990 (Conklin 2004), and a mature specimen at Hørsholm Arboretum in Denmark also died of it in 1986 (M. Frankis, pers. comm. 2007).

Pinus veitchii Roezl from central Mexico has traditionally been included in P. ayacahuite as a variety, P. ayacahuite var. veitchii (Roezl) Shaw (see Bean and Krüssmann: B213, K210) – though one recent author (Businský 2004) treats it as a distinct species. It is much closer to P. strobiformis, however, matching it in cone scale thickness and seed size, and differing from it only in seed wing length and minor detail of curvature of cone apophysis.

[Editorial Note: The combination Pinus strobiformis subsp. veitchii (Roezl) Frankis was published in 2009, the same year New Trees went to press.]