[(U. glabra “Exoniensis” x U. wallichiana) x (U. minor 1 x U. minor 28)] x U. pumila S.2
Istituto per la Protezione delle Piante, Florence, Italy
San Zanobi is one of the Italian programme’s earlier creations, arising from the crossing of the Dutch clone Plantyn with a Siberian elm. It was named for Saint Zenobius, a fifth century bishop of Florence, whose funeral procession apparently caused a dead elm by the way to spring back into growth.
With 19.5% defoliation and 8.5% dieback after inoculation, San Zanobi underperforms several other elm clones but is closely comparable to the Dutch/French Lutece. Elm disease has apparently not caused problems since the tree was commercially released in 2002 any more than it has for Lutece, so it can be regarded as having effective field resistance.
Once established (which can take longer than with some clones) San Zanobi grows very vigorously; annual height increment in the near ideal conditions of the Castellaccio trial site in Umbria was 215.45 cm balanced by an excellent increase in girth. Branching is strongly fastigiate so that the tree develops a pyramidal or columnar shape, reminiscent of Jersey elm. In fact care is needed in removing side branches, since any regrowth will be almost vertical. Co-dominant leaders can spoil a tree’s shape, but on the whole San Zanobi has an interesting tendency to grow through phases of clumsiness back into balance. Lack of stability resulting from poor root development has been a concern, but it seems to have been overcome by better practice in the nursery. Particular care should still be taken in spacing out the roots at planting. The tree copes well with poor soils, but is not tolerant of bad drainage.
The San Zanobi leaf is relatively large, stalked, smooth, and lanceolate. Budburst is broadly in line with European strains of elm, as is the fresh green of the foliage in spring darkening into the summer. Leaves are retained late into the autumn and develop little colour. At no point in the season can San Zanobi’s foliage be described as rich. Depending on tastes, this is either a disappointing feature of the tree, or perhaps accords well with its graceful silhouette.
San Zanobi’s narrow habit and somewhat ethereal appearance mark it out as a tree for ornamental or urban rather than rural planting. It can be considered as an alternative to the more stiffly upright Columella where lesser shade and greater variety of form are wanted.