Ostrya knowltonii Cov.

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Kindly sponsored by
Claire Mansel Lewis


Owen Johnson (2022)

Recommended citation
Johnson, O. (2022), 'Ostrya knowltonii' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/ostrya/ostrya-knowltonii/). Accessed 2023-09-25.


Common Names

  • Knowlton's Hop-hornbeam
  • Western Hop-hornbeam
  • Woolly Hop-hornbeam
  • Wolf Hop-hornbeam


  • Ostrya baileyi Rose

Small tree, to 9 m, with an open, narrowly rounded crown. Bark soon ridged and scaly, grey-brown. Twigs downy (the hairs typically gland-tipped); buds very downy. Leaf ovate to broadly oval, small (2.5–6.5 × 1.5–5 cm), the apex abruptly pointed to rounded; with only 5–8 pairs of lateral veins; downy on both sides, especially along the veins; margin sharply and unevenly double-toothed; petiole 3–6 mm long, with usually glandular hairs. Male catkins short (2–3 cm long), with downy stalks and scales. Fruiting catkin short (2–3 cm), the bladders 10–18 mm long. Nutlet c. 6 mm long, hairy towards the tip. (Furlow 1997; Bean 1976).

Distribution  Mexico Probably present in the north-west of the country United States Arizona, New Mexico, Texas, Utah

Habitat Moist canyons, 1200–2400 m asl; by streams and on rocky slopes.

USDA Hardiness Zone 6

RHS Hardiness Rating H5

Conservation status Least concern (LC)

Ostrya knowltonii was first observed in 1889 on the southern slope of the Grand Canyon by Frank Hall Knowlton, a Professor of Botany at what is now George Washington University in Washington DC (Brian & Spamer 2000; Furlow 1997). This population was refound and collected by James Toomey three years later, and was named after Knowlton (but originally, owing to a misprint, as O. knowtonii) (Brian & Spamer 2000). The first introduction of living material seems to have been made in 1914 to the Arnold Arboretum (Bean 1925), but as a tree from a specialised habitat O. knowltonii was probably maladapted to the cold, wet winters of the north east and is now represented there only by a herbarium specimen (02103189) (Royal Botanic Garden Edinburgh 2022). The species was sent on to Kew in 1916 (Bean 1925) but is unlikely to have survived here for long either, and the reference to this introduction was omitted, for whatever reasons, from subsequent editions of Trees and Shrubs Hardy in the British Isles. In 1981, Rob Nicholson at the Arnold Arboretum knew of no plants left in cultivation in America, and tried but failed to find and collect material in the Great Basin of California and at Grandstaff Canyon in Utah (Nicholson 1982). By 2015, however, O. knowltonii was reported from four ex-situ collections in America (Beech, Shaw & Jones 2015).

Several authorities, including Plants of the World Online (Plants of the World Online 2022) treat Ostrya knowltonii in synonymy with another small-leaved species from similar habits, O. chisosensis D.S. Correll, but the presence of glandular hairs in O. knowltonii seems to be a tell-tale distinguishing feature; a complete chloroplast genome sequence of O. chisosensis in 2019 suggested that this species’ closest ally was in fact the Chinese O. redheriana (Li et al. 2019). The most thorough study of these two species’ discrete native ranges is probably that made by McCauley and Paces in 2015 (2015).

The likely continuation of the ranges of these two species (and others’) into the mountains of northern Mexico is not well understood. Furlow (Furlow 1997) suggested that the Ostrya from western Mexico are closely allied to O. knowltonii.