A semi-evergreen tree so far only known in cultivation, where it has attained a height of almost 60 ft (see below); young stems glabrous, greenish brown in late summer, ageing to greyish brown. Leaves persisting on the tree until spring, rather leathery, obovate to oblanceolate, mostly rounded at the apex, tapering from about the middle to a narrow truncate, sometimes slightly auricled base, 21⁄2 to 5 in. long, 11⁄2 to 3 in. wide, dull green and slightly rugose above, palish grey-green and rather veiny beneath, glabrous on both sides, margins shallowly and irregularly lobulate, the main lateral veins ending in short mucros; petiole glabrous, 1⁄4 to 1⁄2 in. long. Fruits borne singly or in pairs on fairly slender peduncles about 11⁄2 in. long; acorns glabrous, ovoid, about 1 in. long; cup hemispherical, enclosing about one-third of the acorn, with numerous appressed scales which are grey-hairy at the base, glabrous and brown at the tips, and decrease in size from the base of the cup upwards.
Most and probably all the trees of Q. warburgii in this country were distributed by the nurseryman Smith of Worcester in the 1870s, or derive from these. He had received the seeds from the Genoa Botanic Garden in 1869 under the name Q. rugosa, and sent out grafted plants under the name Q. rugosa genuensis (i.e., ‘of Genoa’). In ‘Elwes and Henry’ (Vol. 5, p. 1312), this oak was referred to Q. obtusata, which, like the true Q. rugosa, is a Mexican species. In 1933, E. F. Warburg pointed out that this identification was incorrect and proposed for it the provisional name Q. genuensis Hort. (Journ. R.H.S., Vol. 58, pp. 186-7). Finally, it was given botanical status by Mme Camus in 1939 as Q. warburgii (Les Chênes, Vol. 2, pp. 621-3 and 793). She groups it with Q. obtusata but remarks that it differs in its smaller, more glabrous leaves, longer female catkins, and in the thinner, more appressed scales of the cups.
The status of Q. warburgii is uncertain. Warburg and Camus both suggested that it might be a hybrid, though neither mentioned the possibility that it might have Q. robur as one parent. In fact, its resemblance to the common oak is slight and anyway superficial.
The planting date and origin of the famous tree in the University Botanic Garden, Cambridge, is not certain, but, like the Kew tree, it is grafted on common oak and could be one of the set sent out by Smith of Worcester. It measures 58 × 71⁄2 ft (1969); in 1910 it was 39 ft high (Journ. R.H.S., Vol. 41, p. 8 and fig. 5). The tree at Kew, which came from Smith in 1873, measures 39 × 4 ft (1967). Both trees bear fertile acorns quite frequently.