Quercus ellipsoidalis E.J.Hill

Quercus ellipsoidalis has only in relatively recent times been recognised as a species, and this is still brought into question by taxonomists. One of its common names, Northern Pin Oak, would make it an understudy for Q. palustris; for some, it is a regional form of Q. coccinea; and yet it shares more genes with Black Oak, Q. velutina. The claque of supporters of its species status, led by Andrew Hipp and allies, maintain it is sister species to Q. coccinea, i.e. descended from a recent common ancestor, and that Q. ellipsoidalis remains genetically distinct enough to be a good species (Hipp & Weber 2008; Hipp, Weber & Srivastava 2010; Hipp 2010). Urban forester David Shepard (2009) leads the anti-claque, holding that Q. ellipsoidalis is simply a northern expression of Q. coccinea. But the issue is still not clear cut: even Hipp concedes he is not married to the idea that the two taxa are separate species, and that a good case could be made to call Q. ellipsoidalis a geographic subspecies or even variety. Quercus ellipsoidalis is also very close to Q. velutina, though this is likely due to introgression rather than to shared ancestry: ‘What probably throws things off is that Q. ellipsoidalis hybridises with Q. velutina far more than Q. coccinea does. So if you do a naive analysis … there’s always a tug of Q. ellipsoidalis towards Q. velutina. Tweak things a bit, and the introgression signal wins. Turn the dials a bit in the other direction, and the divergence history wins. They are both embedded in the genome’ (A. Hipp, pers. comm. 2024).

Hill’s Oak (the other common name Quercus ellipsoidalis is known by) is endemic to the Great Lakes region and grows in an area that wraps around Lake Michigan; it is also found in southern Canada west of Lake Superior and north of Lake Erie. In habitat, it ranges from dry sandstone bluffs, oak barrens and sand savannas to seasonally wet sand soils and dry-mesic forest in clayey soils. It is distributed almost exclusively in glaciated terrain, in contrast to Q. coccinea, which is largely confined to unglaciated terrain (Hipp 2010). It thus presents two key advantages: winter hardiness (it is one of the few North American oaks hardy in USDA Zone 3) and tolerance of high pH soils, which means that the chlorosis that so often affects its cousin Q. palustris is usually not a problem with Q. ellipsoidalis. Dirr and Warren (2019) extol its virtues as an excellent candidate for use in high pH, limestone soils (but see below) and recommend it for parks, campuses, commercial landscapes, golf courses and specimen use; as a street tree, it outshines Q. palustris as its branches are less drooping. Sternberg (2004) praises it for maintaining in youth a classic pyramidal form and for its outstanding fall colour.

Despite these merits, its progress up the horticultural charts has been slow. It earned only a brief mention in Bean (1976), who noted a moderately sized tree in the Royal Botanic Garden Edinburgh (since gone), and by the early 21st century it had become, according to Grimshaw and Bayton (2009), only slightly better known in Europe. It is now widely cultivated in the UK and found in many gardens and collections, particularly as the cultivar ‘Hemelrijk’ (see below). In the US, it is also found in most collections, including good specimens at The Morton Arboretum, but it is not as generally popular as might be expected for a species with its qualities, in particular the spectacular autumnal colouring, which rivals that of scarlet oak (Q. coccinea), for many the top performer in that category. Dirr and Warren (2019) remarked that though it is a worthy species, it is ‘seldom known and grown beyond the native range.’

There seems to be confusion regarding its tolerance of lime. While North American authors are unanimous in claiming it is recommended for high pH soils, The Hillier Manual (Edwards & Marshall 2019) continues to mark it as a plant that requires lime-free or neutral soils that will not tolerate alkaline or chalky conditions, a claim that is faithfully echoed in New Trees (Grimshaw & Bayton 2009). This may be due to the fact that the plant may sometimes suffer chlorosis, but presumably for reasons other than intolerance of alkalinity (other causes of chlorosis include drought, poor drainage, and compaction of the soil). Chlorosis has been reported at Eike Jablonski’s Krüchten Arboretum, Luxembourg, in close-to-neutral soils (pH 6.8); similarly, Tom Atkinson (2015) laments that though the species ‘captures his soul,’ when he has tried to grow it in his garden in Toronto it has been slow to grow and soon the leaves are chlorotic and shrivelled. According to Jablonski (pers. comm. 2024), pH tolerance in plants depends to a large extent on precipitation, and tolerance to high pH improves with abundant rainfall. In areas of low precipitation (e.g. 600 mm per year), there is no ‘washing out’, and the presence of nutrients in the soil takes on more importance. In his experience in Luxembourg and Germany, Q. shumardii, Q. texana and Q. velutina have performed better, while Q. ellipsoidalis and other section Lobatae oaks have succumbed to chlorosis.

Quercus ellipsoidalis usually forms a smaller tree than other oaks, such as Q. rubra, but that may be due in part to the fact that it mostly grows on poor soils. It has been known to grow larger in better conditions in cultivation. The champion in the US used to be a tree growing on the campus of the University of Wisconsin-Stevens Point, at 28.7 m × 140 cm, but it succumbed to oak wilt in 2007, at an estimated age of 123 years (Uhlig 2017). The current UK champion for height, according to The Tree Register (2025), grows at Sir Harold Hillier Gardens and reached 24.7 m in 2023, though the record indicates the height is probably overestimated; in 2017, it was 21.7 m high, according to the Sir Harold Hillier Gardens database (Royal Botanic Garden Edinburgh 2024). The contest for girth champion is practically a dead heat between this tree (63.1 cm dbh in 2023) and a tree at Kew planted in 1974 (63.4 cm dbh in 2022, though only 13 m tall).

The species was first described by E.J. Hill in 1899. Though it has been suggested that the epithet may refer to the elliptic outline of the leaf (Heathcoat Amory 2009), the derivation is clearly stated in the protologue as being from the shape of the acorns, which are longer than Q. palustris and have deeper cups (see photo below). The ellipsoidal form of the acorns was the characteristic that first suggesed to Hill that the oak may be a different species, and on the whole, it is the most constant characteristic that sets it apart from its peers (Hill 1899).