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The Butternut has much in common with the White Walnuts of Asia; it is no surprise that their hybrids share a similar look. Most have arisen in North America where J. mandshurica var. cordiformis (Heartnut) in particular is grown for its nuts. Where Heartnut is a parent, the hybrids are given the appropriately hybrid name Buartnut (normally pronounced bew-art nut), or simply Buart.
Well known to North American nut growers, young Buartnut trees are fast growing and rangy, not easily pruned into an attractive shape. They can reach 25 m in height, the crown spreading to 15 m, with a useful life of about 100 years (Crawford 2016). Leaves typically have about 15 large, oblong, shortly acuminate leaflets which are pubescent beneath, but lack obvious axillary tufts (Grimshaw 2004). The nuts are rough, usually pointed at both ends and about 4 cm long; the kernels are almost as oily as butternuts (Crawford 2016). In an orchard situation they are usually pollinated by one of the parent species; some cultivars are self-fertile. Reports of resistance or susceptibility to butternut canker vary and may depend on the individual.
First generation hybrids are very fertile, but may well be pollinated by one or other of the parent species. Even commercially sold buartnuts are often seed raised and may represent backcrosses. Complex hybrids, often closely resembling Butternut, have appeared in natural habitats across much of the wild range of J. cinerea, usually unrecognised (Ross-Davis et al. 2008; Farlee et al. 2010). Their spread has accelerated as Butternut populations are decimated by the emergent fungal disease butternut canker: the disease resistance of many hybrids favours them. J. mandshurica var. sachalinensis and the mainland var. mandshurica are also sometimes grown in North America, and it is entirely possible that these are involved in some hybrids.
In European tree collections, Japanese (and increasingly mainland) forms of J. mandshurica are generally more common than J. cinerea. We should be alert for hybrids between these species, but should perhaps expect backcrossing to be predominantly with J. mandshurica. Grimshaw (2004) concludes that few of the trees labelled J. cinerea in British collections are correctly named, and we should be looking for hybrids among these.
The name Juglans x bixbyi Rehd. is slightly confusing. It applies to hybrids between J. ailantifolia (i.e. both J. mandshurica var, sachalinensis and J. m. var. cordiformis) and J. cinerea. It is widely applied to buartnuts, but those researching invasive hybrids in North America do not use the name, considering that it is not sufficiently capacious to include the range of backcrossed trees which they observe: they refer simply to ‘hybrids’ (Ross-Davis et al. 2008; Farlee et al. 2010). Similarly, it would not cover hybrids involving mainland J. mandshurica, which might be expected in European collections, but would be impossible to distinguish.
‘Mitchell’ is by far the most popular and widely available Buartnut cultivar. It originated in Scotland, Ontario, and has been distributed both by seed (inherently variable) and grafting. It fruits reliably, beginning at an early age and often proves self-fertile. Like other buartnuts it has a hard shell, but is unusual in cracking well. It is resistant to butternut canker (Society of Ontario Nut Growers 2013, Crawford 2016; Grimo Nut Nursery 2019).