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Carya species hybridise widely and freely, but there are limits to their interfertility. Various hybrids exist in collections, some probably unrecognised as such, but few are of much significance. The main exceptions are hybrid cultivars selected in North America for nuts, especially the hicans, which are hybrids between C. illinoinensis and any other Carya species. Many hybrids are poorly described in the literature. The main aim of this account is to alert the reader to what exists, as well as highlight a few more significant varieties. We list in turn: hybrids among true hickories; hybrids among pecan hickories; intersectional hybrids; and finally an alphabetical list of those latinised names which exist. We have not been able to trace any hybrids involving East Asian hickories. Except where otherwise stated, information is taken from Stone & Whittemore (1997) and Grauke (2003).
Among the diploid species, C. ovata and C. laciniosa hybridize to give C. × dunbarii Sargent; nut bearing cultivars include ‘Abundance’ and ‘Weicke’. A specimen at Westonbirt persisted for more than 50 years but is now gone.
Among the tetraploids, C. glabra intergrades with C. texana, C. pallida and C. floridana where their wild ranges overlap; whether or not this involves hybridisation is a moot point. Similarly, C. texana and C. pallida intergrade where they meet. C. texana × C. tomentosa has been named C. × collina Laughlin.
Hybrids between the two ploidy groups are rare at best; it has been suggested that C. ovata × C. tomentosa may exist.
Carya illinoinensis certainly crosses with both C. aquatica and C. cordiformis. C. illinoinensis × C. aquatica is named C. × lecontei Little, and occurs readily when they are grown together. It is intermediate between the parents with rough, flattened, mid-brown nuts. C. illinoinensis × C. cordiformis is named C. × brownii Sargent, and is one of the important groups of hican. Its nuts are typically somewhat flattened with prominent apices, and are four-lobed at the base. Some have bitter nuts, some sweet. Named cultivars include ‘Galloway’ with sweet nuts and ‘Pleas’ which is good in northern areas but has rather bitter nuts (Crawford 2016).
C. illinoinensis gives two more groups of hican by crossing with C. ovata and C. laciniosa. C. illinoinensis × C. ovata seedlings include two of the best hicans (Crawford 2016), ‘Burton’ (hardy, prolific and cracking well) and ‘Dooley’ (with similar qualities). Both the parent species are grown for nuts, and this hybrid has turned up in both the United States and Mexico. There is an example at the Sir Harold Hillier Gardens, Hampshire (6 m × 10 cm in 2007 – The Tree Register 2019).
C. illinoinensis × C. laciniosa is – rather wonderfully – named C. × nussbaumeri Sargent. This the parentage of another much-favoured cultivar, ‘James’, which is precocious, with very large nuts (Crawford 2016). Others include ‘McAllister’, widely grown in North America but a poor nut producer, and ‘Gerardi’, a precocious cultivar sometimes used as a rootstock for pecans.
A putative hybrid between C. illinoinensis and the tetraploid C. tomentosa was named C. × schneckii Sargent.
Hybrids between C. cordiformis and C. ovata are called C. × laneyi Sargent. This cross sometimes occurs in the wild. There are several cultivars occasionally grown for their nuts, of which ‘Laney’ is the type.
Hybrids between C. cordiformis and both laciniosa and the tetraploid tomentosa have been claimed. C. × demareei Palmer is a reputed hybrid between C. cordiformis and the tetraploid C. glabra from a site in Arkansas,
Finally, the name C. × ludoviciana (Ashe) Little refers to C. aquatica × C. texana. Described from a single Louisiana specimen with a flattened nut, this hybrid is heavily disputed: the specimen may be atypical C. texana.
C. × brownii Sargent = C. illinoinensis × C. cordiformis
C. × collina Laughlin = C. texana × C. tomentosa
C. × demareei Palmer = C. cordiformis × C. glabra
C. × dunbarii Sargent = C. ovata × C. laciniosa
C. × laneyi Sargent = C. cordiformis × C. ovata
C. × lecontei Little = C. illinoinensis × C. aquatica
C. × ludoviciana (Ashe) Little = C. aquatica × C. texana
C. × nussbaumeri Sargent = C. illinoinensis × C. laciniosa
C. × schneckii Sargent = C. illinoinensis × C. tomentosa