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Paradox Walnuts are hybrids between Juglans hindsii and J. regia, although the term is sometimes used loosely in North America to include hybrids between J. regia and other black walnut species (Potter et al. 2002). They are intermediate in leaf form and bark colour between the parents, are somewhat variable and cannot all be distinguished with certainty from other regia–black walnut hybrids except by molecular means. Typically they might have (7–)11–15 leaflets, with squarish, thick-shelled, weakly grooved nuts (Jacobson 1996).
These very vigorous hybrids tend to give low nut yields, but produce fine timber. They are planted quite widely in California and sometimes further north on the Pacific seaboard as shade trees, where vigour is an advantage and nuts are considered messy (Potter et al. 2018). However, their greatest value is as rootstocks for grafted J. regia in the Californian walnut industry, thanks to their vigour and disease resistance.
Paradox hybrids have originated in two ways (Potter et al. 2002). Wild J. hindsii growing close to Common Walnut orchards will produce hybrid seedlings. These may establish in the wild and are considered invasive in riparian habitats in the Central Valley of California (Fryer 2015). Secondly, Californian plant breeder Luther Burbank made the cross deliberately in the latter part of the 19th century, first recording hybrids in 1878 and naming them ‘Paradox’ in 1893 (Potter et al. 2018). Burbank was offering seedlings in his 1898 catalogue, noting that they showed a lot of variation (Grimshaw 2004). The name has never referred to a single clone and is now effectively a grex name. There is some confusion in the older literature over whether Burbank used J. hindsii or J. californica as a parent. This is easily resolved: J. hindsii was not recognised as distinct from J. californica at that time, so Burbank used the latter name, but all contemporary evidence agrees that the trees were Northern California Walnut.
Paradox rootstocks for the walnut industry are raised from seed taken from specific wild individuals of J. hindsii, which receive plenty of J. regia pollen. These mother trees may not all be pure hindsii: using molecular markers Potter et al. (2002) demonstrated that while J. hindsii is the main black walnut involved in these hybrids, some have genetic input from J. major, J. nigra and J. californica.
Paradox Walnut can quickly make an impressive, broad crowned tree. Jacobson (1996) mentions a specimen in Whittier, California, which had a spread of about 30 m and girth of 4 m at only 62 years old. A tree of similar size in the suburbs of Portland, Oregon, was defended by local residents when threatened by a housing development in 2014 (monumentaltrees.com 2019).
‘Royal’ is a rather similar hybrid walnut (J. hindsii x J. nigra) with a parallel history. Again raised and named by Luther Burbank, it also occurs spontaneously in California where J. nigra is planted near wild J. hindsii. Despite being fast growing and productive, it has not found wide favour. An enormous specimen (34 m tall with crown spread of around 45 m in 2007) on Sauvie Island in the Columbia River north of Portland, Oregon, was once considered to be the national J. nigra champion, but has since been reidentified as ‘Royal’ (monumentaltrees.com 2019). It has (11–)17–19(–23) dark green, lanceolate leaflets, more slender and less hairy than those of J. nigra, with sharply toothed margins, a pubescent rachis and nuts less strongly grooved than those of J. nigra (Jacobson 1996, Grimshaw 2004 ). The nuts are illustrated by Jacobson (2014). A specimen at Kew is a rare European example.