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Plant Focus

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A guest post by Matt Candeias, host of the In Defense of Plants podcast and blog

Cultivar Close-Up: Bokrijk Cultivars

In 2014, Jef Van Meulder published an article (in Dutch) in the Belgian Dendrology Society’s Bulletin, presenting six oak cultivars that had been selected from trees growing in Bokrijk Arboretum, Belgium, where Jef was Curator of Living Collections from 1978 to 2017. All the selections are from garden-sourced acorns, suggesting the horticultural potential that this type of seed—often disparaged—can have. The cultivars are presented below, with a summary of their history and some of their salient characteristics. Registration of these names is in progress and they will appear in International Oaks, the IOS Journal, in due course. 

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Leaves and aborted acorns of Quercus 'Buksenrake Mystery' © M. Weckx

Quercus 'Buksenrake Mystery'

Four of the cultivars originate from seed collected by Jef in Arboretum Paarl in South Africa, which he visited in 2000. The Arboretum, founded in 1957, is located in Drakenstein Municipality in Cape Province, on the banks of the Berg River. When Jef visited it was in a state of slight neglect, but as he says, “you can learn many things from a neglected arboretum, especially which trees thrive without human assistance.” Quercus canariensis and Q. rugosa had done particularly well, and acorns were collected from these species and also from Q. ilex and from a tree identified as Q. invaginata but not listed in the Arboretum’s catalog. It is a Mexican species, rare to find in cultivation (especially in 2000), but appeared to conform to the characteristics of that species, especially the acorns with their revolute cupule rims.

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The original Quercus 'Buksenrake Mystery' in Bokrijk Arboretum © M. Weckx

Seed collected from this putative Q. invaginata produced an evergreen tree that has shown remarkable growth, reaching a height of 15.5 m by 2014. Another notable characteristic is its winter hardiness: even in the severe winter of 2009-2010, which saw temperatures of -21 °C, it suffered no damage to leaves or trunk. It is clearly of hybrid origin, and the likely pollen donor is a Q. ilex growing next to the female parent. Quercus invaginata is in section Quercus, and intersectional hybrids with Q. ilex (section Ilex) do occur. Other possible parents growing in the Arboretum are Q. robur, Q. canariensis, Q. cerris, Q. suber, and Q. rugosa.

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Corky bark on Quercus 'Buksenrake Mystery' © M. Weckx

As the smooth trunk is reminiscent of young Q. cerris trees, it was initially thought that it might be the hybrid Q. ×crenata (Q. suber × cerris, formerly known as Q. ×hispanica), and that the seed had been mixed up, but when the tree started fruiting, the cupules indicated no section Cerris influence, but rather that of Q. invaginata or Q. ilex. It is interesting to note that the tree at Bokrijk has not produced fertile seed, only aborted acorns that never fully form. As the identity of the parents could not be firmly established, the cultivar has been named ‘Buksenrake Mystery' (Buksenrake is the former name of Bokrijk, it means “a place on the moor where beech trees grow”). It has been propagated by grafting on Q. cerris stock; time will tell whether this was the best choice.

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Quercus 'Buksenrake Mystery' © Jef Van Meulder

Quercus ilex 'Thimon'

Another cultivar of South African origin selected at Bokrijk grew from seed from a Q. ilex at Arboretum Paarl. It was given away as a seedling by Bokrijk and now grows in a private garden in nearby Diepenbeek. It is distinguished by its vigor and winter hardiness. Like Q. ‘Buksenrake Mystery’ it survived unscathed both the severe winter of 2009-2010 and another particularly cold season two years later, despite growing in a backyard without special protection. Again, this was thought initially to be Q. ×crenata, but once it fruited the acorns revealed its true colors as being Q. ilex. The seedling is about twice as high as a dozen other seedlings of the same origin. The tree was a “birth tree”, planted to honor the son of the house, Thimon, so the same name was used as the cultivar epithet.

Quercus 'Kees'

A third cultivar sourced from Arboretum Paarl was grown from a batch of acorns collected from Q. rugosa. Candidates for the male parent include Q. canariensis, of which there several giant specimens in the Arboretum, and Q. robur, widely planted as avenue trees. The grafted plants of this cultivar have turned out to be quite sensitive to oak powdery mildew (Erysiphe alphitoides), a fungal disease that is common on Q. robur but not at all on Q. canariensis. In his 2014 article, Jef concluded that the cultivar is most likely a cross of Q. rugosa and Q. robur, a hybrid named as Q. ×warburgii by Aimée Camus in 1939 and also known as Cambridge oak, due to the fact that a famous specimen grows at the Cambridge Botanic Garden. However, the IOS Registrars are of the opinion that Q. canariensis influence is evident and that Q. rugosa may not be involved, hence the name is due to be registered as Q. 'Kees', with no hybrid epithet.

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Leaves of Quercus ‘Kees’, with reticulate undersides indicating Q. rugosa influence © Jef Van Meulder

Interestingly, powdery oak mildew, which in Europe only attacks young trees of Q. robur and is not much more than a nuisance, in South Africa has proven fatal to the old oaks widely planted by Dutch settlers centuries ago. They are being replaced by Q. cerris and Q. nigra, which according to Jef is a pity, as Q. canariensis grows particularly well there and would be a better choice. Bokrijk had several beautiful trees raised from this seed batch, but all save one succumbed to the dread winter of 2009–2010. The surviving specimen was selected for propagation; its leaves have a reticulated underside, but it is not as close to Q. rugosa as the seedlings that died. It is generally deciduous, but can be semi-evergreen in a mild winter. It was named Q. ‘Kees’, after Mrs. Liesbeth Kees, who had been appointed director of domain Bokrijk shortly before the cultivar was described.

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Quercus  'Kees' in Bokrijk Arboretum © M. Weckx

Quercus ×crenata ‘Mr Lieben’

The final South African–sourced cultivar grew from seed collected at Arboretum Paarl, but records of further details have been lost, so what its parents might be is a matter of conjecture. It is similar to Q. cerris, though its bark indicates it has borrowed some genes from Q. suber. Unlike typical Q. cerris, however, it is wholly evergreen and has kept its leaves almost entirely through Bokrijk’s harsh winters. Acorns might help settle the issue, but they have not as yet been observed.

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Leaves of Quercus ×crenata 'Mr Lieben' © M. Weckx

According to Jef, this is a promising oak that could “create a furor in urban environments” in Northern Europe, where native species usually don’t thrive in the cities’ heat islands. The selection, Q. ×crenata ‘Mr Lieben’, was named after one of Bokrijk’s gardeners.

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The bark on Quercus ×crenata‘ Mr Lieben’ suggests Q. suber parentage © M. Weckx

Quercus ×haynaldiana ‘Bokrijk’

The two remaining Bokrijk cultivars are also grown from garden-sourced seed. Quercus ×haynaldiana ‘Bokrijk’ is a seedling of a Quercus robur Cristata Group at Trompenburg Gardens, from acorns collected at an oak study day held there under the auspices of the International Oak Society. According to Dick Van Hoey Smith, at the time director of the garden, the Q. robur Cristata Group had that year flowered at almost exactly the same time as a neighboring Q. frainetto, so the acorns of the former may turn out to be a cross with the latter.

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Leaves and acorns of Quercus ×haynaldiana ‘Bokrijk’ © Jef Van Meulder

Jef put this to the test at Bokrijk, and though most seedlings were of inferior quality and discarded, two were retained for further evaluation. Of these, one was selected and, based on MrVan Hoey Smith’s comments, was described as a Q. ×haynaldiana, i.e., the cross of Q. robur and Q. frainetto (named by Simonkai in honor of the Hungarian archbishop and naturalist Lajos Haynald). Its leaves have the typical crisped look of Q. robur Cristata Group, but they are much more robust.

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Quercus ×haynaldiana ‘Bokrijk’ at Bokrijk Arboretum © M. Weckx

Quercus ×vilmoriniana 'Limburg'

This selection was grown from seeds collected in 1989 from the ortet tree of Q. ×vilmoriniana at Arboretum national des Barres in France. The mother tree, a famed cross of Q. dentata and Q. petraea planted by Maurice de Vilmorin in 1894, died three years later.

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Leaf-shape variation in Quercus ×vilmoriniana 'Limburg' is to be expected in an F2 hybrid © M. Weckx

It was selected from amongst its siblings for being particularly healthy and for its beautiful dark green foliage, similar to Q. petraea but for the dense felt on the undersides, attributable to its Q. dentata ancestry. Young plants grafted on Q. robur rootstock have done well so far. The cultivar was named after the Belgian province of Limburg, in which Bokrijk Arboretum is located.

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The fringed acorn cup on Quercus ×vilmoriniana 'Limburg' is also found on other Q. ×vilmoriniana seedlings © M. Weckx
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Quercus ×vilmoriniana 'Limburg' in Bokrijk Arboretum © M. Weckx