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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Aesculus + dallimorei' from the website Trees and Shrubs Online (treesandshrubsonline.
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This interesting hybrid was brought to the notice of Kew by the late William Dallimore. He had observed that a tree of A. flava, growing near his home at Bidborough, Kent, bore a branch which in leaf and flower resembled the common horse-chestnut; the tree was grafted, and the branch in question had arisen from the union of stock and scion. It was described by J. R. Sealy in Journ. R.H.S., Vol. 71, pp. 420–3 (Sept. 1956).
The leaves of A. dallimorei are of about the size of those of the common horse-chestnut, and resemble them further in having a rusty-red indumentum at the base of the leaflets and along the midribs of the undersides. In general shape the leaflets resemble those of neither parent, though the long tapering points recall A. flava, as does the white indumentum that covers the surface of the undersides between the main veins. The inflorescence resembles that of A. hippocastanum in general appearance. The flowers are intermediate, having four petals in two dissimilar pairs, the lower pair shorter but broader than the upper (in the horse-chestnut the petals are five in number, all of the same size; in A. flava the petals, although similar to those of A. dallimorei in number and shape, are more erect). As to colour, the majority of the flowers are white with red markings; but some suggest A. flava in having primrose-yellow or greenish-yellow petals with deeper markings at the base.
Sealy considers that A. dallimorei is most probably a graft-hybrid (periclinal chimaera) between A. hippocastanum and A. flava. And indeed this would be certain if the stock used was of the former species. There is, however, the possibility that the seedling stock onto which the scion of A. flava had been grafted was a sexual hybrid between the two species, in which case the anomalous branch could be explained as an outgrowth from the hybrid stock. Such a cross has, however, never been recorded, and, in Sealy’s opinion, union of the tissues of stock and scion is the most likely explanation.
The original tree is in the care of the Kent County Council and its offspring are likely to make attractive ornamental trees, although a long time will elapse before they become generally available.