This lovely tree's natural range is the Ohio Valley and the Appalachian Mountains, USA. It was introduced to the UK in 1764. The tree I have been looking at is on the front lawn of Hadlow College main campus, which is apparently not grafted. I also had a quick look at the grafted and really quite large tree at the Gravesend Cemetery.
The flowers on this tree are a pale creamy yellow, with yellow flashes inside the two upper petals which soon turn pinky-red as the flowers age. As the flowers age further salmon-pinky tinges appear on the outside of the petal tube. However, I do not know if the flowers should not have any red in them at all.
Importantly the stamens are not exerted outside the petal tube, which character distinguishes this species of tree from the Ohio Buckeye, Aesculus glabra, one of the other Aesculus species with largely yellow flowers. Eventually however the style is exserted. On very limited evidence I would say that Aesculus flava flowers with Aesculus indica one or two weeks after Aesculus hippocastanum and Aesculus glabra, but I cannot be sure of this as the Hadlow tree was flowering at the start of June in 2020, while the tree in Gravesend was over flowering and only showing developing fruit. Maybe the leaves are slightly up-folded - cupped in this photo of the Hadlow tree??
These flowers are ageing a bit and their flashes have already turned red. The outer part of the petal tube is beginning to turn a bit salmon-pinky as well.
The leaves are very neat, appearing glaucous (blue-green) and woolly overall underneath and also specifically hairy on the top of the midrib of the leaf, although the top of the leaf is a good mid-green, contrasting quite strongly with the underside. Some authors say that the pubescence of the underside is quite variable. There are five leaflets generally, not seven, at least as far as I could tell on this tree. This seems quite a controversial point, Mitchell sees it as partly diagnostic just having five leaflets, while others think this can vary up to 7. However Mitchell also says that the leaflets are fully stalked, noted to be glossy, and often cupped, none of which I can really fully see in this tree. I don't think the leaves are fully glossy but the stalked leaflets is the thing I most take issue with, as the leaves is the Sibley book look sub-sessile, like the Hadlow tree. Maybe it would be good to walk the hills of Ohio and see the variability of the trees that actually exist in the full population. It would also be interesting to see the "hybrid zone" containing Aesculus x marylandica on the boundary between Aesculus flava and Aesculus glabra. Quite a task!
The feature of "leaf cupping" that is mentioned by Mitchell and on line might be this upward folding of the leaves that I think you can see in this photo of the Hadlow tree.
Looking at the Gravesend tree, which I didn't photograph, it fits the "standard" description much better - the leaves a bit more cupped, they are a little glossier, and the leaflets definitely have noticeable stalklets or petiolules. They are a bit more broadly obovate, while the Gravesend tree has leaves a little bit slimmer and more elegantly pointed at the tip. Could it be that the Hadlow tree is somehow atypical, possibly even containing a small amount of Aesculus hippocastanum?
The large lumpy spineless fruit should normally contain 1-3 seeds (conkers).
The leaves are said by Mitchell to give good bright orange-red autumn colour, Collins and More have it as even redder, being the reason why it is planted, but Braakman at the National Arboretum in Holland says that there is no autumn red colouring in the pure species. All very strange!
The twigs are said to have a bit of an odour, but not the rank offensive smell of Aesculus glabra. The branches are often said to be twisting and ascending.
The bark of the species is said to be a smooth dark brown (Bean's) or grey-brown with some scaly flaking. The trunk of this Hadlow tree has been damaged to the southwest, with flaking bark, but I am not at all sure by what. So I must travel to see other trees planted within striking range.
According to Trees and Shrubs online, there is a form called A. flava f. virginica, in which the flowers are red instead of yellow. This form was supposedly collected by George Washington for his garden in Mount Vernon. This source views such trees, "if in cultivation", as possibly distinguished from A. × hybrida, the reddish-flowered cross between Aesculus flava and Aesculus pavia by the absence of glands from the edges of the petals. However the Holden Arboretum
https://holdenarb.org/horticulture/plant-profiles/aesculus-x-hybridia/ views these two taxa as synonyms for the hybrid - in other words all these trees are the hybrid, with presumably the glands present on the sides of the petals. The hybrid has exserted stamens.
Another possible variety is f. vestita, which is pubescent under the leaves and along the young branchlets. Found in various localities in the wild (Bean's)
It is said to prefer acid to neutral soil. In the UK it is suggested that shelter may help to get it established and that it does better long term when the top of the canopy can be in full sunshine.
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