Aesculus chinensis Seeds | Chinese Horse Chestnut
LATIN NAME: Aesculus chinensis
COMMON NAME: Chinese Horse Chestnut
Plant Profile
The upright panicle is a pollination machine with a signal built in. Each white flower opens with a yellow patch at the base of the petal, and once a bee has worked it and the nectary is drained, the patch turns red. Bees read the colour and skip the red flowers, which saves them time and gives the tree a tidy way of directing traffic to the flowers that still have nectar.
Fertilised flowers make a leathery capsule that swells through summer and bursts in autumn to release one to three glossy brown seeds, each with a large pale scar where it was attached. The wild type has a spiny capsule wall; the Wilson form has a smooth one, a difference that has caused plenty of argument about whether the two are species or varieties.
Compared with Aesculus hippocastanum, the Chinese species is a slightly smaller tree with narrower leaflets, usually five to seven in a palmate hand rather than seven or more, and its panicles are more slender and a little later. The seed is smaller too, and the capsule less fiercely armed in the Wilson form. In leaf alone the two are easy to confuse, which is why the fruit is the reliable test.
Science & Biotechnology
The seed that children in Europe know as a conker is, in China, a drug. Dried seed of this species is sold as Suo Luo Zi, an item with its own entry in the Chinese pharmacopoeia, dispensed for stomach and chest complaints. The tree that produces it grows wild in northern China and is planted around temples, where the upright flower candles are treated as part of the spring display. One plant, two entirely different careers.
The active fraction is escin, also written aescin, a mixture of triterpene saponins built on protoescigenin-type aglycones and the same complex that made the European horse chestnut a venous remedy. Seed, leaf and bark all carry the saponins, which foam in water and act on cell membranes. That activity is why the raw seed irritates the gut and why it is processed, not eaten.
The bark and twigs carry a second group of compounds, the coumarin glycosides esculin and fraxin, which break down to aesculetin and its relatives. Esculin has a long history as a laboratory reagent and as an ingredient in skin preparations. Its presence is also the reason a strong infusion of young bark glows blue under ultraviolet light, a property that has been used to detect the material in mixtures.
The genus runs on forty chromosomes, and species cross within it readily when their flowering times overlap. The best-known garden product of that promiscuity is Aesculus x carnea, the red horse chestnut, a hybrid between the European species and the American Aesculus pavia. Chinese material enters breeding programmes as a source of later flowering and of tolerance to drier summers.
Because the saponin fraction is commercially interesting, tissue-culture routes have been explored within the genus: callus and cell-suspension lines, and somatic embryogenesis from seed and leaf explants, used both for clonal propagation of good forms and for studying how the saponins are assembled.
Uses & Benefits
The whole plant is toxic raw. Saponins and a glycosidic fraction make the seed dangerous to people and to livestock, and cases of poisoning usually involve children who treat the seed as a chestnut or animals that browse fallen fruit. The seed is safe only after processing, and the processing is a pharmaceutical matter, not a kitchen one. Squirrels and deer are more tolerant than we are, which is why the seed often disappears before you find it.
Growing from Seed
Sow the seed fresh in autumn, as soon as the capsule splits. Use a free-draining mix of loam, leafmould and grit, set the seed on its side with the pale scar facing down and just cover it, then leave the pan outside under mesh through winter. The seed is large and the shoot comes up from a stout stem, so give each one its own deep pot once it has rooted, and lift the pots off the ground so the taproot is not disturbed by worms.
Young trees grow quickly in deep soil and full sun. Water well through the first two summers, mulch with compost each spring, and keep the area under the crown clear, because the tree casts a dense shade and the seed litter is significant. Watch the leaves for the blotch miners that trouble horse chestnuts across the genus; on this species damage is usually cosmetic, but pick up and bin fallen leaves where an infestation is heavy, as that is where the insect overwinters.
Our Take
Grow it for the flower candles in a park or a large garden, and know the seed is a medicine and a poison rather than a snack. I keep mine beside the European buckeyes I raise from Aesculus seed, which makes the difference in capsule and seed plain every autumn, and I have been tempted more than once by the smaller shrubby species from Aesculus parviflora seed for a border where a full-size tree would not fit. The smooth-fruited form appears in catalogues as Aesculus wilsonii seed, and growing the two side by side is the quickest way to see what the argument is about.
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