Wang’s Horsechestnut

Uoc Huu Le, my guide in Vietnam since 1999, holding a conker near one of the few ancient surviving specimens of Aesculus near Khoang, Vietnam in 2016. (Photo DJH)

The genus Aesculus, whose Latin roots suggest ‘an oak with edible fruit’, was first published by Linnaeus in 1753 and applied to Aesculus hippocastanum, a tree that was certainly not an oak and whose fruit was sickeningly inedible.  The resulting common name of Horse Chestnut only added another layer to an already confused gastronomy, resulting in countless episodes of dyspepsia over many years from roasting its seed over an open fire.  

Aesculus wangii foliage, in my opinion, possesses a discernible refinement of textural characterisitics over other Aesculus species. Horsechestnuts have palmately compound foliage held opposite in pairs. Castanea (edible Sweet Chestnus) possess simple leaves alternately arranged along the stem. (Photo at Windcliff by Claire Takacs)

Recent DNA barcoding has concluded that the approximate 16 species of Aesculus to be first cousins to an exhaustive inventory of mostly tropical and subtropical genera in the Sapindaceae.   Hardier outliers in this family, besides Aesculus,  include horticultural rock stars, such as maples (genus Acer) and the Golden Rain Tree (genus Koelreuteria).    

Although the genus Aesculus holds many ‘horticonic’ trees that hail from both Eurasia, North America as well as south to Baja California, the focus of this post is on one remarkable, misunderstood and unpublished taxon,  Aesculus wangii.  It is found naturally, and rarely, in a highly restricted area of northern Vietnam, in the uplands between the Black and Red Rivers (Sông Đà  and Sông Cái respectively).  Its specific epithet honors Prof. Wencai Wang (1926-2022), a famous Chinese botanist from the Beijing Institute of Botany. (personal communication, Dr. Donglin Zhang)

Aesculus wangii beginning to blosssom in situ, northern Vietnam, in April, 2018 (photo DJH)

By late 2007, the year I was first introduced to this species, I had already under my belt four extended forays into the botanically opulent forests of the mountainous areas north of Hanoi, in relative proximity to the border with China’s Yunnan Province. Furthermore, I did not become aware of its existence in the usual manner, that is, of stumbling upon a specimen along a trail or reading of it in some outdated flora of the region.   

A small remaining slice of old growth forest (Tsuga, Aesculus, Illicum, Huodendron, Acer, Rhododendron, Nyssa et al) only miles from the border with China’s Yunnan Province. With enhanced logging equipment and a burgeoning economy, the rate of deforestation since my first trip to this region in 1999 and present day is startling. (photo, DJH)

Instead, quite unromantically, it was in a somewhat seedy hotel in Taipei, Taiwan, in October of that year, that my friend and celebrated Welsh nurseryman, Bleddyn Wynn-Jones, gifted me a parcel of extremely large, glossy, mahogany-colored seed, each the size of baseballs. He had just collected these in northern Vietnam prior to the time we would spend together in the mountains of Taiwan.  It should be noted that though Bleddyn’s seed may have been the first ever exported, U.K. plantsman Keith Rushforth et al had already established at least four specimens in England by grafting scions collected as early as 1991. (the first of these to flower was at the Ness Botanic Garden in Liverpool in 2019) 

A day’s walk from the closest road, a young cardamon farmer carries his crop to a crudely constructed kiln nearby where the fruit will be dried before later selling to spice merchants Cardamon plantations throughout the mountains of northern Vietnam are creating vast dead zones of biodiversity due to the shade they cast on the forest floor. (photo, DJH)

Its scarcity in the wild is due, in part at least, to ubiquitous cardamom plantations throughout the entirety of northern Vietnam.  Black Cardamom, Amomum tsao-ko, produces dense 15’ thickets of banana-like foliage, preventing successful regenerative growth of any other plant species.  Fortunate for the few remaining and impressive specimens of this tree (and other rare tree species), the crop is fully dependent on shade for successful cultivation.  Equally threatening is, well, its rarity; fewer individuals equate to inbred genetic pools resulting in loss of vigor and fertility.  More threatening still, Donald Trump is President of the United States.

The flowers of Black Cardamon (Amomum tsao-ko) presented at the base of the plant in April. It is a member of the ginger family, Zingiberidaceae.

An magnificent specimen of Aesculus wangii just beginning to leaf in mid-March. On this particularly fine day in the spring of 2022, I climbed atop a rock outcropping that rose above the towering canopy of cardamon.

The seed I was gifted by Bleddyn in 2007 might have been included in the collections of Windcliff, Washington Park Arboretum and Heronswood as early as the summer of  2008. Troublingly, its biology and the rules and regulations of the USDA-APHIS got in the way.   

The large conkers of Aesculus wangii are fully primed to germinate immediately upon dropping in autumn.  The shipment of seed I sent from Taichung, Taiwan that year, cuddled in the warmth and darkness of an insulated cardboard box, arrived in Seattle as fully germinated plants; rejected as such under my seed permit by the inspectors, they were duly incinerated.

Conkers from my garden specimen, showing the three-parted husk surrounding the extremely large seed. (photo, DJH)

It was not until 2010, when, on a particularly noteworthy dry and sunny day in October, we seemingly flew up the Khoang Valley where we found a trove of the seed we sought.  Armed with better data, one of these, planted at Windcliff in 2011, blossomed and set fertile seed for the first time in 2022, presumably the first to blossom in North America. 

Seeds now collected from my original 2010 collection germinate in mid-winter in my greenhouse. The brilliant red stems of the seedlings are sadly not repeated in the new growth of mature trees. (Photo, DJH)

I have made the same trek up that slope many more times since, most recently in the autumn of 2023, not to collect additional fruit but to witness and record the health of the remaining population. It is by continuing to study these few remaining trees, both in situ and ex situ, that a more accurate taxonomic picture will unfold.  Aesculus wangii was never published as a valid name due to an error in presenting voucher material.   Is this tree, as the Flora of China suggests,  simply an extension of Aesculus assamica, a horsechestnut species common to the north and east?   If so, by rules of priority, the honoring of Wencai Wang’s contributions to botany will be abandoned.  

My 2010 collection of Aesculus wangii in autumn color in early December at Windcliff (photo DJH)

The flowers of Aesculus wangii on DJHV 10039, flowering for the first time in June, 2022, thirteen years after germination. Self-fertile, Tiger Swallowtails appear to be its primary pollinator at Windcliff.

Never gone, however, will be my experiences surrounding this magnificent tree, brought to life each time I admire its foliage in summer and autumn, or stoop in autumn to retrieve one of its astoundingly large seed.   

DJH 4/6/25

Aesculus wangii is available for sale at at Heronswood, Ravenna Gardens of University Village in Seattle, and Bay Hay and Feed on Bainbridge Island.. Additionally, Cistus Nursery (Portland) offers this collection through mail order.

Windcliff Plants( updated inventory here) is open to visitors Tuesday, Thursday and Saturday (appointments can easily be made by emailing  windcliff2@centurytel.net ) If you are visiting for a specific plant, please ask regarding its current availability prior to your visit.  

Windcliff Nursery and Garden will host is first Garden Open of 2025 on June 14th, 9:00 AM – 2:00 PM. Entrance is free and the nursery will be available for shopping.