The yellowhorn (Xanthoceras sorbifolium) is surprisingly underutilized in our North American landscapes although its availability is certainly associated with its obscurity (and vicea versa). Also called goldenhorn, white raintree and Chinese flowering chestnut, this small, deciduous tree from northern China is hardy in Zones 4-7 and is surprisingly durable and long-lived once established. First collected and named in 1833, this member of the soapberry family (Sapindaceae) initially became cultivated in Europe (Paris) by 1868. Joseph Hooker, Director of the Royal Botanic Gardens - Kew (1865-1885), described this plant in 1887 as “one of the most attractive and interesting hardy garden shrubs that has been introduced in many years.” Yellowhorn has also received the Royal Horticultural Society Award of Garden Merit which should hint at its potential value and impact in the landscape.
Yellowhorn features lustrous green, compound leaves that resemble the foliage of mountain ash (Sorbus), hence the specific epithet of sorbifolium. The pinnate foliage features very narrow leaflets that are quite glossy. This plant certainly contributes a fine texture in the landscape and the fall color can range from a muted gold to a clear yellow. Yellowhorn will reach heights between 8-25 feet with a width of 10-15 feet in time and should be considered slow to moderate in terms of growth rate. This size fits right in between the loose definitions of a large shrub and small tree. With age, the form becomes more upright with a stiff, coarse branching habit although the significant flowers (mentioned below) may bend the branches to a certain degree when at peak bloom.
Specimens are frequently multiple trunks although some are single trunks and branched fairly low to the ground. Yellowhorn may slowly colonize a space with some root suckering which is not a characteristic of every specimen. The fleshy, fibrous roots make this plant challenging to transplant which is noteworthy. The commonly observed high transplant mortality seems to be associated with a sensitive root structure. Gentle handling is prudent during all phases of planting.
Blooming in May, the fragrant flowers, appearing on terminal racemes, are white with very light green streaks and a center that age from yellow on the newest flowers to a gorgeous red orange on older flowers. These proliferous flower clusters can be up to 10 inches long and individual flowers are roughly 1 inch in diameter with five petals.
Another common name for this plant is “popcorn shrub” due to the appearance of the flowers upon opening. The duration of bloom is only about two weeks, but it is a gorgeous and memorable display. Plants as young as two to three years old will start to bloom readily. William (Ned) Friedman of the Arnold Arboretum (Harvard University) writes about the value of the yellow to red color shifts in the flowers of yellowhorn and other woodies with a similar flowering characteristic. Ecologists have shown that insects have an innate preference for yellow flowers over red and by targeting the younger flowers with yellow accents, they are assured of more nectar and pollen. Those that have faded to red have likely been visited already. Friedman mentions that this yellow to red color shift has evolved to help steer insects to newly opened flowers.
The fruits, more common on older specimens, are 2 ½-inch, pear-shaped, leathery capsules. The capsules are initially green and resemble a black walnut husk but later age to a brown and split open in to three chambers that contain the glossy, pea-sized black seeds. The half-inch seeds are edible and when roasted, have the flavor of macadamia nuts. The seeds are also used to produce a quality cooking oil and aside from being roasted, can be boiled or dried and ground into flour. Apparently, this plant also has edible flowers and foliage which are traditionally boiled in advance of consumption.
Yellowhorn can be found in Beijing, China as a small urban tree and is also commonly found throughout a wide range of other urban settings. There are actually large plantations of yellowhorn in China as the seeds are showing great promise as a highly suitable biofuel.
Yellowhorn is commonly propagated from seed or cuttings (stem and root suckers). A higher germination rate for the seeds has been observed with three months of cold stratification. Soaking the seeds for 24 hours before sowing combined with scarification is also recommended by some sources.
The first challenge in growing yellowhorn initially starts with sourcing it. Finding this plant will certainly be a quest but one ultimately worth the time. This unique and beautiful woody plant has impressed those that have grown it over the many years since it was introduced into cultivation.
In the landscape
Yellowhorn doesn’t mind slight dampness but is quite sensitive to excessive moisture or heavier soils that stay wet. In general, this plant isn’t overly picky about soil although again, drainage is important. A loamy soil would be ideal but isn’t essential for the success of this durable woody plant and slightly acidic to slightly alkaline soils are just fine. A pH range between 5.5 to 8.5 is recommended. Yellowhorn is very sensitive to too much shade where it will simply not thrive or flower well. A full sun location is ideal as is plenty of summer heat associated with that exposure. This plant has no significant insect or disease problems although coral spot fungus has been observed on occasion.
Keep in mind that yellowhorn also flowers on old wood so any pruning should be accomplished immediately following the bloom cycle similar to the approach with lilacs (Syringa). An isolated yellowhorn can still flower and fruit although multiple specimens will assure more significant fruiting.
There is a variety called Clear Creek (‘Psgan’) selected by Green Acres Nursery in Golden, Colorado, and promoted by Plant Select, which markets a brand of plants designed to thrive in high plains and intermountain regions. This selection has all the same features of the species but is thought to have increased cold tolerance and hardiness.
Whether yellowhorn is used as a specimen plant, in a mixed border or as spring feature in a prominent location, it certainly deserves broader awareness, availability and enjoyment of its attributes.
Mark Dwyer was the Director of Horticulture at Rotary Botanical Gardens in Janesville, Wisconsin, for 21 years. He has degrees in landscape architecture and urban forestry and now operates a private consulting practice, Landscape Prescriptions by MD. www.landscapeprescriptionsmd.com