After centuries of searching the world for new plants, it would seem unusual that there remains a ‘new plant’ yet to be found by botanists. Furthermore, it is all the more unusual if this plant was a small tree that bloomed during mid-summer, just when flowering trees are much in need. Imagine my surprise when I first saw a 30’+ tall specimen of Chinese Pearlbloom, Poliothyrsis sinensis, with its white flowers dramatically displayed against the late July sky at the Scott Arboretum (as seen behind the observatory on the right)! How had I never seen, no less heard of this beautiful plant?
Poliothyrsis sinensis is a member of the Willow or Salicaceae Family and is classified as a monotypic genus since only one species is found within this genus. The plant was originally discovered in the late 1800’s by the Irish plant explorer Augustine Henry (1857-1930) who sent samples back to Kew Gardens in England. Daniel Oliver (1830-1916) was the Keeper of the Kew Herbarium from 1864-1890 and officially described and named the plant in 1885. The genus name is from the Greek Poli or Polio for grey and Thyrs for panicle, which refers to the panicles of white flowers. The species epithet has its roots in the Greek word Sinai, meaning China.
The plant did not appear in North America until the English plant explorer, Ernest Henry Wilson (1876-1930) brought back seeds to the Arnold Arboretum in 1908. One might than wonder how the plant could possibly be new to American Gardens if the plant has been at the Arnold all this time. For reasons unknown, the plants at the Arnold were removed in 1933 and the plant was not reintroduced to the Arnold and North America until 1981 when seeds were received from the Shanghai Botanical Garden. Now, after over a century, it is finally getting the recognition it well deserves!
Although many references describe this plant as a large shrub, it is best grown as a small tree since it can reach lofty heights of 50’ with time. It remains a rather upright or columnar tree with widths reaching 20-25’, making it an ideal candidate for narrower planting sites. It should also be trained as a single-stemmed rather than multi-stemmed plant, since this develops a stronger branch scaffolding over time. The foliage is a very glossy and attractive deep green, and it is nicely complemented by the red petioles or leaf stems (see image below). In spring, the unfurling foliage displays dusky reds and burgundies before changing to green and finally a clean yellow come autumn. The 6-8” long by 3-4” wide flower panicles are produced on the ends of the current season’s growth and yield flowers that are not only showy, but fragrant! The flowers only crack open slightly throughout the length of their bloom, with each flower appearing much like a ¼” diameter pearl that provided the inspiration for its common name of Pearlbloom (see image below on left)! From a distance, the plant actually bears a strong resemblance to the May blooming Japanese Tree Lilac. The flowers are monoecious, meaning that on any given panicle, some of the flowers only form the pollen shedding anthers while other flowers will generate the seeds. The showy portion of the flowers are actually not petals, but modified leaves called bracts, a trait that is also witnessed on Flowering Dogwood. The flower bracts are initially a bright white and age to a buff or yellow-white over the course of a month. The female flowers give rise to gray seed capsules come fall, which again bear a strikingly strong resemblance to the Japanese Tree Lilac! It is the white flowers transitioning to gray seed heads that gave rise to its genus name of ‘gray-panicles’.