Botanical Oddities: Gingko Biloba ‘Thelma’

Botanical Oddities: Gingko Biloba ‘Thelma’

By Alex Henderson, Curator of Living Collections, Royal Botanical Gardens

The ginkgo tree is one of the oldest living tree species in the world, as it is the sole survivor of an ancient group of trees that outlived the dinosaurs. Such plants or animals are termed “living fossils,” remaining almost unchanged from deep geological time and resembling species only known from the fossil record. Fossils of ginkgo tree leaves, dating back more than 200 million years, have been discovered, and they are almost identical to the leaves of those alive today.

Native to Southeast and South-Central China, the International Union for the Conservation of Nature (IUCN) lists Ginkgo as endangered in its native habitat due to severe ecosystem degradation, logging, and wood harvesting, which has caused a continuing decline in mature individuals. The oldest recorded ginkgo tree is thought to be an astounding 3,500 years old, and it is likely that wild species were first domesticated and planted in temple gardens in China and Japan. Today, they are planted in cities worldwide due to their tolerance of urban conditions, pollution, and lack of pest issues, as all of their natural pests are extinct.

Ginkgo is the only member of its genus (Ginkgo), which is the only genus in its family (Ginkgoaceae), which is the only family in its order (Ginkgoales), which is the only order in its subclass (Ginkgoidae)—meaning that, in evolutionary terms, it is truly a solitary species. Ginkgo is also the only living connection between ferns and conifers.

In general, male trees are preferred for cultivation, as female trees bear fleshy, yellowish fruit that, when they fall and decay in October, release a rancid, foul-smelling odor. For those who like living on the edge, female ginkgo trees can be found in the Rock Garden and its parking lot, so best of luck, and godspeed should you choose to experience the malodorous scent firsthand.

The foul smell of the fruit is due to the production of butyric acid. This chemical attracts carnivores, tricking them into thinking the fruit is a tasty piece of meat. However, once they try to ingest the seeds, they quickly realize it’s not meat and spit them out. As carnivores are highly mobile, this results in a handy seed dispersal method, spreading the seeds beyond the mother tree, where they can successfully germinate under optimum conditions. This showcases the fascinating ways plants form reciprocal relationships with other organisms to further their own survival.

If all this makes you think that ginkgo is a botanical oddity, let’s dive deeper. Many cultivars (cultivated varieties) of ginkgo are available from garden centers, but perhaps none are stranger than Ginkgo biloba ‘Thelma’. At first glance (to my eye), this small, compact tree, which reaches a mature height of 10 feet, looks as if its leaves have been stripped from the branches. This is because the foliage is incredibly variable, producing typically fan-shaped leaves (as one would expect) but also leaves that are fringed, rolled, tubular, needle-like, or even lacy (see picture). From a distance, this gives the impression (again, to my eye) that the tree is in distress. Upon closer inspection, it initially confounds expectations, as the foliage appears severely distorted or mutated, overwhelmed by some strange malady.

The fact that this plant is readily available in the market suggests that some ginkgo aficionados appreciate these odd characteristics as a desirable ornamental trait, worthy of addition to ginkgo collections or suitable for general planting in smaller gardens. Personally, I feel that the variable traits of the leaves spoil the stately appearance of this much-loved tree. While aficionados may describe the foliage as charming and favor such hybridization traits, I prefer the more traditional, iconic leaf shape. As the old adage goes, why fix something that isn’t broken?

Another oddity about this cultivar is that nurseries list it as a male tree. As mentioned earlier, male trees are generally more desirable for garden culture since they do not produce the rancid-smelling fruit. So why, then, has a male ginkgo cultivar been given a female name? Even the American Conifer Society makes no mention of this peculiar naming convention, leaving it a mystery.

If you’d like to visit Ginkgo biloba ‘Thelma’, you can find it near the entrance to RBG Centre at Plains Road by what is affectionately known as the banana bed. I recently introduced a colleague to it for the first time, and their response was short and succinct: “That’s wild!”

I wonder what you’ll think?