Washington DC Department of Urban Forestry nips stinky seeds in the bud
By Liza Lester, ESA communications officer.
A male Gingko biloba in Lafayette Park, flanking the White House. Credit, Liza Lester April, 2012.
As an urban arboreal companion, the ginkgo has much to recommend it. Its tall branches bring welcome summer shade, the fans of its leaves turn a lovely gold in the fall, it copes well with city pollution, lives for thousands of years, and isn’t prone to disease or insect infestation.
But it has a serious drawback. In the fall, mature female ginkos produce fleshy seeds (not a “fruit” in the parlance of botany, as the ginkgo is not an angiosperm, or flowering plant), and unlike cherry season, the height of ginko reproduction is not a time of celebration. The seeds drop all over city streets, smelling “like dirty socks and vomit.”
Some city dwellers hate the trees so much that they are willing to cut them down rather that endure the annual mess. Rather than massacre female ginkgo trees all over the city, this week DC’s Urban Forestry Administration will spray the trees with “Shield EC” aka “Sprout Nip” aka “chlorpropham,” an herbicide that interferes with the division of plant cells during growth. Agricultural distributors typically use chlorpropham to discourage potatoes from sprouting after harvest. Buds and shoots – anywhere the plant is actively growing – are hotspots of cell division, and the incipient ginko seed buds fall off before they can grow stinky. At least, that’s the idea. Not all customers are satisfied.
Since only female trees are a problem, it would make sense to plant only male trees. But male and female trees look identical when their reproductive parts aren’t hanging out. It can be a good two decades before a tree matures and begins to produce either pollen cones or seeds. Botanist CL Lee’s argument for an X/Y sex determination scheme (like the human mechanism), pointing to a subtle chromosomal difference between the sexes, has not been confirmed in the fifty years since he proposed it. Genetics has not provided an easy solution. Although Chinese scientists have been looking for molecular signatures that would allow botanists to sex young saplings, there is no easy test as of yet.
Instead, nurseries now take cuttings of mature male trees to create “clones” of the male tree, either inducing root growth, or grafting the cutting to the roots of a young tree (sometimes this backfires when the graft fails and the root stock turns out to be female, hence reports of male trees turning female). But in the meantime there are robust, mature female trees all over the eastern seaboard. If you’re blessed or cursed with ginkgo seeds this fall you might make the best of it and eat them.
Ginkgo biloba is the last representative standing of a once mighty family. Jurassic era Ginkgo fossils appear all over the world, but the trees lost ground over the millennia. Within the last few million years, the fossil record only finds them in central China. There are only a few “wild” refuges left, some of which were probably cultivated by Buddhist monks.
But in the last few hundred years, the ginkgo has regained a worldwide range – in our cities and gardens.
Ginkgos are the lone living representatives of an old evolutionary branch, and relatively closely related to the flowering plants (angiosperms). There is an old argument about the relationships of the seed plant lineages, and the evolutionary innovation that led to an explosion of flowering species in the last 100 million years. Molecular phylogenies of seed plants differ from morphological analyses that place the weird Gnetales (Ephedra and Welwitschia) closer to the flowering plants, based on the appearance of flower-like reproductive organs. Figure from Bowe et al. (2000) Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales’ closest relatives are conifers PNAS 2000 97 (8) 4092-4097; doi:10.1073/pnas.97.8.4092.