Each spring, the bloom of nearly 4,000 cherry trees captivates Washington D.C. The spectacle of pink and white blossoms amid the backdrop of monuments attracts more than a million locals and tourists alike to the shores of the Potomac every year.
Many of these trees are Yoshino cherry trees, a variety that sprouts tufts of fragrant white flowers. At full bloom, they create the illusion that clouds of mist are swirling above Potomac Park. But the Yoshino cherry trees themselves are more complex than they seem. While their gnarled trunks look like just any old tree, Yoshino cherries are actually the hybridized product of two different species of cherry trees that have been replicated through an ancient technique known as tree grafting.
Horticulturists have been employing variations of grafting for thousands of years. The practice seems deceptively simple: the grower takes a branch from a desired tree, known as the scion, and connects it to the root system, known as the rootstock, of a different tree. Using a variety of cuts, experienced cultivators are able to piece together the two trees like a puzzle. If the plants prove compatible, the two parts of the plants fuse together and grow as a cohesive tree.
According to botanist Richard Hodel, a Peter Buck postdoctoral fellow at the Smithsonian’s National Museum of Natural History, this artificial technique is possible because plants are naturally predisposed to fuse with their leafy brethren. In fact, cherries and their fruity kin like peaches and apricots began hybridizing long before humans started snacking on their fruit.“It's not magic that we are able to hybridize these plants,” said Hodel, who studies the diversity of cherries and other plants in the genus Prunus. “Being able to crossbreed has been a really important part of these plants’ evolutionary history dating back 60 million years.”
Plants naturally hybridize through sexual reproduction, where the pollen from one tree fertilizes the flower of another species with the help of wind and pollinators. This complex process continually churns out new varieties of plants. But asexual practices like grafting remove the variability of natural reproduction. By continuing to graft the branches of a desired tree onto different rootstocks, you are essentially creating carbon copies of the original tree.
Grafting has been a boon for agriculture because it allows particularly productive trees to be replicated across a field. While the thought of eating cloned fruit may make some squeamish, the practice is so common in modern agriculture that virtually all the fruit at the grocery store comes from some kind of clone. “It's no accident that we've cultivated them so much,” Hodel said of cherries and their kin. “Their history of rampant hybridization has left them particularly amenable to being manipulated by humans.”
Grafting also serves an important purpose for propagating ornamental trees like Yoshino cherries. “The genes are important because that carries the information about how to flower,” said Jake Hendee, an arborist for the Smithsonian Gardens. “We love to reproduce our Japanese cherries because we love their flowers.”As a result, the groves of blooming trees that capture the capital’s zeitgeist each spring are a “cohort of clones,” as Hendee calls them. Japanese gardeners have been refining these pastel palettes for centuries and each variety of cherry blossom has a slightly different hue. Potomac Park is home to twelve different varieties of Japanese cherries that bloom in shades ranging from rosy pink to ghostly white.
While we cherish the beauty of these flowering trees, overplanting a single lineage can have grave consequences. Because they are all genetic clones of each other, hybrids like Yoshino cherries are all susceptible to the same pests and diseases, meaning a single pest could ravage an entire grove of cherry trees.
Washingtonians found this out the hard way. When the first gift of 2,000 cherry trees arrived in Washington in 1910, every plant was riddled with parasitic pests. To prevent the pathogen from spreading to native trees, horticulturists were forced to burn the infected trees in a bonfire on the National Mall.
Thankfully, cherry blossom advocates on both sides of the globe were persistent and in 1912, another gift of 3,020 cherry trees from Yukio Ozaka, the mayor of Tokyo, arrived in Washington pest-free. Once the trees were established, they quickly became a seasonal staple of the capital. Each spring, more admirers converged around the Tidal Basin and by 1935, the Cherry Blossom Festival was established.
According to Hendee, this annual fascination with flowering hybrids leads to a greater appreciation for the rest of Washington’s trees. Although they may lack the pink blooms, the city’s trees provide us with clean air and green spaces. “It’s a very gratifying experience to work in a city that loves these hybrid trees,” Hendee said. “These charismatic trees speak for the entire urban forest.”
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