Each spring, the blooming of nearly 4,000 cherry trees captivates Washington DC The spectacle of pink and white blossoms against a backdrop of landmarks draws more than a million locals and tourists to the banks of the Potomac each year.
Many of these trees are Yoshino cherry trees, a variety that produces tufts of fragrant white flowers. When in full bloom, they create the illusion of clouds of mist swirling over Potomac Park. But the Yoshino cherry trees themselves are more complex than they appear. While their gnarled trunks look like any old tree, Yoshino cherries are actually the hybrid product of two different species of cherry trees that were bred through an ancient technique known as tree grafting.
Horticulturists have used grafting variants for thousands of years. The practice seems deceptively simple: the grower takes a branch from a desired tree, known as a scion, and connects it to the root system, known as a rootstock, of a different tree. By using a variety of cuts, experienced growers are able to piece the two trees together like a puzzle. If the plants are found to be compatible, the two parts of the plants merge and grow as a cohesive tree.
According to the botanist Richard HodelPeter Buck Postdoctoral Fellow at Smithsonian National Museum of Natural History, this artificial technique is possible because plants are naturally predisposed to merge with their leafy brethren. In fact, cherries and their fruity cousins like peaches and apricots began hybridizing long before humans started munching 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 of the genus. prunus. “Being able to interbreed has been a very important part of the evolutionary history of these plants that goes back 60 million years.”
Plants hybridize naturally through sexual reproduction, where pollen from one tree fertilizes the flower of another species with the help of wind and pollinators. This complex process continually produces new varieties of plants. But asexual practices like grafting remove the variability of natural reproduction. By continuing to graft branches from a desired tree onto different rootstocks, you are essentially creating carbon copies of the original tree.
Grafting has been a boon to agriculture because it allows particularly productive trees to be reproduced in a field. Although the idea of eating cloned fruit may make some people fussy, the practice is so common in modern agriculture that virtually every fruit in the grocery store comes from some sort of clone. “It’s no coincidence that we grew them so much,” Hodel said of the cherries and their relatives. “Their history of rampant hybridization has left them particularly susceptible to manipulation by humans.”
Grafting also serves an important purpose for the propagation of ornamental trees like Yoshino cherries. “Genes are important because they contain information about how to flower,” said Jake Hendee, arborist for the Smithsonian Gardens. “We like to reproduce our Japanese cherries because we like their flowers.”
As a result, the groves of flowering trees that capture the capital’s zeitgeist each spring are a “cohort of clones”, as Hendee calls them. Japanese gardeners have been honing 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.
Although we cherish the beauty of these flowering trees, overplanting a single line can have serious consequences. Because they are all genetic clones of each other, hybrids like Yoshino cherries are all susceptible to the same pests and diseases, which means a single pest could ravage an entire grove of cherry trees.
Washingtonians found out the hard way. When the first gift of 2,000 cherry trees arrived in Washington in 1910, every plant was riddled with pests. To prevent the pathogen from spreading to native trees, horticulturists were forced to burn infected trees in a bonfire on the National Mall.
Fortunately, 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 without pests. Once the trees were established, they quickly became a seasonal staple in the capital. Each spring more admirers converged around the tidal pool and in 1935 the cherry blossom festival has been established.
According to Hendee, this annual fascination with flowering hybrids leads to a greater appreciation for the rest of Washington’s trees. Even though they lack pink flowers, the city’s trees provide us with clean air and green spaces. “It’s been a very rewarding 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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