Among plant breeders, it is commonly understood that developing better trees takes time, perhaps a dozen years, even whole careers. After attempting new crosses—meticulously transferring pollen from one plant to another—seeds must be collected and propagated. Plants must then be evaluated as the seedlings mature, and if crosses are made between this first generation, the whole process restarts. At the Arnold Arboretum, one of the most prominent plant breeders was Karl Sax, who was appointed director of the Arnold Arboretum in 1946, after almost two decades as a professor of plant cytology at the Arboretum. Nowhere is his horticultural legacy more evident—at least in spring—than on the slopes of Peters Hill, within the crabapple (Malus) collection, where about thirty trees date to his breeding work in the 1940s and early 1950s.

The crabapple collection will soon decorate the slope of Peters Hill with confectionery shades of pink and white. Among these trees—not yet flowering, but the buds swollen with promise—is an unnamed Sax hybrid between the American crabapple (Malus coronaria) and the European crabapple (M. sylvestris). The tree (accession 691-52*A) is visible, looking uphill, near the intersection between Peters Hill Road and Poplar Gate Road. The floral buds are balls of bright pink fondant, waiting to unfurl in the next several days. When they do, the petals will turn delicate white. Elsewhere on the hill, other crabapples will bloom brighter—swabs of fuchsia and cotton candy—but, for me, this tree is all about form and texture. The emerging foliage achieves a pointillist quality that would send Georges Seurat—the French post-Impressionist—running for his paintbrushes. These small daubs of green are clustered into fist-sized dollops, layered one atop the other.

Taken together, the composition of foliage and flower buds forms a low dome, much broader than tall, as though a parasol from Seurat’s iconic painting A Sunday Afternoon on the Island of La Grande Jatte had floated away without its handle. About a dozen of the primary branches twist like gimlets from the squat trunk, suspending a finely textured canopy that will later bear small green fruits, less than a centimeter in diameter, blushing red on the cheeks. The inner canopy is open, with an elfin mix of branches arising along the outstretched branches. The tree looks like it has been carefully tended by a bonsai master, snipping and bending each branch just so.

Karl Sax working in his research nursery in 1959. Notice the roof of the Bussey Institution in the background. Photo from Arnold Arboretum Archives.
Karl Sax working in his research nursery in 1959. Notice the roof of the Bussey Institution in the background. Arnold Arboretum Archives

When Sax fully launched his breeding work on ornamental crabapples in 1939, he raised several thousand hybrid seedlings. From these, three thousand were distributed in 1940, to more than three hundred individuals and institutions for ongoing evaluation. Several hundred more were reserved for testing at the Arboretum. In 1947, Sax noted that while several promising hybrids emerged from the trial, many, as expected, had proven unsatisfactory. “As a partial compensation to our cooperative friends who carried on this testing work, we hope to be able to supply you with some of the better varieties in a few years,” he wrote. “Meanwhile if you have any outstanding trees from the hybrids which we distributed in 1940, please let us know so that we can propagate them.”

Now, some sixty years after Sax’s hybrid (Malus coronaria × sylvestris) was planted on Peters Hill, in the spring of 1952, the tree has more than proven its worth. In 2007, grafting material was collected from this plant, and two progeny now grow on the slope above the Bradley Rosaceous Collection, not far from the former research nurseries at the Bussey Institution, on South Street, where Sax conducted his work. In the most recent issue of Arnoldia, living collections fellow Jared Rubinstein wrote about another unnamed Sax lineage: hybrid pines (accessions 266-46) growing behind the Hunnewell Building. The pines are now towering trees—an attractive mix of their parental species—and in these cases, I find it remarkable that the success of the hybrids has been recognized so late. Sometimes an experiment is decades in the making. This is the plant-breeding long game.