It's been a dull spring for cherry blossom watchers so far. Blossoms in Philadelphia did not open until early April. Usually, the flowers appear anywhere from mid- to late March. Festivals in New York and Washington are experiencing a similar delay due to the ongoing cold and wet spring.
"Cherries and other early spring blooming plants are highly variable as to when they bloom, and it's driven totally by warmth," said Paul Meyer, executive director of the Morris Arboretum at the University of Pennsylvania.
This means that in a single city, different cherry trees will flower at different times. Those that are downtown tend to flower earlier than suburban trees — there's more pavement downtown, which creates an "urban island" heating effect when the sun heats the ground up.
Also, plants on the south side of a building tend to bloom earlier than those on the north side, because the south siders receive more sunlight.
Flowers, like all biological processes, are driven by chemistry. One chemical rule of thumb — true in most cases, but not always — is that for every 18 degrees Fahrenheit (10 degrees Celsius) increase in temperature, the amount of chemical reaction taking place, say, in a plant, doubles as particles interact more frequently. This increase in chemistry makes the flowers bloom.
During the winter, the closed cherry buds can tolerate below-freezing temperatures, Meyer said. "But as things start to warm, they will break dormancy and they will start to swell," he added.
So the lingering cold is a signal to the blossoms to stay closed for a while longer, as the reactions that will make them bloom aren't yet humming along.
The cold spring may at least produce one positive side effect: longer-lasting blooms. If exposed to high temperatures — say, in the 60s or 70s — a flower will bloom quickly and then disappear within four to five days. However, a prolonged colder temperature will cause the flowers to slow their blooms, making them last between seven and 10 days, Meyer said.
Cherry plants are pretty short-lived for trees, typically living around 40 or 50 years. Their longevity, however, depends on the species. There are some types in Japan that have been bred to last more than a century.