Soon, trees and shrubs in every neighborhood will be opening their buds to unfurl their new leaves.
As plants emerge from winter dormancy, they are coming into their superpower, like the first time an action movie’s protagonist appears in colorful tights and a flowing cape. Spreading their leaves enables plants to start doing something no animal can do: capture the sun’s energy and use it to make food.
The moment when the tough bud scales that shelter a tender young leaf through the winter first spread apart to let it unfold is called bud break or bud burst, said Christy Rollinson, forest ecologist at The Morton Arboretum in Lisle. She and her team study the timing of events such as bud break (and the leaf out that follows) as they research the effects of climate change.
Like other researchers, she relies on the observations of volunteer citizen scientists at the Arboretum and across the United States, reported to the USA National Phenology Network.
Although bud break sounds dramatic, it can actually be hard to spot, Rollinson says, because buds vary so much among species of woody plants. “It’s not always clear,” she said. You may catch it if you look closely at a shrub or a low-lying tree branch and see the scales of a bud spread apart to show just a glint of green.
On taller trees, you may only become aware that buds have opened when a green cloud seems to appear around the bare branches above your head. Some species, such as maples, actually open their flower buds and bloom before they unfurl their leaves. That’s because they depend on the wind to carry their pollen from flower to flower, and leaves would get in the way.
Every species opens its leaves on its own schedule: lilacs earlier, oaks later. The timing of bud break also is affected by the weather. In general, cold weather in spring delays bud break and warm weather accelerates it. That’s why the overall, long-term trend that shows earlier bud break is of interest to climate scientists such as Rollinson.
The unfurling leaves make springtime visible to us in a dramatic way. But like an action hero who has been training in secret, trees have been at work for months. Underground, roots have been growing. Just beneath the bark, water and sugar have been lifted up the trunk from the roots into the buds. Inside each bud, cells have been dividing as the hidden leaf grew, causing the bud to swell.
“On a beech, you may see the bud swell and swell until it’s more than an inch long before it bursts,” Rollinson said.
As long as the buds stay closed, the tree has to rely on food stored up last fall. Once the green solar collectors are open for business, the tree can create its food and support itself. The process of photosynthesis can begin, turning sunlight, water and oxygen into sugars.
“Bud break is when the plant starts standing on its own two feet,” Rollinson said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice or [email protected]). Beth Botts is a staff writer at the Arboretum.