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After Winter’s Chill Has Gone: Can Trees Tell Time? *Updated*

Trees have a lot to teach us, if only we pay attention.

Last fall, we moved three seedlings from the windowsill here at ForestPlanet World HQ. We “out-planted” them in a corner of the demonstration orchard at historic Peirce Mill here in Washington, DC. At a depth of about eight inches, the roots of the sourwood, maple, and sycamore seedlings were hopefully deep enough to avoid freezing. While winter storms passed over them, the little sprigs slept. Or did they…?.

From left to right: Sourwood, Maple, and Sycamore

Trees go “dormant” during the winter months. There are two types of dormancy: endo-dormancy and eco-dormancy. The term “endo” has its roots in Greek; it means “inside.” Shorter days and cooler temperatures trigger endo-dormancy when the inside of the plant (versus weather conditions) directs the plant to conserve energy, to hibernate. What is happening at a cellular level is that during autumn, most deciduous trees produce abscisic acid, a hormone that initiates the process of dormancy.

Come Spring, we excavate

How does the tree know when to awaken? While in endo-dormancy, a tree keeps track of time—specifically, hours of time during which the weather is above freezing. These are called “chilling units” or “chill hours.” Trees and other plants wait until the right amount of time has passed during which the air is above freezing. When those hours have accumulated, plants begin their eco-dormancy period, “eco” coming from the Greek term “oikos” which means “home” or “place to live.” Inside the tree, the cold (but not freezing) weather breaks down the abscisic acid and the tree begins to produce leaves. In other words, when the weather gets warm enough, the plants come out of their faux sleep and begin producing new growth, to “leaf.”

Fully emerged from winter’s chill.

Around the globe, deciduous trees experience this cycle of endo- then eco-dormancy, then reawakening in the spring. It’s happening right here at ForestPlanet World HQ. In late March we brought our saplings out of the earth and back to their spot on the window ledge. We hope that they survived the winter, but only time will tell for sure.


How many times have we heard that getting a good amount of deep sleep is key to good health? As discussed above, on March 21, 2021 our trio of trees emerged leafless and groggy from a deep winter slumber. After spending just two months on the warm window sill here at ForestPlanet World HQ the leaves are back, new branches are forming, and the planet is incrementally cooled. This season we’re applying lessons learned from our friends at Peirce Mill, mostly the importance of a ‘cover crop’ to help retain moisture. A small selection of grasses and clovers have been added to each pot, and we think the additional cover is making a big difference.

Check back in a few months when we once again outplant our forest for the winter once again…this time maybe permanently?


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