Written by: Rome Marinelli, Horticulturalist at The Inn at Honey Run
Is it Spring yet? Let’s ask a tree!
We are all aware of the common signs of spring; more greens than browns, ephemerals sprouting from the soil, buds appearing on trees… but how do plants, like trees, know winter is over and spring is approaching? For trees, it turns out, it depends on one molecule within them to determine the arrival of spring.
Before knowing when to “wake up,” trees must undergo a specific number of chill hours. Chill hours begin at 44 degrees F and cut-off around 30 degrees F. If temps get below 30 degrees F (as they did this year…), chill hours will not register; it is merely the time spent between 44-30 degrees F. Once the chill hour requirements are met, the tree begins counting. Yes, counting. Well… sort of.
One would think that the tree begins to detect the amount of sunlight and warmth as spring approaches, but it actually depends more on the length of the nights. Trees have a molecule called phytochrome. As you continue to read, imagine phytochrome as an hourglass rather than a molecule (just for the simplicity). Once the day turns to night, the molecule begins tracking the amount of time until daylight. If the metaphorical sand in the phytochrome hourglass runs out, the tree knows warmer weather is not yet approaching. However, once we hit the vernal equinox (March 20), the sun stops receding and begins climbing, causing longer sun exposure. At some point, once the length of the night shortens, that metaphorical sand does not empty completely prior to daylight and indicates to the tree that warmer days are approaching.