Longenecker Horticultural Gardens in early fall.
As fall begins and day length gradually shortens, trees are being cued that winter is approaching. In response, deciduous trees (those that lose their leaves) stop producing chlorophyll – the pigment that gives leaves their green color – and begin to move sugars and other nutrients into cold-hardy plant parts. As this occurs, yellow and orange pigments called carotenoids, which have been present in the leaves all along but hidden by the green chlorophyll, become visible.
Some tree species display red, purple, or maroon colors every fall due to the pigment anthocyanin, yet for most species these displays depend on seasonal weather. For the most intense color, we need warm sunny days and cool nights. The warm sunny days increase the sugar production in leaves, while cool nights slow the movement of these sugars through narrowing leaf veins and petioles.
In 2003, UW–Madison Department of Horticulture researchers found that trees respond to the trapping of these sugars by producing extra anthocyanins, which act like a sunscreen to protect the sugars from the harmful effects of the sun in the absence of chlorophyll (link paywalled).
While day length and temperature are the primary drivers of fall color, other factors also play a role. Soil moisture can affect color development – especially summer droughts which can delay the onset of color, more so when combined with abnormally warm late summer and fall temperatures. Tree health is also a factor. Those in poor or declining health often respond to drought by coloring earlier.
Given the Madison area’s late summer heat wave and low rainfall, many stressed trees started displaying fall color over the past few weeks. For most trees however, current fall color predictions are for a later-than-usual unfurling of color in the Madison area, with peak color predicted for late October into the first week of November. An early hard freeze could disrupt this by killing leaves before they fully change color. At present, the Arboretum is about 10 percent full color. We will share updates on fall color throughout the fall.
– David Stevens, Ed Hasselkus curator, Longenecker Horticultural Gardens
