For the Week of March 17, 2025
By: Bruce Spangenberg, Horticulture Outreach Specialist
Plants Patiently Wait for Spring
Spring officially arrives on the calendar this week. Recent weather has fluctuated between some very spring like days back to more like winter across Wisconsin. Plants are not likely to get fooled into growing too soon by those early warm days thanks to internal processes that control growth.
To survive winter extremes, trees and shrubs need to go dormant before cold sets in and as spring returns, need to wait until the threat of cold has lessened to resume growth. Hormones, organic compounds produced within plants, regulate these growth processes. Hormone levels increase or decrease as conditions change around plants, allowing events such as breaking bud, putting on new growth, flowering, and going dormant to occur at proper times. This control protects plants from growing at the wrong time, including unusually warm days in winter or early spring.
Five major plant hormones include auxin, gibberellin, cytokinin, ethylene, and abscisic acid. Auxins, cytokinin and gibberellins promote growth and development and allow plants to actively grow under favorable conditions for much of the growing season. Ethylene is the ripening or cell aging hormone, which promotes processes such as leaf drop, petal fall and ripening of fruit. Abscisic acid controls dormancy, both of plant buds and seeds.
Hormones readily respond to light and darkness in the environment around plants by changing concentration levels. To ensure trees and shrubs went dormant before winter set in, shortening days, or increasing darkness, over several consecutive weeks was the triggering signal last fall. Darkness levels are consistent as seasons change, regardless of the weather. As a result, growth promoting hormone concentrations dropped, shutting down active growth. Ethylene increased to break down cells and force leaves to drop, followed by increasing abscisic acid levels to keep buds dormant.
Temperature also influences hormones to respond in the best interest of the plant. Once dormant, native trees and shrubs require a certain length of cold period, to signal winter is over, before they can resume growth in spring. Even unusual winter warm spells do not allow growth promotion to start until abscisic acid levels drop, which requires a specific cold period.
Once cold exposure requirements are met, accumulating warm temperatures, not just a warm day or two, are needed to break dormancy. Accumulating warmth in spring is measured by degree days, with 50 degrees usually the base temperature. Every degree above 50 for daily average temperatures is an accumulated degree day. Once enough degree days accumulate for each species of tree or shrub, processes begin to break bud, leaf out, and flower. This is why we have early, mid, and late spring flowering trees and shrubs.
Bruce Spangenberg is a Horticulture Outreach Specialist with UW-Madison Division of Extension. Get answers to your lawn, landscape, and garden questions anytime at www.go.wisc.edu/GardenQuestions
