Signs of autumn are found throughout the yard and garden. Leaves turn color, fruit ripens, and plants go dormant. The calendar tells us fall is here, but how do plants know when to undergo these changes and be ready for the dormant winter season ahead?
In northern climates like Wisconsin, plants need to match active growth with optimum growing conditions and inactive growth, or dormancy, with adverse conditions. This is controlled through hormones, organic compounds produced internally, that change in concentration from season to season. There are growth-promoting and growth-inhibiting plant hormones. Levels of each respond as conditions around the plant change, with temperature and light being key factors.
Early sunsets and shortening days are becoming very apparent to us now, but plants began reacting to this weeks ago. The process of leaves turning yellow, orange, and red in fall started as the active growth hormones, which promote chlorophyll (green pigment) in leaves, started to decline due to longer dark periods accumulating each day. Once chlorophyll fades away, other colors show up.
Ethylene is the aging and ripening hormone of plants. Concentrations of ethylene increase at the base of leaf stalks, degrading the cells that hold leaves on twigs, thus leaves fall to the ground. Ethylene also drives ripening processes of apples, crabapples, tomatoes, and other fruits. The best way to ripen green tomatoes off the vine, such as all those picked before fall freezes, is to maintain them at room temperature in a paper bag or shallow box. Ethylene will do the rest.
Once plants have finished active growth in fall, it is important not to resume active growth before optimum growing conditions of the next growing season return. Enter abscisic acid, the dormancy hormone. Buds in trees will develop high concentrations of abscisic acid in fall. The same holds true for seeds of many native plants. While these concentrations remain high, abscisic acid prevents growth-promoting hormones, such as auxins, from increasing and stimulating active growth again.
Plants need to be exposed to a certain amount of cold to lower abscisic acid levels. This safeguard prevents plants native to cooler climates from being fooled by a warm spell in winter. Even though it may be warm and moisture is available, growth cannot start again until abscisic acid has dissipated in buds, or in seeds.
Finally, another safeguard exists in spring. Once the cold period has been met, inhibiting abscisic acid may be gone, but growth does not immediately resume. Accumulating warmer temperatures, measured by degree days, are required before growth-promoting hormones start to increase.
By Bruce Spangenberg, Horticulture Outreach Specialist, UW-Madison Division of Extension
