By Blake Jackson
Autumn brings noticeable changes to yards and gardens. Leaves shift from green to vibrant shades, fruits ripen, and many plants gradually prepare for winter dormancy. But what tells plants that the growing season is ending?
According to Bruce Spangenberg, a horticulture outreach specialist with the UW-Madison Division of Extension, plants rely on environmental signals and internal chemical processes to recognize seasonal changes.
In northern regions such as Wisconsin, plants must carefully balance active growth with periods of dormancy. They use hormones, naturally produced compounds that regulate growth and development, to respond to changing conditions.
Some hormones encourage growth, while others slow or stop it. Temperature and the amount of daylight are among the most important environmental factors influencing these hormonal changes.
Although shorter days become obvious to people as autumn progresses, plants begin responding to declining daylight much earlier. As nighttime periods become longer, levels of growth-promoting hormones that support chlorophyll production decrease.
Chlorophyll gives leaves their green color, so as it breaks down, other pigments become visible, creating the yellow, orange and red colors associated with fall.
Another important hormone is ethylene, which plays a major role in plant aging and fruit ripening. As autumn approaches, ethylene levels rise near the base of leaf stems. This causes the cells connecting leaves to branches to break down, eventually allowing leaves to drop. Ethylene also helps ripen fruits such as apples, crabapples and tomatoes.
For gardeners with green tomatoes remaining on plants before the first freeze, Spangenberg recommends allowing them to ripen indoors. Keeping them at room temperature in a paper bag or shallow box can help the natural ethylene process complete the ripening.
Plants also need a mechanism to prevent new growth during temporary winter warmups. Abscisic acid, often called the dormancy hormone, helps provide that protection.
During fall, trees can accumulate higher levels of abscisic acid in their buds, while many native plant seeds do the same. This hormone suppresses growth-promoting hormones, including auxins, and keeps plants dormant.
Cold temperatures are necessary to reduce abscisic acid levels. This requirement prevents plants adapted to northern climates from beginning growth during an unusually warm winter period.
Spring brings one final safeguard. Even after sufficient cold exposure reduces dormancy signals, plants do not immediately resume growth. They also require enough accumulated warmth, measured through degree days, before growth-promoting hormones rise, and active development begins.
Photo Credit: university-of-wisconsin-madison
Categories: Wisconsin, General