Lee Valley Tools Gardening Newsletter
Vol. 2, Issue 2
April 2007
How Plants Know When to Rise and Shine

Unusual weather in several regions this winter has many gardeners concerned about the fate of their perennials; specifically, whether or not plants, shrubs and trees came out of dormancy and got zapped by the subsequent return to low temperatures.

Crocus blooms
In climates that experience four seasons, crocus blooms commonly signal the first signs of spring.

While nobody welcomes the negative effects of abnormal weather, at the least it shakes us out of our complacency about how nature works and inspires questions such as, "How do plants know that winter is coming? What do they do to survive? How do they know when it's time to wake up in the spring?"

In temperate climates, perennial plants have adapted over the millennia to survive freezing weather, so what exactly is the problem? Basically, when water freezes, not only does it not flow, it also expands. This process is hazardous for plants because their main component is water. So, when a plant cell freezes, it does what a frozen water pipe does: it bursts.

To avoid this situation, perennials follow a to-do list for shutting down their systems in preparation for the cold. It includes stopping growth, setting buds for next year, dropping leaves, moving water from inside cells to spaces in between cells (leaving within the cells a mixture of sugars and other substances that tolerate cold without freezing) and storing energy reserves in roots. To know when to start these preparations, plants rely on environmental signals such as falling temperatures and/or shortening days. These conditions trigger changes in plant hormones (for example, increases in a growth inhibitor called abscisic acid or ABA) that kick-start the fall to-do list and result in plant dormancy.
  Bud growth
In fall, plants store energy reserves in their roots as they enter a dormancy period for the winter. In spring, when conditions are right, the plant will utilize those reserves to kick-start growth.

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