Give Your Garden Seedlings Some Tough Love
Lee Reich Associated Press | 10/23/2019 | Via Morning AgClips
Seedlings raised on windowsills or in greenhouses have been coddled to some degree and aren’t ready to face the great outdoors. A temporary period called “hardening off” can prepare these plants for more intense sunlight, wind and varying temperatures.
Make this transition gradually, over the course of a week or two. A good place to harden off seedlings is in a somewhat sheltered spot outdoors, such as in a coldframe (basically an open-bottomed box with a clear, removable cover) or near a wall in dappled shade. Or offer the seedlings full exposure for limited, but increasing, periods.
ACCLIMATION TO TEMPERATURE
The changes that lower temperatures during the hardening-off period will induce in coddled seedlings depend on the nature of the seedlings themselves.
Seedlings of cabbage, lettuce, snapdragon, pansy and other plants that can eventually laugh off cold even well below freezing develop that tolerance for cold by building up sugars in their cells. Cold also changes the composition of their cell membranes.
Seedlings of tomatoes, marigolds, zinnias and other plants that cannot tolerate temperatures much below freezing suffer from so-called chilling injury even at temperatures below 50 degrees F. Changes in plant membranes from chilling injury interfere with sunlight driving photosynthesis, so instead damaging toxins build up in leaves.
As a tomato or other warmth-loving plant becomes hardened off through gradual exposure to cooler temperatures, it becomes better able to repair and prevent such damage.