Liz Maynard, Clinical Engagement Assistant Professor of Horticulture, Purdue University
A freeze signals the end of the growing season for many gardeners and farmers. Yet some vegetable crops, like the kale in Figs. 1 and 2, can survive freezing. When temperatures rise above 32°F again, one can pick some of the leaves for fresh greens and let the rest remain for a later harvest. If protected from wind by a hoophouse and/or row cover (Figs. 3, 4), crops like spinach, kale, mustards, and lettuce can provide fresh greens throughout the winter and into early spring. In the case of a hoophouse, the environment can even be warm enough on sunny days for plants to grow.
Plants have a number of mechanisms to survive freezing. They create more compounds that act as ‘antifreeze’ so plant cells don’t freeze as readily, switch to cold-hardy versions of key chemical building blocks that make up plant cells, and make proteins that protect cells from the dehydration that occurs as water between plant cells freezes. Plants acclimate to cold by making these changes as they grow at cool temperatures above freezing. Unacclimated plants that have grown only at warm temperatures can’t survive freezing as well.
Sucrose, or sugar, is one of the important ‘antifreeze’ compounds. In spinach, amounts in the leaf can increase 10 to 20-fold during acclimation. No doubt that’s why many people (including this writer) find winter-grown spinach so tasty.