How is it that some plants survive temperatures below zero, when my tomatoes turn to mush at the first hint of frost?
Plants that live in cold climates have developed the ability to withstand below-freezing temperatures without damage. When you consider that each living cell is filled mostly with water, this is an amazing feat.
Plants such as your tomatoes—as well as other tender annuals and tropical plants—can’t survive freezing. When the water in their cells turns to ice crystals, cell walls rupture and the tissues die.
Hardy evergreens, on the other hand, fortify their cells with sugars and other dissolved substances as winter approaches, turning water into an “antifreeze” of sorts. The foliage of both broadleaf and needle evergreens usually has a waxy layer that offers extra protection. Hardy deciduous trees and shrubs can also freeze-proof the sap in their woody tissues.
You’ve probably noticed an interesting side effect of the autumn buildup of sugars in plant cells. After the first light frosts of fall, cool-weather vegetables like broccoli, kale, and Brussels sprouts begin to taste sweeter—evidence that the plants are bracing for cold.
But even antifreeze has its limits, and that’s where the USDA Plant Hardiness Zones come in. These hardiness ratings give gardeners a good idea of just how much winter cold each species can tolerate. —Doug Hall