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They compartmentalize freezing by moving most of the water outside of their plant cells to freeze (which is why frozen plants appear wilted in the morning, but quickly spring back to life once they thaw).
In most of the United States, it’s simply desiccation: the drying effects of cold winter winds. These plants have already moved water out of their cells, then the low humidity and harsh winds of winter dry out their tissues even further.
You can push the minimum limits of those frost-tolerance tables simply by giving your plants some protection, either in low tunnels (what I personally use) or cold frames.
If you want to give winter gardening a shot with these cold-protection tricks, don’t baby them. Just as the cellular changes and sugars they accumulate are induced by cold weather, they’re quickly lost when the plants are exposed to heat.
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