White frost crystals on a Brussels sprout. (Susan Robinson)

The equation seems elementary: Sun and warmth yield tasty ripe fruits such as tomatoes, cucumbers and melons. Its obverse: Cold and ice yield a blackened garden and the start of supermarket season. But a veteran gardener would say, “Not so fast.” With the coming of frost, the sweetest season begins.

Although it’s true that the crops we grow for their fruit, like those seductive tomatoes, thrive in warm weather, there are many more for whom heat is unnecessary and sometimes fatal. Fall is the time for crisp lettuce salads, and for the arugula to toss into them, free of the stippling of flea beetle holes that mar this peppery green in summer. Its flavor is also more mild and pleasant now.

But the real satisfaction comes when frost hits. Yes, frost. Although lettuce and arugula can only stand so much of it, a long list of other garden favorites are transformed. Most of them are brassicas such as cabbage, broccoli, Brussels sprouts, kale, collards, turnip greens and a host of Asian greens such as tatsoi, bok choy, mizuna and mustard. A few frosts have the magical power to replace that cabbagey edge with sweetness.

Greens from other plant families such as spinach, beet greens, chard and the chicory relatives endive, escarole and radicchio behave the same way, losing any curse of bitterness. I recall standing on a February day in the snow-dusted fields of Even’Star Organic Farm, Brett Grohsgal’s fascinating operation in Lexington Park, Md., and nibbling on the sweet green shoots of his totally unprotected brassica crops.

The most dramatic change comes in root crops such as carrots, parsnips, turnips and radishes. Carrots and parsnips, especially, become pure candy.

To reap the cold season harvest, the gardener must think ahead: Most hardy veggies are started in the heat of August and September.

This seasonal overlap gets us thinking about the marvelous diversity of plants: Many summer crops hail from tropical places and don’t need to survive a freeze, while fall and winter crops can shrug off a freeze, to varying degrees.

But why does the cold make them sweet? There’s a scientific explanation, of course. Just as a sugary syrup poured into your car’s radiator prevents freezing by displacing water molecules and hindering the formation of ice crystals, sugars in plant cells prevent or delay the icing that causes cell breakdown.

A Japanese study published in 1996 demonstrated that when cabbage plants were gradually acclimated to cold, they turned their stored starches into sugar and were thereby more able to withstand freezing. This revelation occurred in a lab, but you can replicate the experiment quite simply in your garden by growing fall cabbage and enjoying the tasty result. With carrots for dessert.

Damrosch is a freelance writer and author of “The Garden Primer.”

Tip of the week

In containers, replace tired summer annuals with cool-loving pansies or their miniature cousins, violas. Add fresh soil as needed and a little balanced fertilizer. Pinch out the blooms as they fade to promote more flowering. The show should last until the end of the year, at which point the pots can be emptied and brought inside for the winter.