The author used a clever trick to protect these zinnias from frost. (Barbara Damrosch)

One of the hardest things to learn about gardening is when to quit. In fall, a bed of once-radiant annual flowers may still sport a few spots of bright color. But if the effect is marred by dead leaves and the remaining buds are unlikely to open, there’s nothing to do but pick the few surviving blooms and collect them in a vase. Notice that they do not last as long as they might have if cut at prime time. Pull up all the fading annuals, put them on the compost pile and admire the bed’s new, tidy look.

Meanwhile, the tomatoes are slowing down. There are still a small number of red-ripe fruits to pick, plus a few more that show some reassuring pink and a great many more green balls that could certainly ripen — as long as winter never arrives. Pick the red ones, and the pink ones if you must, because tomatoes do color up and soften on the kitchen counter.

If there are enough red ones, make tomato sauce and can or freeze it. Make a salad if there are only a few. Taste the salad, and admit that the tomatoes lack the rich flavor they had during long, warm, sunlit days. Most likely, the nutritional content will have suffered as well. Resist the urge to wrap greenish tomatoes individually in paper and store them in boxes, in hopes of an afterlife. Look forward to pumpkin pies, buttered Brussels sprouts and other late-season treats.

On the other hand, what if an early frost warning is posted, followed by a 10-day forecast of temperate days and nights? Who can stand by and allow that premature and wholly unjust event to cut the season short? Perhaps you waited too long to make the pesto, so out comes an old sheet to throw over the basil before it can blacken and die. The winter squash will not keep well through the winter if touched by frost, and now it’s suddenly too dark to pick it. So more old bedding is dragged out, or maybe a big sheet of plastic that can cover the whole patch.

Seasoned gardeners have a pretty good bag of tricks for frost prediction, based on years of observation. Just before a full moon is a dangerous time, especially if the sky will be clear and the air very still. If, as bedtime nears, the mercury is at 40 degrees, watch out. Even if your root cellar is full and your cold-tolerant salad greens are nestled safely in a cold frame or greenhouse, there is always something in the garden that calls out for rescue.

Recently, just such a night loomed, and I was damned if I was going to lose a large bed of gorgeous zinnias. They had survived what we call a “grass frost,” where only the ground shows a touch of white, but this time the signs were not good. I looked around for large sheets of this or that to throw over them, knowing that the tallest flowers could be damaged by the sheets’ weight but that they could still provide a tent to shield the ones lower down. Even a big piece of floating row cover would help.

My husband had another thought: Fruit crops, woody nursery stock and even some vegetable crops can be frost-proofed up to a point with irrigation. Sometimes it’s done in spring to protect against a late freeze and sometimes in fall to thwart an early one. If the sprinklers you set up cover the plants with a skin of ice, this can actually prevent damage to the cells within. The fact that freezing generates a bit of heat plays a part, too.

Because we have a good water supply and the sprinklers were already in place, a solution that involved nothing more than pulling up the handle of a frost-free hydrant was appealing. The bed needed watering, anyway, so I let the sprinklers go all night.

In the morning, there was a bona fide frost, but the zinnias were fine. The sprinkler trick worked! With the next frost warning, I might use it again, but then again, maybe I’ll let nature take its course. The long-range forecast is for winter, and that’s that.