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.