Four o'clock is a tender perennial, meaning that the plant doesn't tolerate cold weather. Other tender perennials include dahlia, canna lily, impatiens, coleus and tuberous begonia. Four o'clock is perennial in U.S. Department of Agriculture hardiness zones 10 and above, which means that four o'clock may survive temperatures as low as 30 degrees Fahrenheit with proper care, but will be killed by colder temperatures. In borderline climates of zones 8 and 9, four o'clock is often winterized in the ground. In colder climates, four o'clock's long tubers are stored during the winter.
Winterizing Four O'Clock
Four o'clock survives in the ground with a good protective winter covering in hardiness zones 8 and 9. To winterize the plant, water the soil deeply enough to saturate the tubers before the first frost in autumn. Clip off dry and dead growth to within 3 inches of ground level after the ground freezes, then cover the plant with 4 to 5 inches of mulch such as straw, pine needles or dry, chopped leaves. Avoid using wet leaves as mulch, as the leaves become compacted and prevent water and air from reaching the tubers. Remove the mulch as soon as shoots poke through it in the spring.
Digging Four O'Clocks
Four o'clocks have sturdy underground tubers that make them good candidates for overwintering and replanting in spring. To overwinter, cut the plants back to 3 to 4 inches above ground level after the foliage is nipped by a light frost. Dig the tubers carefully with a garden fork. Dig deeply, as the carrot-like tubers can measure more than 12 inches in length.
Rinse the tubers with a garden hose, then place them in a shady, well-ventilated spot to dry for two weeks. Bury the tubers in a cardboard box filled with peat moss or vermiculite, then store them in a dry room where the temperature will be maintained between 35 and 40 degrees. Replant the tubers after all danger of frost has passed in spring.