As their name suggests, four o'clocks (Mirabilis jalapa) flower in the late afternoon. Although grown as an annual in cold climates, four o'clocks are perennials in U.S. Department of Agriculture plant hardiness zones 10 and 11, and they may also overwinter successfully with protection in USDA zones 7 through 9. Seedlings grow quickly and will flower the first year after sowing. The plants are toxic if eaten.
Sowing indoors four to six weeks before the last expected spring frost gives four o'clocks a head start on the growing season, or you can wait and sow them directly in the garden bed after the last frost. The large seeds sprout more quickly if you soak them in a bowl of warm water overnight before planting. Soaking softens the seed coats so the seeds absorb water more readily. Plant the seeds immediately after soaking or they will begin to rot.
Small 2- to 3-inch diameter seedling pots or seedling cell pack trays with bottom drainage work well for starting the seeds indoors. Soak the pots and any planting tools in a solution of 1 part bleach to 9 parts water for 30 minutes to disinfect them, and then allow the pots to air dry before planting. Sow two seeds per pot in moist, sterile potting soil, planting them 1/2 inch deep. Cover the pot with a plastic bag to keep the soil moist until the seeds germinate, and remove it as soon as the sprouts emerge in seven to 10 days. Provide the four o'clock seedlings with all-day sun in a sunny window, and water them when the soil surface feels dry. If both seeds in a pot germinate, pinch off the weaker seedling when they grow in their second set of leaves.
Well-drained soil of almost any quality and full sun provide for the healthiest growth, but four o'clocks can tolerate partial shade outdoors. Sow the seeds 6 inches apart in all directions, and keep the soil moistened to 6 inches during germination. Direct-sown seeds should sprout within one or two weeks in moist soil. After they emerge and grow in their second set of leaves, called the true leaves, thin the seedlings so they are 12 to 18 inches apart.
Four o'clocks grow well in most soil conditions without additional fertilizer, but they needs regular watering to bloom fully. Water four o'clocks approximately once a week, supplying about 1 inch of water. During hot, dry weather they may need more frequent watering. Four o'clocks self-seed easily which can make them invasive. Pinching off blooms after they fade can minimize seed production.