The Four O’Clock plant, Mirabilis jalapa, is a vibrant, fragrant addition to many gardens, known for its trumpet-shaped flowers that open in the late afternoon. This plant is botanically a tender perennial, meaning it possesses the ability to return year after year, but only in warmer climates with mild winters. In areas that experience cold, freezing temperatures, the Four O’Clock is cultivated as an annual, completing its life cycle in a single season. The process of cutting back the plant is necessary to maintain its size during the growing season and to prepare the plant for its mandatory winter dormancy.
Determining the Right Time for Major Pruning
The primary cutback for the Four O’Clock plant is dictated by the environmental shift of autumn and the onset of cold weather. This major pruning should be timed to occur after the above-ground foliage has been completely killed back by the first hard frost. The frost signals the end of the growth cycle and prompts the plant to withdraw energy into its subterranean tuberous roots. While the plant is actively growing throughout the spring and summer, minor pruning can be performed to manage its sometimes sprawling shape and size. Lightly trimming back overgrown stems helps maintain a tidy, bushier appearance throughout the season. Waiting until the foliage is entirely brown ensures the plant has successfully moved all available nutrients back down into its tubers for winter storage.
Step-by-Step Pruning Technique
Once the foliage has succumbed to the cold and turned brown, proceed with the physical cutback of the plant’s dead stems. Use a pair of sharp, clean pruning shears to make precise cuts. Sanitation is important to avoid introducing pathogens into the plant base before it enters dormancy. The goal is to remove all the dead, above-ground biomass, cutting the stalks down to a height of approximately 4 to 6 inches above the soil line. Removing the dead material is an important step in garden hygiene, as it eliminates potential overwintering sites for pests and fungal diseases. The remaining root crown, with its short stems, is then ready for winter preparation.
Preparing Tubers for Winter Survival
The long-term survival of the Four O’Clock depends on its carrot-like tubers, which function as storage organs for its winter energy reserves. In mild climates, specifically USDA Hardiness Zones 9 through 11, the tubers can be left in the ground to survive the winter naturally. Gardeners in Zones 7 and 8 can increase the survival rate by applying a thick layer of protective organic mulch, such as straw or shredded leaves, over the dormant root crown before the ground freezes solid.
For gardeners in colder regions, the tubers will not tolerate sustained freezing temperatures and must be lifted for indoor storage. After cutting back the stems, carefully dig up the tubers using a garden fork. Once lifted, gently brush off the excess soil.
The freshly dug tubers must then be cured, or dried, in a cool, shaded, and well-ventilated location for about one to three weeks. After curing, store the tubers in a cool, dry, and dark location where the temperature remains between 40 and 50 degrees Fahrenheit (4–10°C).
The best method for storage involves layering the tubers in a cardboard box, wooden crate, or mesh bag, surrounded by a light, dry medium:
- Peat moss
- Vermiculite
- Sawdust
Avoid using plastic bags or closed containers, as these trap moisture and increase the risk of rot, which would compromise the tuber’s viability for replanting the following spring.