The passion flower (Passiflora genus) is a vigorous, often woody vine admired for its intricate, ornate blooms. While cultivated for beauty and sometimes edible fruit, the plant’s success depends entirely on the local climate and the lowest temperatures it can tolerate. Determining the appropriate hardiness zone is the most important step for ensuring the plant’s long-term survival outside of tropical regions.
Understanding Cold Tolerance and Hardiness Zones
Successful perennial gardening relies on the USDA Plant Hardiness Zone Map, the standard reference for determining a plant’s ability to survive winter temperatures. This system divides North America into zones (1 being the coldest, 13 the warmest) based on the average annual extreme minimum winter temperature. Zones represent 10-degree Fahrenheit differences in minimum temperature.
Each numbered zone is further broken down into two 5-degree subzones, designated by ‘a’ and ‘b’, allowing for a more precise assessment of cold tolerance. For example, a plant hardy to Zone 7b can withstand slightly colder temperatures than one rated for Zone 8a. This designation indicates whether a specific passion flower variety will survive the winter and return the following spring.
Identifying Hardy Passion Flower Species
To grow passion flower as an outdoor perennial in cooler climates, select a cold-tolerant species. The two most common and cold-hardy varieties are Passiflora incarnata and Passiflora caerulea. These vines die back to the ground in colder winters but regrow vigorously from the root crown when spring arrives.
The Maypop (Passiflora incarnata) is the most cold-tolerant species, reliably surviving in USDA Zone 5 or 6, where winter temperatures can dip down to -10°F. Native to the southeastern United States, it produces three-lobed leaves and lavender-to-purple flowers with a delicate, shredded appearance. Its common name, Maypop, refers to the hollow, yellow, egg-shaped fruit it produces, which makes a popping sound when crushed.
The Blue Passion Flower (Passiflora caerulea) is widely cultivated and hardy to USDA Zone 7, tolerating average minimum temperatures down to 0°F to 10°F. It is recognizable by its showy, saucer-shaped flowers, which feature white or pale pink petals and sepals topped by a distinctive corona of blue and white or purple filaments. Its stems remain semi-evergreen only in the mildest climates.
Overwintering Care in Marginal Zones
For gardeners attempting to cultivate a hardy passion flower at the northern limit of its zone tolerance, such as growing a Zone 7 plant in Zone 6b, specific winter protection is required. Passion flower vines typically retreat their energy into their roots, allowing the above-ground growth to die back after the first hard frost. This natural process concentrates the plant’s resources in the crown, which must be protected from freezing.
After the foliage has blackened, the vine should be pruned back to a short stub of about 8 to 12 inches above the soil line. The most effective strategy involves applying a thick, insulating layer of material over the root zone. A mound of loose mulch, such as straw, shredded leaves, or pine needles, should be piled at least six inches deep over the crown.
This heavy mulching helps to prevent rapid freeze-thaw cycles, which can heave the root crown out of the soil and expose it to damaging cold. Planting a hardy passion flower against a south-facing wall or near a building foundation creates a sheltered microclimate that provides a few extra degrees of warmth, improving its chances of winter survival.
Growing Tropical Varieties Indoors or in Containers
Many of the most colorful and fragrant passion flower varieties, such as the scarlet-flowered Passiflora coccinea or the main commercial fruit producer Passiflora edulis, are tropical and cannot survive any frost. These non-hardy species must be grown in containers and treated as annuals or overwintered indoors. The key to successful indoor care is to manage the plant’s growth and energy use during the colder, darker months.
Before the first frost, the vine should be pruned significantly, often cutting back the long, sprawling stems by one-third to make it manageable for indoor relocation. Once inside, the plant can be kept actively growing by placing it in a bright, south-facing window, or under supplemental grow lights, with temperatures maintained between 55°F and 65°F. Alternatively, the plant can be forced into a state of semi-dormancy by moving it to a cool, dark location, such as a basement or garage, where temperatures are consistently near 50°F.
During the winter rest period, whether active or dormant, the care routine must change dramatically. Fertilizing should stop completely, and watering should be reduced to only what is necessary to prevent the soil from drying out entirely. This reduced watering and cooler temperature allows the plant to conserve energy and rest until it can be moved back outside after the danger of frost has passed in the spring.