The Passiflora genus, commonly known as passion flowers, is a diverse group of vines celebrated for their unique blossoms and, in many species, their edible fruit. Originating primarily from tropical and subtropical regions, these plants have an inherently low tolerance for freezing temperatures. Understanding hardiness zones is important for ensuring their winter survival. Selecting a plant suited to a specific climate, or protecting a less-hardy species, determines whether the vine will return each spring.
Understanding Plant Hardiness Zoning
The USDA Plant Hardiness Zone Map helps gardeners determine which perennial plants can survive the winter in their location. This system is based on the average annual extreme minimum winter temperature, calculated from 30 years of historical weather data. The map divides North America into numbered zones, each representing a 10-degree Fahrenheit range. Zones are further divided into ‘a’ and ‘b’ half-zones, representing a 5-degree difference.
Gardeners use this zone designation to compare their local climate’s coldest expected temperature against a plant’s listed cold tolerance. To survive and return each year, the plant’s roots must be able to withstand the minimum temperature of the designated zone. This system indicates the average low, meaning a rare, extreme cold snap could still damage plants growing at the edge of their hardiness range.
Primary Hardiness Zones for Common Passion Flower Species
The hardiness of a passion flower vine is dependent on the specific species, ranging from relatively cold-tolerant to strictly tropical. The native Maypop (Passiflora incarnata) is the most cold-hardy species, generally thriving in zones 6 through 10. In colder areas, the above-ground growth will die back completely, but the root system can survive temperatures as low as -20°F and regrow vigorously in the spring. This root-hardy nature makes it suitable for temperate regions where more tender passion flowers would not survive the winter.
Another widely cultivated and relatively hardy species is the Blue Passion Flower (Passiflora caerulea), which is generally listed as hardy in zones 7 through 10. Some sources suggest this vine can even survive in zone 6 with adequate winter protection, though its foliage may be killed by frost. In colder climates, it is often grown as a root-hardy perennial, meaning the underground parts survive to sprout again even if the leaves are lost.
The vast majority of the four hundred Passiflora species are tropical or subtropical and require warmer conditions. These tender types, such as Passiflora edulis (common passion fruit) or many ornamental hybrids, typically demand a minimum hardiness of zone 9 or 10. In these warmer zones, the vines can remain evergreen or semi-evergreen throughout the year and are not forced into a dormant state.
Tropical species cannot tolerate freezing temperatures for any extended period and are commonly grown as annuals or container plants in regions below zone 9. Any temperature below freezing is likely to cause damage or death to the plant tissue. Even within a designated zone, microclimates, such as the warmth retained near a south-facing wall, can slightly extend the growing range for a species.
Cultivation Strategies for Marginal Zones
Gardeners in marginal zones—those areas just slightly colder than a plant’s listed range—can employ specific strategies to increase the passion flower’s chance of winter survival. Heavy mulching around the root crown is effective for vines planted in the ground. Applying a thick layer of organic material, such as straw, shredded leaves, or wood chips, helps insulate the soil and protect the shallow root system from deep freezes.
The mulch should be applied several inches deep in late fall after the plant has begun to go dormant. Planting the vine in a sheltered location also helps mitigate cold damage by reducing exposure to harsh winter winds. A spot against a building foundation or a solid wall, especially one that faces south, will absorb and radiate warmth, creating a slightly warmer microclimate for the plant.
For tropical species, or for gardeners living in zones too cold for any ground survival, container growing is the simplest solution. The potted vine can be moved indoors before the first frost, allowing it to overwinter in a dormant or semi-dormant state in a cool, dark location like a basement or unheated garage. Before moving a container indoors, the vines should be pruned back to a manageable size, and watering should be reduced significantly to encourage dormancy.