Clematis, often called the “Queen of Climbers,” is a popular perennial vine cherished for its spectacular, star-shaped flowers. To successfully cultivate this plant, understanding its temperature requirements is necessary for winter survival. The longevity and prolific flowering of a clematis vine depend heavily on matching the plant’s natural cold tolerance to the local climate.
Understanding the USDA Hardiness System
The standard tool for determining a plant’s ability to survive winter temperatures is the USDA Plant Hardiness Zone Map. This map divides the United States and Puerto Rico into 13 zones based on the average annual extreme minimum winter temperature. Each zone represents a 10-degree Fahrenheit difference in this average lowest temperature.
The map further refines these distinctions by dividing each zone into two sub-zones, labeled ‘a’ and ‘b’. These sub-zones indicate a finer, 5-degree Fahrenheit increment. For instance, Zone 6a is colder than Zone 6b, helping gardeners determine which perennial species can withstand the coldest local temperatures.
The Optimal Hardiness Zone Range for Clematis
The majority of popular clematis cultivars are reliably hardy across USDA Zones 4 through 9. This broad range means the average clematis vine can withstand minimum winter temperatures from approximately -30°F (-34°C) up to 20°F (-6°C).
Clematis are deciduous woody vines that enter a dormant state in winter, allowing them to tolerate significant cold. However, the plant’s root crown, where the stems meet the roots, is the most vulnerable point during deep freezes. Some exceptionally cold-tolerant species can thrive in Zone 3, where temperatures drop to -40°F (-40°C).
Selecting Specific Varieties Based on Zone
Selecting a clematis variety based on your specific zone is crucial for long-term gardening success, especially at the edges of the plant’s range. Gardeners in colder regions, such as Zones 3 and 4, should seek out cold-hardy types like the Atragene group, which includes species such as Clematis alpina and C. macropetala. These varieties are known for their exceptional tolerance to harsh winters.
For those in Zone 3, choosing Group 3 clematis, which bloom on new wood, is often the safest bet. Examples like ‘Jackmanii’ or C. viticella varieties perform well because winter damage to the top growth will not prevent them from regrowing and flowering from the roots in spring.
Conversely, in warmer climates like Zones 9 and 10, heat tolerance is the limiting factor. Varieties like C. texensis and C. viticella types are recommended, as they are better adapted to high summer heat. The popular evergreen clematis (C. armandii) is best suited for warmer Zones 7 through 9, as its foliage is less tolerant of prolonged freezing.
Seasonal Protection for Zone Extremes
Clematis planted at the coldest margins of the range, such as Zone 4 or exposed sites in Zone 5, benefit from specific winter protection measures. Once the ground has frozen, a thick, four-to-six-inch layer of organic mulch should be applied around the base of the plant. This deep mulch layer insulates the vulnerable root crown from drying winter winds and temperature fluctuations.
In hot climates like Zone 9, the challenge shifts to heat management, particularly protecting the root system. Gardeners must ensure the roots remain cool and moist, following the adage, “feet in the shade, head in the sun.” This is achieved by applying a three-inch layer of mulch to shade the soil and by planting low-growing perennials at the base of the vine. Consistent, deep watering and planting clematis where they receive afternoon shade prevent the flowers and foliage from wilting or fading due to intense sun.