Roses are prized garden features, but their delicate nature raises concerns about survival during colder months. These plants face significant stress when temperatures drop below freezing for extended periods. Understanding the specific temperature limits and taking appropriate action is key to ensuring these popular shrubs survive the winter and return to bloom in the spring.
Defining Cold Tolerance and Hardiness Zones
Cold tolerance measures a plant’s ability to withstand low temperatures without sustaining fatal damage to its tissues, particularly the roots and the crown. To standardize this measurement, gardeners rely on the United States Department of Agriculture (USDA) Plant Hardiness Zone Map, which divides North America into zones based on the average annual minimum winter temperature. Each zone represents a 10-degree Fahrenheit range, providing a reliable estimate of which plants are most likely to survive in a given geographical area.
For many common garden roses, the temperature range separating a hardy variety from a tender one often falls between -10°F and 20°F. Roses rated for USDA Zone 7, for example, are expected to survive average minimum temperatures of 0°F to 10°F, generally requiring no special winter protection. Conversely, a rose rated for Zone 5 is capable of surviving colder lows, down to -20°F, though supplemental protection is often recommended to prevent cane dieback.
How Tolerance Varies Among Rose Types
A rose’s natural hardiness is determined by its genetic background, leading to vast differences in the minimum temperatures various types can tolerate. Modern Hybrid Teas and Floribundas are often the least cold-tolerant, generally requiring protection in regions colder than USDA Zone 7. The delicate canes and vulnerable graft unions of these types make them susceptible to dieback when exposed to prolonged freezes.
In contrast, shrub roses and certain species roses possess greater natural resistance to extreme cold. The Canadian Explorer series and Rugosa roses, which trace their lineage to hardy wild species, can often survive in frigid climates down to Zone 3, where temperatures may plummet to -40°F. These exceptionally hardy types frequently have thick, resilient canes and are often grown on their own roots, meaning they can regenerate even if the above-ground growth is killed back completely. Grandifloras and some modern climbers tend to fall into a medium tolerance range, surviving in Zone 6 or Zone 5 with minimal protection.
Identifying Cold Weather Damage
Recognizing the signs of cold damage is important for assessing a rose’s survival and determining where to prune in the spring. One common symptom is cane dieback, where the stems turn brown or black, indicating that the vascular tissue has been killed by freezing. This damage often progresses from the tips of the canes downward until it reaches a point of living tissue.
A simple method to check for life is the scratch test, where a small section of the outer bark is scraped away. If the tissue underneath is bright green, the cane is alive; if it is brown, gray, or black, the section is dead and will not produce new growth. The most severe damage occurs when the cold penetrates the crown or the graft union, which can result in the entire plant failing to revive. Harsh winter winds can also cause desiccation, or “windburn,” which dries out the moisture in the canes, leaving them brittle and susceptible to splitting.
Essential Strategies for Winter Rose Protection
Gardeners in colder regions must employ specific strategies to help their roses survive temperatures below their natural tolerance threshold. One of the most effective methods is soil mounding, or hilling, which involves piling 10 to 12 inches of soil, compost, or mulch directly over the base of the plant. This mound insulates the vulnerable graft union and the crown, which are the most critical parts for the rose’s survival and spring regrowth.
Physical Protection Methods
Beyond protecting the base, the canes themselves can be shielded from damaging winds and extreme cold. Techniques include wrapping the canes with burlap or covering the entire shrub with a specialized rose cone made of styrofoam or insulating material. In extremely cold climates, below Zone 5, the most tender roses may require the “Minnesota tip” method, where the plant is carefully detached from its support, bent to the ground, and completely buried under a layer of soil and mulch for maximum protection.
Pre-Winter Preparation
Successful winterizing depends on proper environmental preparation that begins well before the first hard freeze. Growers should cease fertilizing by late summer to prevent the stimulation of tender new growth that would be killed by cold weather. Ensuring the plant is well-hydrated before the ground freezes solid helps the canes resist desiccation and windburn. Allowing the rose to experience a few hard frosts will help the plant achieve full dormancy, which is necessary before applying any physical winter protection.