|ACCLIMATION||Cooling air temperature; decreasing day length||Canes harden in preparation for frost (moisture withdrawn from tissues, cell walls thicken)||Early frost||Speed of hardening|
|MID-WINTER HARDINESS||Freezing temperatures; soil is frozen||Plants survive or die||Cold-hardy genes inherited from dozens (or hundreds) of ancestors||Too little snow; extreme cold|
|DE-ACCLIMATION||Soil thaws; air temperature rises; days lengthen||Leaf buds on canes begin to show growth||Growth hormoes (auxins)||Late frost can kill plant|
Climbing roses are exposed to wind chill and lower temperatures, and ‘John Cabot' (Zone 3) is an Explorer rose with great hardiness. An arching shrub that can be trained as a climber, it reaches three metres, carrying semi-double, medium red flowers fading to deep orchid pink. Another Explorer, ‘John Davis' (Zone 3), is a trailing rose with old-fashioned, quartered flowers and spicy fragrance, best tied to a pillar and allowed to cascade. The Kordes rose, ‘Alchymist' (Zone 4), is a once-blooming climber with fragrant, very double blossoms in suffused shades of yellow and orange. Pale pink ‘New Dawn' and her sister, ‘Coral Dawn', are tall shrubs growing to two metres in Zones 4 and 5, and climbers up to three metres in warmer zones.
Ultimately, hardiness in woody plants is a three-part process (see chart above). Unfortunately, well-meaning gardeners (like myself) may also contribute to the demise of a favourite rose by applying fertilizers too late in the growing season. Manufactured rose fertilizers (though not compost or manure) applied after the end of July can cause a spurt of late-season growth, interrupting the acclimation phase, delaying hardening of wood and jeopardizing winter survival.
But now that I understand the family dynamics of this winter-hardiness issue and the factors in nature that can influence it, I'm no longer perplexed by the unexpected resiliency of my floribunda roses with tender Hybrid Tea genes in their ancestry. I discovered all their secrets through Internet research (www.helpmefind.com/roses). ‘Sue Ryder' has an impressive 27 generations of antecedents in her breeding, with 159 roses contributing genes to her performance, including nine old species-and the cold-resistant genes of R. rugosa, R. kordesii and R. roxburghii. ‘Nearly Wild' is true to its name, with a much shorter pedigree of nine generations and 24 ancestors, including a Zone 4 seedling of R. multiflora in the third generation and the likely source of its winter endurance. Armed with my new-found knowledge of rose pedigrees, and fingers crossed, I'm ready to stock my garden with dependable, winter-hardy roses and get growing.