Lee Valley Tools    Gardening Newsletter
   Vol. 8, Issue 3
   March 2013
   Gardening on the Hardiness Edge
Abyssinian gladiolusAbyssinian gladiolus
Abyssinian gladiolus (Acidanthera) flourishing in a cooler-climate garden
Over the past 35 years, I've had success in my garden with fringe tree (Chionanthus virginicus), silverbell tree (Halesia carolina), yellow wood (Cladrastis lutea), sweet gum (Liquidambar styraciflua) and my favorite shrub, dwarf fothergilla (Fothergilla gardenii). Though many of these are considered warmer-climate plants, they've managed to do well, even thrived, as have many of their offspring that I've given to friends as far away as Quebec's Eastern Townships (Canadian zone 5a, American zone 4a). This raises the question, what's happening with plant hardiness? Is it global warming, special microclimates, new hybrids and cultivars, or improved over-wintering techniques? While the answer may be somewhat unclear, what is obvious is that only through experimentation can we discover the extensive range of some of these plants.

Plant labels and nursery catalogs commonly indicate a plant's hardiness by listing its likely northern limit or, alternatively, the range for its cold and warm limits. Plant hardiness zones are based on the average climatic conditions of the area, including winter temperatures, frost-free periods, maximum snow cover, maximum wind speeds, amount of rainfall, etc. Zones with lower numbers represent colder areas, those with higher numbers, warmer ones. There are, however, certain significant "swing factors". In central and Southern Ontario, for example, the winter of 2012 was mild and almost snow-free. Unfortunately, however, a cold snap in the early spring had significant implications for fruit and magnolia blossoms, which were badly frozen. It's important to use plant hardiness zones as a guideline for plant selections, since they could have implications for plant guarantees; however, they are open to interpretation and debate.
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