gladiolus (Acidanthera) flourishing in a cooler-climate
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.