I often find, when I’m browsing through books or catalogues, that the zone recommendations for a plant are wrong. I have spent my gardening life in either zone 2 or zone 3. When I come across plants that are recommended for zone 4 or 5 and I have them growing happily in my zone 2 gardens, I wonder what is going on.
After reading the section on hardiness zones in, Flora a Gardeners Encyclopaedia, I realized that hardiness zones span the globe, and not all areas in the same zone have the same ecosystem let alone the same climate.
Tony Avent over on Plant Delights Nursery, points out that the zone maps do not tell us how long an area receives the cold temperature only that the area has received that cold temperature. He says the length of time may differ from a few hours to a few days to a few weeks. A plant may be able to survive a temperature drop over night but it will die if that same low temperature goes on for a week.
As well, Avent talks about the concept of provenance, where the parents live. Seed taken from a hardy tree in a northern area will be hardier than seed taken from the same tree in a more southern region.
The instructor who taught the master gardener course here in Prince George is all for pushing the zone barriers, and trying to grow whatever she feels like in the clay soil of her zone 3 garden. For her it's a challenge and a triumph to succeed with a plant, rated as being hardy in a much warmer zone.
I’m beginning to wonder how relevant hardiness zones are, as a guide to what will grow in my garden.
Kathy Purdy, over on Cold Climate Gardening, says the reason plants get labelled with the wrong zone information is because of statistics. Zones 2, has fewer gardeners than a more temperate zone 7 climate so less is known about what grows there. The only way to find out what grows in your garden is to experiment and document you successes and failures.
As my master gardener instructor said, it all comes down to knowing the plants requirements and realizing that in experimenting with different plants you will kill some of them.
Differences in soil, drainage, predators, snow cover or lack of snow cover, and the amount of wind may have more effect on the plants survival than how cold it gets. You may have to compensate, ie wrap a plants susceptible branches in burlap to protect it from drying winds, when it is dormant. Mulch or pile on snow over the roots of herbaceous perennials, or, accept the fact that a plant will never fully reach its potential, since it winterkills to the ground every year and can only grow so much from its roots before it is killed back yet again.
Microclimate and soil are two big factors determining plant survival. Clay soil is harder for plants to adapt to than a sandier soil. A plant grown next to a brick wall will be less affected by temperature change and drying winds than the same plant grown in the open garden.
Keeping the plant healthy, giving it adequate amounts of sunshine or lack of sunshine, fertilizer and water will definitely help its survival.
If you are a plant collector enthusiast then go ahead and experiment. I am past the wide eyed, fervent beginner stage. I have had may share of disappointment when plants die, even after I thought they were well established. Nowadays I just plant the tried and true not because I want to avoid disappointment but rather I don't want the work it takes to coddle a plant into survival.