According to the map of hardiness zones published by Agriculture Canada in 1967, I live in hardiness zone 5b. The hardiness zone is a classification of the factors that affect a plants survival. These factors include the minimum winter temperature, the length of frost-free periods, wind velocity, humidity, and precipitation. Plants are given a hardiness rating that a gardener can use as a guide to determine if that plant will survive in their garden. Plants will not survive in a zone with a lower number then their hardiness rating.

It is important to remember, though, that even within a back yard there will be some variation. There will be areas of micro-climate created by the sheltering effects of the house and other structures and plantings, or by the lack of snow on a wind-swept hillside. Some parts of your yard may be as much as a full zone either way from the overall rating, so the siting of plants is important. In our yard, on the south side of the house, up against the wall, I have had parsley and fennel (normally too tender to survive our -20C winters) survive the winter while the same year we lost a cedar on the north side of the house to wind damage.

There are at least three different hardiness zone maps commonly encountered in books and magazines. One was published several decades ago by Harvard University. In the 60's, the USDA brought out a new map, and Agriculture Canada published one in 1967. In 1990, the USDA issued a new, more detailed map. In 2000 Agriculture Canada published another update. From what I've read, the Agriculture Canada map remains more accurate for Canada. It is a good idea to find out what your hardiness zone is before you invest in that favourite plant.

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