In the garden: Growing ‘in the zone’ is only a guideline
Are you zoned out? I ask because the U.S. Department of Agriculture has revised its map of plant hardiness zones, causing a lot of excitement among gardeners, who are the largest users of zone classifications.
Unlike the Canadian zone map, which is based on average climatic conditions and plant survival data, the U.S. map is based only on average mean low temperatures, so that each zone, from one to 13, increases by 10 degrees Fahrenheit. (Time for them to join the Celsius club!)
The new U.S. map has received a lot of attention because it has meant a shift in zones for many areas. Central Michigan, for example, has warmed up from zone five to zone six.
Some may fear these changes are due to global warming, and they well could be, but the producers of the map make it clear these are climatic changes based on only a 30-year cycle using more accurate data than was previously available. In some cases, given areas have slipped down a zone. Sadly, unlike the old map, the new one doesn’t include Canada, even though data from Canadian weather stations was used for the border states.
And how does this affect us in Waterloo Region?
To use the U.S. map, we can only extrapolate. Since central Michigan has a similar climate to south central Ontario, that would place us in zone six, too. The U.S. zone six ranges from -23.3 to -17.8 C (average annual extreme minimum temperature) and we have had much colder winter temperatures than this. But seeing as -16 C is the lowest I’ve recorded in my own garden this winter, it would appear things are changing. Based on that one reading, I could be looking for tender plants suitable for a warmer zone seven.
But remember, zones on Agriculture Canada’s map, updated in 2000 from the 1967 map, are not based solely on lowest temperatures. A newer map underway will show the potential range for individual species of trees, shrubs and perennial flowers with a climatic profile for each plant.
The current version shows this area as zone five. Zones are further divided into ‘a’ and ‘b,’ ‘a’ being cooler. The Kitchener and Waterloo area is roughly on the dividing line. To the south is zone 5b, which includes Cambridge, and to the north is 5a, taking in Guelph. Just north of Fergus it switches to the cooler zone 4b. Hardiness zones, however, are only lines on a map and don’t allow for variations such as cities being warmer than rural areas.
Confused yet? Are we really zone five, or six or seven? The other problem with zone numbers is they have a touch of arbitrariness about them: Who made the decision to label a plant as suitable for a particular zone? Is the plant from a U.S. source or from a Canadian source? And if it is a Canadian source, which zone map was the label based on — the older maps or the latest ones — Canadian or U.S.?
Yet there are other factors to consider besides a number on a plant label. Perhaps the biggest is snow cover. Plants that can’t make it here will often do fine up north where there is deep insulating snow, even though the location may be a zone cooler. Elevation must be considered, too. My garden is on the “mountainous” side of Waterloo, with a somewhat cooler and more exposed location than you would find, for example, riverside in Cambridge.
In any garden there are microclimates — places that are warmer, cooler, or sheltered. A south-facing corner close to the house is far warmer than an open, north-facing slope, while frost pockets form in low spots that can freeze harder than other places in the garden. Established plants are better able to withstand cold temperatures than newly planted material.
If a plant is labelled as zone five, chances are it will do just fine in this part of Ontario, but for the gardener who likes a challenge, go ahead and plant a zone six or even a zone seven plant. Just be prepared to take extra precautions or risk losing it over winter.
The bottom line is that zone numbers are only a guide and should be treated that way. The best guide is the gardener’s own garden, plus advice from knowledgeable staff members at local nurseries.
