Climate changes in Calgary: gardening in Zone 4a

planter, hellebores, and a slug

Based on climate data collected from 1981 to 2010, most of Calgary now lies in the warmer plant hardiness zone 4a. This means we can be more adventurous in our plant selection: at first this sounds like heaven - but not quite. Tree budburst dates are earlier, but the dates of late frosts have not changed so much, so fruit tree blossom will suffer more from late frosts. There are two different methods to identify what zone we garden in. The simplest is that published by the U.S. Department of Agriculture, based solely on the extreme minimum temperature of the coldest day of the year: the second is the more sophisticated Canadian method that takes several variables into consideration, including minimum daily and monthly temperatures and precipitation. One difficulty for us is that growers’ plant labels often don’t say which of these two systems the growers are using, and they differ, but both the US and the Canadian methods demonstrate the warming of Western Canada’s climate.
A third process is being developed that uses climate envelope maps that indicate what plants we may grow in a specific region – see .These Canadian climate “envelopes” (i.e. plant distribution maps) have moved an average of 57 km northwards since the middle of the last century. The latest figures we have shows the last frost typically occurs on May 21 and the first fall frost on September 16 an average of 117 consecutive frost free days. The average precipitation has also increased to 419 mm (16”). Extremes seem to be changing, as seen in the floods of June, 2013, but the recorded daily maximum rainfall was in 1927, and the highest temperature was back in July, 1919. Will our gardens have more hellebores … or more succulents: bigger slugs … or more grasshoppers? Only time will tell! Glynn Wright. August 10, 2015