Article by Mark Godlewski
Plant Hardiness Zones are very important to planning a successful garden. They are a relatively simple concept that can become a little more complicated when you look into the details.
Let’s start with the simple part. If you want to know your nominal plant hardiness zone in Canada, you can simply type in your municipality into the following Government of Canada website.
Typing Kelowna into the website gives you a hardiness of USDA 7a for the most recent period of evaluation (1981-2010). For Osoyoos you get a hardiness of 6b. Osoyoos is half a zone colder and this difference probably has something to do with the proximity of Kelowna to Lake Okanagan.
If you look up a Purple Ice Plant on the OXA website, it tells you that it is hardy to zone 6. This means that normally it should survive the winter in Kelowna. Any plant that is zone 5 or lower should definitely be able to survive winter in Kelowna according to this rating.
PLANT HARDINESS COMPLICATIONS
The first complication is global warming. We have all experienced warmer winters in Canada over the past decades. Lake Okanagan has not frozen over since 1969, whereas it used to freeze over more commonly as in both 1950 and 1949. The Government of Canada website mentioned above compares the period of 1961-1990 to the period 1981-2010 and the zones are all higher on the website for the later date. The zones are usually higher by half a zone to a full zone. This overall warming trend will probably continue into the foreseeable future.
Climate change adds another complication, however. As our climate warms on average, it is also becoming more variable. This means that we are more likely than before to have occasional extreme cold snaps. The winter of 2022/23 is a good example of this pattern. We had 2 days at zone 5 temperatures and 2 days at zone 4! This resulted in an unusual amount of winter kill for our plants in the Okanagan. Looking at the last 12 years in Figure 1 you can see that these extreme low temperatures are becoming more common.
Fig. 1 – Lowest Historical Temperatures for Kelowna
Another complication is that there are two methods for calculating hardiness zones. One comes for the USDA and it is based simply on the lowest temperature experienced in a given area. The Government of Canada uses a more refined and complex formula which incorporates six other winter weather variables in addition to the lowest temperature. The Government of Canada website listed above actually provides USDA type plant hardiness but sometimes you will run into maps which show the more refined Canadian values (see Figure 2). The refined Canadian value for most of the Okanagan including Kelowna is zone 6a. For most other cities the USDA and the Canadian value were quite similar. This means that the scientists at our federal government think that we should really be using 6a rather than 7a as our plant hardiness zone.
The final complication is the variation of plant hardiness zones introduced by variations in elevation. Looking at the zone map you can see that the Okanagan is a narrow valley surrounded by high hills and mountains. The higher elevations around our valley have the much lower zone rating of 3a to 3b. The general trend is to move down one zone for every 400m of elevation gain. This means that a community like Joe Rich would be closer to a USDA 5b compared with our USDA 7a.
Local variations in elevation and exposure can create microclimates that may be somewhat better or worse than your official zone designation. This will be the subject of a subsequent blog post.
Fig. 2 – Canadian Plant Hardiness for BC
Most of the nurseries and plant databases categorize their plants based on USDA hardiness due to the close commercial ties between the two countries. Unfortunately, like everything associated with nature, there is some variation in the measurements for a single species and variety of plant between sources.
Plant hardiness is a relatively simple and important concept for Okanagan gardens, but it is not entirely reliable. Gardeners should take this uncertainty in account in their planting plans. If you want to try out an interesting perennial which is rated at close to your maximum zone, then it might be worth the risk. On the other hand, if you are planting a tree or hedge that you want in place for a long period of time, it is better to choose a species that is two or three zones colder than your maximum.
To avoid disappointment in your garden give careful consideration to plant hardiness.