The following is based on an article that appeared in YCGC's Spring 2011 (April 2011) newsletter, the original article was written by Dwayne Wohlgemuth. Supplemented by Dave Taylor in the years since.

How many frost-free days do we have? 

Fifteen years of Environment Canada weather data from 1996 to 2010 for the YK airport says that we have an average of 123 frost-free days. The last spring frost averages May 22, while the first fall frost averages Sept 23. That’s not much different than most places on the Canadian Prairies! In 2010, the frost-free period for Calgary, Grande Prairie, and Yellowknife were 128, 125, and 115 days, respectively.

What are growing degree days and how are they used? 

Growing Degree Days (GDD) are an accurate way to determine the production potential, flowering date, and growth of plants. GDD is a very good indication of plant growth and maturation if there are no other significant limitations such as water and sunlight. GDD is calculated as the difference between the day’s average temperature and the basic temperature required for growth of a given crop, such as 5.0C. Therefore, if a day’s average temperature was 8.5C, the GDD would be 8.5 – 5 = 3.5. For an annual value, GDD is summed for all days in the year. A few locations and their annual GDD based on 30 year averages are given below (Environment Canada, 2010): 

Growing Days

In this chart GDD are calculated only from after the last spring frost until before the first frost in the fall. The 30 year stats on the left probably calculate all GDD for a year. The number of GDD before the last frost or after the first fall frost varies from 10-90 in the years shown.

So Yellowknife has 21% fewer GDD than Edmonton and 10% fewer than Fort Smith. Warming trends have meant, however, that average Yellowknife GDD from 2006 - 2010 was 1141, almost as high as the 30 year average for Fort Smith. Supplemental light can shorten the time to maturity and increase yields though, so our vegetable harvest shouldn’t be reduced by as much as our growing degree days would suggest, thanks to our longer summer daylight hours!

Danger: Clear Nights! 

Note that frost damage to plants may occur on clear nights even when the air temperature is above 0oC due to an effect called “radiative cooling”. Plants lose heat to the sky and can freeze even though the air temperature is above 0oC. This doesn’t happen when there’s cloud cover, however, and it doesn’t happen to plants that are well shaded by houses and trees. Some vineyards warm their precious plants on these types of nights through the use of large fans that look somewhat similar to wind turbines.