Climate Change and the Garden

What’s the Effect of Global Warming on Gardening?

Over the past 10 or 15 years, gardeners have been adjusting their activities to changing environmental conditions. They are aware that the changes are caused by global warming, but wonder what their magnitude might be and whether they are permanent. Is the growing season really getting warmer and longer? And is the rainfall not what it used to be as well? Here is what the data recorded by Environment Canada at the Airport in Dorval are telling us.


The temperature of the period relevant for gardening, that is, the five months from May through September, varies considerably from one year to the next. Since 1942, the minimum of the period’s average was experienced at 15.9 oC in 1956 and the maximum at 19.6 oC in 1999. The moving average and the ‘least-squares’ trend line both indicate a cooling until the late 1970s and a rather rapid warming since. It seems therefore not unreasonable to assume that noticeable global warming began around the late 1970s, early 1980s.


To get an idea of the changes caused by global warming, we compare the 30-year period from 1986 through 2015 with the years before. It turns out that the growing season’s average temperature is now 0.6 oC greater than it used to be. It was 18.0 oC for 1986-2015 compared to 17.4 oC for 1942-1985. The increase is not uniformly distributed over the season. It is with 0.9 oC greatest in May and with 0.4 oC smallest in July.


It comes as no surprise that the rise in temperature has resulted in a longer frost-free period. Defined by the days between the median dates (those with as many observations before as after) on which the latest and the earliest frosts occurred, its length has increased from 163 to 172 days. The median date of the last frost was still the same (around 27 April) and the nine extra days were all added in October.


While the absolute earliest date of the first frost has moved very little (it was last experienced on 24 September 1992), as the graph shows there is still a 10 to 15% chance for the first frost to occur before the end of September. In October, the first frost of any probability now occurs at least a week later than it did before global warming. As for the latest frost, the extreme for 1942-1985 was recorded on 31 May 1961 and in the era of global warming, on 13 May 2005.

Although the frost-free season has essentially been extended in the fall, the most significant change for gardening is taking place in the spring, with the extreme date of the latest frost in May now being a week or two earlier. There is of course always the possibility of a very late occurrence of the last frost, but it looks like the old rule that wants you not to plant anything before June 1st does not apply any more. It is now pretty safe to plant after the middle of May, and the actual planting day may depend more on when the soil is dry enough to be prepared.


Since higher temperatures favor evaporation and warmer air holds more moisture, global warming brings with it an increase in rainfall. Over the period from 1942 through 1985 an average of 413 mm of rain fell during the growing season. That average has risen to 453 mm for the 30-year period starting in 1986.


The increase in rainfall is most pronounced in May (17 mm on average) and June (11 mm). It is less for July and August, and insignificant for September.


The number of rainy days in the growing season has increased as well, but not by very much. Before 1985, there were on average 61.3 days on which rain fell. Over the years since 1986, that average has grown to 62.0 days. Like the increase in the amount of rain, the average increase in the number of rainy days is concentrated in May (0.9 days) and June (0.5). In September, the average number of rainy days has decreased by a day, from 12.1 to 11.1 days.


That a small number of additional rainy days resulted in the observed increase in the quantity of rain means that rainfall has become more intensive. Before global warming, in the month of May for example, an average of 12.9 rainy days produced an average of 68.7 mm of rain, or 5.3 mm per rainy day. Since 1986, 13.8 days produced 85.3 mm, or 6.2 mm per rainy day.

Of particular interest are of course the days of heavy rainfall. The graph shows the increase in their number for the 5-months growing season. The number of days on which rainfall exceeded 15 mm, for example, has grown from 8.0 to 9.2 days. More than 25 mm of rain used to be observed on 2.8 days on average; global warming caused their number to climb to 3.7 days.



The recorded climate data confirm that the growing season has become considerably warmer and longer. The rise in temperature is particularly noticeable in the spring, while the extension of the frost-free period is most apparent in the fall. But we also see a significant shift in the date of the extreme latest frost in the spring, indicating that it is now pretty safe to plant as early as the middle of May.

The increase of rainfall is concentrated in the spring as well. Rainy days have not become much more numerous, however, so that the greater amount of rain is produced by more intensive precipitation. The changes to rainfall may for now be less important for gardening than the earlier occurrence of the latest frost in spring. What we must not forget though is that all changes observed so far indicate a trend that will to lead to stronger effects as global warming progresses.

(Article written by Paul Egli)

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