I may have lost some tomato plants last night, even though I had them covered. No big problem - I knew it was a gamble, and I went ahead and set some out because I had lots of extra transplants to keep as replacements. But it got me thinking about last frost dates.
We had some very early warmth this spring. Farmers were out seeding in late April, and I would say that's about a month ahead of the traditional seeding time for this area. The ground dried out, and I got worried about drought. Then it started to rain, and I don't trust my cracked gauge, but Mom says they have had 5 inches in the hills since May 9th, while the average for the whole month is only 2. But before all that happened, at the end of April, I was gardening with drought in mind. I refused Garth's offer of rototilling and in fact refrained from digging the beds at all, to conserve moisture. As I started planting early vegetables, I went to fetch water from the ditch near our yard and found that it was already dry. I took my bike and trailer and hauled a tub of water from a vernal pool over on the east side of the 604. I was worried, but full of hopes that my earliest-ever garden planting might help get plants well established before a summer drought hit hard.
Last year I talked to many people in this area who said the crops turned out better than they expected, considering how dry it was in the latter part of the summer. One farmer friend agreed with my speculation that the early spring and early planting had allowed the crops to get a better root system established, so they weathered the drought better than in former years.
But as my tomatoes show, early planting is a gamble. I don't know much about frost tolerance of the common field crops in this area, but I know there are times when fields have to be reseeded because an early crop was killed by frost. As our springs get warmer, and droughts become more frequent (as predicted due to climate change), farmers may try to adapt by planting earlier. But what about frost? How are the last frost dates changing?
I pulled out my CD of climate data for Western Canada and checked what was available for our area. Carlyle had max/min temperature data from 1922 to 1996 (with one year missing, 1962). I plotted last spring dates with temperatures less than or equal to 0ºC.
Now, where is my old statistics text book? I vaguely remember a lot of cautions about interpretation and testing where time series are concerned, but to me this graph seems to confirm my suspicion that last spring frost dates have not simply moved earlier. They have become more variable, with earlier dates contributing almost all the new variation, but late dates still occurring frequently.
What does that mean for adaptation? In my garden, I can plant things in succession, I can hold back extra transplants as replacements, and so on. But in a farm operation with thousands of acres under cultivation, what is to be done? Can farmers absorb the costs of having to reseed fields one year in six, say, or one year in four? Can they choose their planting dates strategically and be prepared for reseeding operations, or do they have to get it all in as quickly as possible and get back to their off-farm jobs? I wonder.
I also wonder about spring weather patterns in terms of soil temperatures and growing degree days and the overall shape of the warming. My second planting of peas has not appeared yet, and I think it's been ten days. Did the recent cold and cloudy spell chill the soil and rot the seed? How does this spring compare to the springs we used to get? Was a steady warming more the norm? I will have to ponder how to analyse my climate data for that.
UPDATE: Oh, yeah, and speaking of adaptation, I forgot to mention that whole area of adaptation that most people never think about, where there will definitely be problems: adaptation by every other species except human beings and their little collection of manipulated plants and animals. Are we already seeing the effects? What about that die-off of poplar in the hills in the '80s? The experts blame insects, but the locals say it was a combination of drought stress and a late frost. What about the decline in bees across North America? Insects can adapt fairly quickly because of their short generation time, but pollinators are closely co-evolved with plants, and some plants take a long time to reproduce. These changes are happening incredibly rapidly when compared to previous evolutionary history. Can genetic adaptation keep up?
2 weeks ago