In my context, frost can be a danger to saskatoons, particularly once bloom has begun. In "Growing Saskatoons" by St-Pierre (the main body of research done on Saskatoons by the University of Saskatchewan), he mentions that a killing frost is defined as -2.2〫C during active growth or flowering. Well, this spring, we had several late frosts. I had heard that we were typically safe from frost after the first full moon after May long weekend, but it definitely wasn't the case this year, with -2 on both May 24 and 25. (And a frost warning in June - yikes!)
This mobilized my research efforts, and I found a great handbook by the ag department in BC. Click on the handbook link for an interesting read on how frost is formed, and an evaluation of different frost protection. While fire can be useful (many small fires are better than one big bonfire), smoke can actually be detrimental. Long-wave radiation (heat from the ground) passes right through smoke, unlike clouds, so it does not trap or reflect heat back towards the plant, but in the early morning it will block the short-wave radiation from the sun. Wind machines can be effective but are very costly and must be set up correctly. Irrigation can be a great way protect trees, but another frost protection guide from the Iowa State University notes that when dewpoint is more than 5〫below the predicted low temperature irrigation can actually cause evaporative cooling and increase the frost damage (which was the case this May).
So, we ended up protecting them in one of the more labor-intensive ways - direct cover. We used stakes as a support, and covered our flowering fruit with blankets from friends, and for the second frost managed to pick up a few more from the local MCC thrift store. Since we had three nights in a row with frost warnings, we had to put them up and carefully close them off with clothespins or rocks at the bottom, and take them off again in the morning. We had enough to protect the 48 "fullest-bloomed" saskatoon trees (out of ~240), along with our veggies and cherry trees. It was a LOT of work. I later estimated that the covered trees had around 25% less frost damage, but significantly less frost kill. The cherries didn't fare quite as well, losing up to 90% of its blossoms to frost kill.
Honeyberries are virtually indestructible to Manitoba frosts, which is one reason I really like them. 0% damage to the post-bloom crop with no protection - these super-hardy fruits from Siberia can handle -7〫C in full bloom, and lower temperatures when they are tight. The crop looks perfect so far - I may be expanding this if I get good customer feedback this spring!
If you'd like to comment, tell me about your frost protection methods and how well they work. And if anyone knows the folklore about when to put your tomatoes in, do tell. Maybe Manitoba doesn't follow that rule anyway - I'm thinking of just waiting until June every year! (Although almost all of our frost-damaged tomatoes grew back this year)