Today, we have a guest post from my husband, the man who puts in most of the sweat in this operation, Aaron Wills.
We have been cautiously hoping we'd have enough berries to be open for u-pick this summer. The recent cold spell has us a little on edge as to whether there will be any blues at all to pick and eat this summer. The -10 degree morning on Tuesday is not our primary concern. Even our less hardy varieties should be able to withstand up to -20 degrees. Our primary concern is the nearly 50 degree drop in temperature we experienced from Saturday afternoon to Monday morning. Here's why.
During the winter, our blueberry plants are dormant. They aren't growing. However, when the temperature warms above freezing (32 degrees) the plants can start to grow again if their "chilling hour" requirements are met. Chilling hours are when temperatures are between 32 and 50 degrees. Ironically, our hardier varieties need less chilling hours and thus are often ready to start growing again earlier in the winter than our less hardy varities. This means a big winter warm up followed by a rapid drop in temperature can hurt our Minnesota bred plants more than our less hardy highbush varieties. This is what we think happened last February when we had a few days in the 50s. Our hardy Minnesota varieties started putting out flowers while our highbush plants stayed dormant. And when spring came our northblue plants had very few flowers left to produce fruit on.
Even if the plant doesn't start growing again when the temperature rises above 32 degrees, it naturally acclimates to the warmer weather. A blueberry plant can adjust to a 20 degree drop below freezing in a day but a drop of over 20 degrees or more in a day can kill the fruit buds or severely weaken the plant. And if we've had a couple days above freezing before the big temperature drop that's when things get serious for a fruit grower. This sensitivity to temperature fluctuation is why many good fruit growing regions in the Midwest are near large bodies of water. The Bayfield area on Lake Superior in Wisconsin is an example of this. As is the main blueberry growing region in the Midwest, which is in Western Michigan along Lake Michigan.
Shakespeare had it right. When it comes to blueberries, those rough winds of winter shake those delicate buds of May.