While some Iowa winters are relatively mild, others are particularly severe with lots of snow and more important, extremely low temperatures. The winter of 2021 is certainly one of the latter. So, not surprisingly, we frequently hear the question: How are the grapes doing?
The short and most accurate answer is that we don’t know yet, and we won’t know for sure until the vines bud out in late May and early June.
Grapevines are different from Iowa’s two main agricultural crops: corn and beans. Corn and beans are both annual crops that are planted as seeds in the late spring and they are harvested in the early fall of each year. Therefore, they don’t have to survive our winters. Grapevines, on the other hand are perennial. They are planted as rooted cuttings in mid-May after the chances of a late frost are minimal. Over the next two years, they grow a root system, a trunk that reaches up the top trellis wire, and two arms that grow in both directions from the trunk. During this time, if they do set any fruit, that fruit is usually removed in order to allow the vine to focus on vegetative growth. After the vines have leafed out for the third time, they are ready to produce a fruit crop. It is usually small that third year after planting but it gets bigger in subsequent years.
So, survival of winter weather becomes a major consideration in determining which types of grapes to grow in Iowa.
Because people are accustomed to equating snow on the ground with cold, they see snow and automatically think cold! Actually, snow is a pretty good insulator for our grape varieties. Six or eight inches of snow will allow our grapes to withstand moderately cold winter temperatures.
The real issue is the actual air temperature. Most cold climate grapevine varieties are usually resistant to damage from temperatures down to about -25°F. Colder temperatures can cause damage to the trunk and the arms that prevents the vine from bringing up the carbohydrates in the sap from the root system when spring comes and weather warms up. This damage is not immediately obvious and that is why we have to wait to see if the vines bud out properly. The good news is that extreme cold in February when the buds are just starting to form is not likely to damage the bud as will cold temperatures in April, for example. By April, the buds have formed and the primary bud is very susceptible to cold temperature damage.
Another factor that may protect buds from forming too fast if temperatures warm up considerably in early March is that they seem to have a cold weather memory. In other words, the colder the winter, the slower the development of the buds in the spring regardless of spring temperatures. This tends to protect the vines from spring weather that warms up to 70–80°F in late March only to descend again to freezing temperatures for a couple of weeks in April.
If the extreme cold weather damages the trunk of the vine, the vine will use the carbohydrates stored in the roots over the winter to create suckers that emerge from the soil or from the bottom of the trunk. In effect, they will grow a new vine to replace the one that was damaged. So, we would lose the existing vine and harvestable fruit for this year but if we train the suckers up, we can expect at least a light harvest next year.
So, we will start pruning in early March and will carefully watch bud formation on all of our varieties. With any luck, the primary buds will not have been damaged and the vines will start budding out in mid-May and the 2021 grape growing season will be off to the races!