Snow provided an insulating blanket for asparagus crowns this winter

While air temperatures were frigid this winter, snow cover helped buffer soil temperatures to protect herbaceous perennials like asparagus from the brunt of the cold.

Asparagus crowns in a trench at planting in spring 2013 prior to being covered by soil. These plants grew during 2013, but this fall would have become cold-adapted and buffered from cold temperatures by their position beneath the soil surface.

Asparagus crowns in a trench at planting in spring 2013 prior to being covered by soil. These plants grew during 2013, but this fall would have become cold-adapted and buffered from cold temperatures by their position beneath the soil surface.

We have recently gone through one of the coldest winters on record in Michigan. This begs the question, “How have our perennial crops fared?” A grower concerned about her asparagus crowns recently asked me this. Because asparagus is an herbaceous perennial and survives winter as roots buried in the soil, soil temperatures are more critical than air temperatures. While we won’t know for sure if the crowns were damaged by cold winter temperatures until spears emerge, it is true that our record snowfall provided an insulating blanket that would have buffered the soil and crowns from cold air temperatures.

How does this work? When the furnace turns on in your house, it heats the air to make it more comfortable in winter. Because heat is conducted from warmer to cooler materials, heat from your house moves from the warm house to the cooler air. Thankfully, materials used for insulation are designed to be poor heat conductors, so they slow heat loss, keeping your house warmer and your heating bills lower.

By analogy, solar radiation is a furnace that heats up the soil in summer. In the winter, this heat is conducted from the soil to the cooler air. When the soil is bare, this heat loss occurs more rapidly and the ground can freeze more deeply. However, when snow is present, it acts as insulation, reducing heat loss from the soil just like the fiberglass insulation in your house.

Comparing snow, soil and air temperatures from the Hart National Weather Service station between this winter and the winter of 2012-13 can help drive home what this all means. (Air temperature and snowfall rankings are based on data for winters between 1893-2014 during December and January, while soil temperature data are from the nearby MSU Enviro-weather station.) This winter, long johns likely got much more use than in 2012-2013 as average air temperatures were 19 degrees Fahrenheit, the fourth coldest on record. In contrast, last winter’s mean temperatures of 27 F were the 16th warmest. While this suggests that soil temperatures would have been much cooler this winter, snow depths during December 2013 and January 2014 were also the 11th deepest on record and last winter’s mean snow depth was the 14th lowest on record.

As a result, even though mean air temperatures were about 8 F cooler this winter compared to last, mean soil temperatures at crown depth (around 6 inches) were only 1 F cooler (33 F this winter compared to 34 F last winter), in part because our increased snow cover helped insulate the soil by reducing heat loss (soil temperatures are from the nearby Hart Enviro-weather station).

According to Michigan State University Extension, this suggests that the impact of this past winter’s cold air temperatures on herbaceous perennials like asparagus were not at all as harsh as the cool air temperatures would suggest. 

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