By Thomas Todaro
The 2017 grape growing season ended by fall frost in northwest Michigan with fruit still on the vine. What can this mean for harvest and cold acclimation?
Growing season length is a critical parameter for vineyard site selection, and coupled with vineyard management practices, determines the ultimate success of the vineyard. In past viticulture research by Jordan et al., 1980, the growing season has been defined as the number of consecutive days where the minimum temperatures are above 29 degrees Fahrenheit (-2 degrees Celsius). A more conservative and recent growing season definition by Jorgensen et al., 1996, uses 31 F (-0.5 C) as a temperature threshold below which frost damage will occur.
Grape cluster on leafless vine
However, Michigan viticulture, like regions of Europe such as Austria, Germany and Burgundy, is classified as cool-climate viticulture with average growing season length ranging from 165 (northwest region) and 180 (southwest region) days, and is characterized by relatively short growing season length, cooler summers (relatively low heat unit accumulation) and potential frost or freeze damage during fall, winter and spring, according to Perry and Sabbatini, 2013.
For this reason, frost-free days, defined as consecutive days with minimum temperatures above 32 F, are a more accurate and conservative determination of growing season length, and allow classifying sites as either unsuitable (less than 165 days), marginal (166 to 180 days) or preferable (more than 180 days).
By this definition and classification, Old Mission and Leelanau Peninsulas achieved 172 and 180 frost-free days, respectively (Fig. 1). This places the 2017 growing season length for the two peninsulas in the marginal range classification. Although the growing season length is within normal historical range for the region, in 2016 Old Mission Peninsula (198 frost-free days total) and Leelanau Peninsula (202 frost-free days total) had 26 and 22 more frost-free days than in 2017, respectively. In 2017, the lowest minimum temperature to date on Old Mission Peninsula (19.4 F) and Leelanau Peninsula (18.7 F) was recorded on Nov. 9, while last year temperatures did not drop that low until Dec. 10 (Figs. 1-2).
Figure 1. Minimum seasonal temperature of Leelanau and Old Mission Peninsulas from April 1 to Nov. 9, 2017. Horizontal black line indicates freezing temperature (32 F).
Figure 2. Minimum seasonal temperature of Leelanau and Old Mission Peninsulas from April 1 to Dec. 31, 2016. Horizontal black line indicates freezing temperature (32 F).
This means that 2017 has a significantly shorter growing season (fewer frost-free days) by roughly two to three weeks and earlier and colder minimum temperatures going into the winter than in 2016, resulting in a relatively short or non-existent post-harvest foliated period. In fact, some grape producers have clusters on leafless vines that can have negative effects on the fruit and wood.
Introduction to frost damage in the vineyard
There are two main types of frost events according to Jorgensen et al., 1996: advection event, which is a horizontal movement of a cold air mass; and radiation event, which is a loss of heat into the atmosphere. Regardless of the cause, cool-climate viticulture is especially subject to frost damage at any time in the growing cycle (according to Gladstones, 1992), and previous research by Nemani et al., 2001 has determined the number of frosts per season can significantly impact wine quality.
Generally, frosts occur at the beginning (spring) and end (fall) of the growing season. According to Trought et al., 1999, spring frosts after bud development has started will decimate the potential crop tonnage for the season, while fall frost will directly damage the canes and maturing berries. Damage to maturing berries impacts fruit quality, and damage to canes and leaves leads to premature leaf fall and a lack of post-harvest assimilate storage (nitrogen, carbon, etc.) to support the following season’s spring growth, according to Shaulis and Pratt, 1965, and Trought et al., 1999.
Frost damage to grape berries
With low temperatures comes damage to the cells of the grape skin, a challenge that has been overcome in the past with careful and hasty harvest, and even adjusting the intended use of the grape with the production of ice wine. However, when low-temperature damage to the grape skin is followed by an increase in temperatures to the 55-60 F range, the susceptibility of the fruit to fungal infections, and in turn fruit deterioration, increases dramatically. This is something to monitor in the vineyard and may influence your harvest operations.
Post-harvest foliated period for overwintering vine components
Grapes still on the vine after the leaves fall (fall frost) by definition eliminate the post-harvest foliated period. The post-harvest foliated period is considered critical in cold acclimation and achieving maximum cold hardiness during mid-winter for vine components including the buds, canes, cordons and trunks. A long post-harvest foliated period is associated with increased nitrogen and carbon resource assimilation to the roots, which provide energy for early shoot growth the following season, according to Greven et al., 2016.
Although there have been no reports of bud and cane cold damage and injury in the region following the minimum low temperatures of 19.4 F (Leelanau Peninsula) and 18.7 F (Old Mission Peninsula) on Nov. 9, assessing cold damage is something to conduct in the vineyard throughout the winter via assessing the bud and cane tissues for damage.