Oct 31, 2018
Harvesting grapes after fall frost

Last year, the 2017 growing season was ended with low fall temperatures and fall frost before fruit was harvested, leaving the grapevines without a post-harvest foliated period. This year, a greater number of heat units were accumulated than in 2017, but sub-freezing fall temperatures were reached at an earlier date. What does this mean for vineyard operations, fruit quality and cold acclimation?

Because there has been very few additional growing degree-days (GDD) accumulating in the region since the sampling date (Oct. 16) and a vast number of regional vineyards have either partial or complete leaf fall resulting from low sub-freezing temperatures on Oct. 18, it is likely that any marked increase in sugar accumulation (Brix) is likely due to berry desiccation where the berries lose water and sugars are concentrated.

Table 1. Regional fruit maturity report.
Location Cultivar Pruning type # Fruiting canes/ vine Collection date GDD 100 berry wt. (g) SS%


pH T.A. (g/L)
Old Mission Merlot Cane 2 Oct. 16 2,674 136.00 21.80 3.64 4.99
Old Mission Cab. franc Cane 2 Oct. 16 2,674 122.00 19.90 3.44 6.53
Leelanau (Cedar) Riesling Cane 2 Oct. 16 2,838 143.00 18.73 2.96 12.79
Leelanau (Cedar) Chardonnay Cane 2 Oct. 16 2,838 174.00 20.15 3.26 9.94
Leelanau (Cedar) Teroldego Spur-cordon trained 2 Oct. 16 2,838 175.00 19.70 3.04 13.76
Leelanau (Cedar) Merlot Cane 2 Oct. 16 2,838 143.00 19.43 3.40 7.95
Leelanau (NWMHRC) Riesling Cane 2 Oct. 16 2,838 123.33 18.70 3.02 12.68
Leelanau (NWMHRC) Riesling Cane 4 Oct. 16 2,838 120.17 18.20 2.95 12.18
Leelanau  (Manitou trail) Pinot noir Spur-cordon trained 2 Oct. 16 2,838 NA 21.00 3.20 7.50

Pinot noir in Leelanau County 10-16-18
Pinot Noir in Leelanau County at harvest before fall frost, Oct. 16, 2018.

Riesling grapevines 10-16-16
Riesling fruit zone, grapevine and clusters at the Northwest Michigan Horticulture Research Center after fall frost, Oct. 22, 2018.

A review of growing season parameters

Growing season length is a critical parameter for vineyard site selection, and coupled with management practices, determine the ultimate success of the vineyard. In past viticulture research from 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 from Jorgensen et al., 1996, uses 31 F (-0.5 C) as a temperature threshold below which frost damage will occur.

However, Michigan viticulture, like regions of Europe such as Austria, Germany and Burgundy, is classified by Perry and Sabbatini, 2013, as cool-climate viticulture with average growing season length ranging from 165 days (northwest region) and 180 days (southwest region), 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.

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), preferable (more than 180 days). By this definition and classification, Old Mission and Leelanau Peninsulas achieved 160 and 171 consecutive frost-free days, respectively (Figure 1). This places the 2018 growing season length for the two peninsula AVAs in the unsuitable (Old Mission) and marginal (Leelanau) range classifications and this is due to early subfreezing temperatures in October (Figures 1 and 2).

Although the 2018 growing season accrued more GDD at a higher rate than in 2017 and the 2018 growing season length is within normal historical range for the region, fall frosts have continued to encroach into the growing season over the past two growing seasons (Figures 1 and 2). This year (2018), the lowest minimum temperature to-date on Old Mission Peninsula (26.0 F) and Leelanau Peninsula (30.2 F) was recorded on Oct. 18, while last year temperatures did not drop that low until Nov. 7 (Figures 1 and 2), according to Michigan State University Enviroweather.

These earlier and colder sub-freezing temperatures have led to widespread leaf fall and means 2018 has continued the trend with significantly shorter growing season (fewer consecutive frost free days) by roughly 10 days than in 2017. That difference is even greater when compared to 2016, resulting, again, in a relatively short or non-existent post-harvest foliated period. In fact, some grape producers have clusters on leafless vines which can have negative effects on the fruit and wood.

Figure 1. Minimum seasonal temperatures of Old Mission Peninsula from April 1 to Nov. 7 in 2016, 2017 and 2018. Source: MSU Enviroweather.

Figure 2. Minimum seasonal temperatures of Leelanau Peninsula from April 1 to Nov. 7 in 2016, 2017 and 2018. Source: MSU Enviroweather.

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 and post-harvest vineyard operations

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.

Since harvest season is winding down, it is a good time to refresh on post-harvest vineyard operations that may include assessing cold damage via assessing the bud and cane tissues for damage and the practice of hilling-up where soil is mounded to cover the base of the scion portion of the rootstock scion graft and insulate the base of the grapevine from damaging low winter temperatures. Information on grapevine cold hardiness, hilling up, assessing bud and cane tissue cold damage, and pruning strategies in response to cold damage can be found by clicking the links below.

–  Thomas Todaro and Jasmine Hartm Michigan State University

Photo at top: Pinot Noir in Leelanau County at harvest before fall frost, Oct. 16, 2018. All photos by Thomas Todaro, MSU Extension.

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