Ames, Iowa -- The winter of 2009-10 started early in Iowa. On Oct. 10, temperatures dropped well below freezing to as low as 22°F. The freeze was a harbinger of more winter problems that would follow in December and January.

On Dec. 10, temperatures were recorded between minus-6° and minus-11°F, and on Jan. 2, even colder temperatures occurred across the state, plummeting from minus-18° to minus-21°F.

Beginning in March, viticulture researchers at Iowa State University began to assess the amount of damage caused by these significant freeze events. According to Dr. Paul Domoto, professor in the Horticulture Department at Iowa State University, the early October freeze played a major role in the bud injury suffered by state vineyards.

Domoto told Wines & Vines, “With the cool growing season in 2009 and the late harvest, the grapes (vines) didn’t have the chance to harden off. The early freeze had a real impact, and we saw a lot of damage. The previous winter had even lower temperatures, but the grapes were better prepared and more acclimated.”

There are four grape research sites in Iowa; grape cultivar trials are being conducted at all locations. The Horticulture Research Station (HRS) is located in Ames, the home of Iowa State University (ISU) in the center of the state. The other three sites include ISU’s Armstrong Research and Development Farm (ARF) in Lewis in southwestern Iowa, the Southeast Research Farm (SRF) in Crawfordsville, and the Northeast Research Farm (NRF) in Nashua. These locations represent three different climatic and four different soil conditions. Table 1 shows the minimum temperature at each site for the three severe freeze events. Sites and cultural conditions have an effect
Not surprisingly, less cold-hardy varieties suffered more damage at each of the sites. However, Domoto noted that among sites the severity of injury for a given cultivar was not always closely associated with the minimum temperature recorded. Cultural conditions appear to have had an influence on the extent of injury.

The Horticulture Research Station has the most vigorous site, and the Northeast Research Farm the lowest vigor. The cooler-than- normal growing season in 2009, the lateness of harvest in relation to the Oct. 10 killing frost and the crop load also appear to have been factors. Table 2 shows the percentage of bud damage for each varietal at the four research vineyards. Ten winegrape and five table grape varieties were planted in 2002 as part of a trial to evaluate the adaptation of grape cultivars by three different management systems: 1) a conventional system that relies on herbicides for weed control and the regular application of insecticides and fungicides; 2) an IPM/best management system that uses herbicides as needed and monitors the vineyard to determine the need for insecticides and fungicides; and 3) an organic-approved system that relies on alternative methods for weed control and the use of organic-approved insect and disease control strategies.

In 2003, 20 additional winegrape varieties were planted to evaluate their adaptability, productivity and winemaking quality.

The full extent of the winter injury will not be known until the vineyards leaf out. By April 15, grapevines in Iowa were just beginning to push, and how productive the vines are in 2010 will depend on the viability of the secondary buds on the different varietals. Domoto noted, “We’ll adjust our pruning and see what happens.” Spring frosts may also be a problem, with the average last date for frosts varying from May 5 to May 10 across the state.