On the mornings of April 10 and 11, most of Iowa experienced freezing temperatures low enough to injure emerging grape shoots. The charts below from Harry Hillaker, State Climatologist, report the temperatures recorded at various recording stations throughout Iowa and surrounding states. Temperatures in vineyards were probably different, but similar based upon the vineyard site characteristics.
The extent of injury was dependent on the temperature that occurred in the vineyards, and the stage of bud and shoot development. Vineyards in southern and southwest Iowa were further ahead, but generally did not experience as low temperatures as those that occurred in central and northern portions of the state.
At the ISU Horticulture Research Station, we recorded 24degrees on April 10 and 20 degrees on April 11, and I did a preliminary assessment of the injury Wednesday afternoon of the cultivars in our NE-1020 grape cultivar trial plot. The results of that assessment for named cultivars are presented below along with a chart on the critical temperatures for 10% and 90% of grape buds and shoots.
| Location | Cultivar | Growth Stage | % Dead buds & shoots |
|---|---|---|---|
| Least advanced | Most advanced | ||
| Marquette | Full swell | 3rd leaf | 90 |
| La Crescent | Full swell | 2nd leaf | 80 |
| St. Croix | Full swell | 2nd leaf | 80 |
| Frontenac | Full swell | 1st leaf | 20 |
| Corot noir | 1st swell | Bud burst | 25 |
| Petit Ami | 1st swell | Bud burst | 10 |
Based upon these preliminary results and critical temperatures for grape buds and shoots, the most advanced cultivars such as Marquette, La Crescent and St. Croix suffered the greatest injury, while those that broke bud later, such as Frontenac, Corot noir and Petit Ami suffered much less injury (See images of Marquette, La Crescent and Frontenac, below). As the injured buds and shoots become drier, it will be easier to determine what is alive and dead.
So what's the prognosis and what can be done? Many have already left longer spurs than normal with plans of double pruning to take advantage of the delay in emergence of buds closer to the base of the spurs.
If the entire shoot is dead, secondary shoots should emerge at the base of the dead shoots in about 10 to 14 days which will put the vines on a near-normal schedule. Depending upon the cultivar, those secondary shoots will vary in productivity from being almost as productive as the primary shoots to something less than half as productive as the primary shoots. Generally, the secondary buds of lubrusca (American)-type cultivars are less productive that the interspecific hybrid wine cultivars. For many cultivars, information on the productivity of the secondary shoots is available in the Review of Cold Climate Grapes Cultivars that is posted on the ISU Viticulture Home Page.
If the base of the injured shoots remains alive, more than likely a non-productive lateral (axillary) shoot will emege from the green tissue and if this occurs, it will inhibit the emergence of the potentially productive secondary shoot. Therefore, to improve the potential productivity of frost-injured vines, any injured shoots that remain alive at the base should be removed.
Many cultivars produce basal shoots that develop from adventitious buds on the cordon at the base of spurs and spur arms. For some interspecific hybrid wine cultivars, these basal shoots can be productive and normally break bud a little later that the buds on the spurs and canes. In normal years, these adventitious shoots are removed early in the growing season to avoid over-production and excess shading. With the frost injury that has occurred, basal shoot removal should be delayed long enough to determine if there will be clusters on the shoots.
Where the frost killed just a portion of the primary shoots on various cultivars, it may be necessary to do some crop adjustment at veraison to even out the maturity of wine cultivars. Normally, a "green drop" (such as removing under-mature clusters) would be practiced at this time, but if only a few primary shoots survived, it many mean removing the most advanced clusters on the vines.
Minimum Temperatures around Iowa, April 10, 2012
Minimum Temperatures around Iowa, April 11, 2012
Freeze damage to Marquette grapes; ISU Hort Farm April 11
Freeze damage to La Crescent grapes; ISU Hort Farm April 11
Freeze damage to Frontenac grapes; ISU Hort Farm April 11
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