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The Easter Freeze of 2007: Extent of Damage and Strategies for Managing Freeze-Injured Vineyards

Vineyard alert posted April 18, 2007

R. Keith Striegler, Ph.D., Andy Allen, and Eli Bergmeier;
University of Missouri-Columbia, Institute for Continental Climate Viticulture and Enology
Justin Morris, Ph.D.; University of Arkansas, Institute of Food Science and Engineering

The weather conditions of April 4 to 9, 2007 caused a significant amount of freeze damage in Missouri and Arkansas vineyards, as well as those in many eastern U.S. states. What made this freeze so devastating? Figure 1, below, tells the story well: a near-record period of warm weather, which caused grapevines to de-acclimate and begin growth, followed by a five-day period of record-breaking cold temperatures.

Figure 1. Temperature departure from normal during the March 21 – April 9, 2007 period at Columbia, Mo. (Image provided by Pat Guinan, Missouri State Climatologist)

The Institute for Continental Climate Viticulture and Enology (ICCVE) personnel conducted an extensive survey of the Missouri grape industry during the week of April 9. As one would expect given the recorded low temperatures in Figure 2, below, the damage we observed was severe. According to Justin Morris, Ph.D., at the University of Arkansas, "In my 40-plus years of experience in the grape industry, I have never witnessed damage as extensive as this." Most Ozark Mountain Region vineyards have suffered complete or nearly complete primary bud/shoot loss (Fig. 3), and some appear to have suffered significant injury to the secondary buds as well (Fig. 4).

Figure 2. Minimum temperatures recorded at various Missouri locations during the April 4 to 9, 2007 period. (Image provided by Pat Guinan, Missouri State Climatologist)

At this point, the tentative estimate is that the Missouri and Arkansas grape crops have been reduced by approximately 95 percent. However, we must add several important caveats:

This level of crop reduction will likely not be observed uniformly in all vineyards, as injury may vary by cultivar and stage of bud/shoot development. Some vineyards will likely produce little, if any crop, and others might produce a significant yield. The cultivar being farmed, site macro- and mesoclimate, observed temperatures, 2006 growing conditions and the time of pruning are all variables that will determine what level of injury has been suffered in individual vineyards.
The estimate is based upon a cursory view of visible injury. Although we did examine some swollen and "tight" secondary buds, we were not able to perform a widespread viability study of dormant buds. This estimate of crop loss might increase or decrease as the approaching warm weather and the resumption of vine growth make the full extent of injury more apparent.
The estimate was also developed upon the premise that regional vineyards have not suffered trunk or cordon injury. This is presently not a safe assumption, in that winter injury to semi-permanent vine tissues may not be evident until approximately bloom, when growers may begin assessing trunk and cordon injury by making shallow, near-parallel cuts into these structures and viewing the subtending phloem and cambium tissues.
Further crop reduction could occur. Tragically, another late frost/freeze is not beyond the realm of possibility, and poor conditions during fruit set could further reduce the crop.

Fig. 3. Young developing primary shoots on Chardonel killed by freezing temperatures during the Easter '07 freeze. (Photo by Andy Allen)

Faced with crop reduction of this magnitude and the potential for significant vine injury, grape producers farming bearing vineyards should carefully evaluate their management strategies to assure vine health and recovery.

Fig. 4. Examination of buds on Chardonel showed no live secondary buds on any of the buds examined. (Photo by Andy Allen)

A partial listing of considerations to be made should include the following:

Economics: Prior to developing management plans for the 2007 season, growers should carefully evaluate the potential crop of each cultivar in their vineyards by performing cluster counts as new shoots emerge. If vineyard blocks containing the same cultivar were treated differently during the previous year or feature different mesoclimates, these blocks should be surveyed independently. We recommend surveying 20 representative vines per acre to aid in developing an accurate field or block average. Multiplying average cluster number per vine by historical average cluster weights and again by the number of bearing vines per acre should develop a useful yield estimate. If it is noted that the emerging clusters are below average in size, an appropriate adjustment to the estimated cluster weight should be made.

Once the yield estimate has been developed, growers should contact prospective buyers and discuss fruit pricing. It is reasonable to expect that a moderate price increase above normal may be obtained due to the reduced crop, but growers must resist the urge to recoup lost revenue potential by demanding unreasonable prices. Doing so could seriously jeopardize growers’ relationships with processors and their long-term financial viability.

With estimated yield and fruit pricing information in-hand, growers can then make a revenue projection to determine whether the potential crop is sufficient to cover the variable costs of production. Information on the cost of production for grapes in the Ozark Mountain Region is available from the University of Arkansas’ Agricultural Experiment Station Research Report 976 entitled Production Budgets for Arkansas Wine and Juice Grapes (PDF).

If it is determined that the expected returns are insufficient to cover the anticipated variable costs of production during 2007, the grower should consider removing the vineyard or block from production for the season and manage it on a reduced maintenance basis for the season. Such a strategy does not imply the vineyard should be neglected for the season, but that it will be managed to assure vine health and recovery at the lowest possible cost. Cost control through reduced fertilizer, weed control and disease and insect pest control inputs may be appropriate. However, if the expected returns are high enough to cover the variable costs of production, growers are encouraged to farm the crop as normal to deliver a sound crop of ripe fruit.
Need for vine renewal: As suggested above, it is presently not feasible to determine whether significant trunk and/or cordon injury has occurred. Under these conditions, it is highly advisable to retain a limited number of suckers and well-positioned shoots developing in the head region of the vines to serve as replacement structures should trunk or cordon failures occur. Accordingly, growers should confine their early season suckering and "trunk stripping" treatments to those shoots that would be unusable as renewals and perform these operations later in the season when replacement needs are better known.
Weed control: The need to retain suckers and shoots emerging low on the existing trunks should have a significant impact on growers’ weed management strategy. With the exception of a small cadre of grass-selective herbicides, all available post-emergent herbicides are injurious to vine foliage and place suckers and shoots near the ground at great risk. If these tissues are needed for vine renewal but subsequently are injured by herbicide, vine rejuvenation might be delayed and the subsequent year’s yield reduced.

Accordingly, growers are strongly advised to adopt a weed management strategy more compatible with sucker retention. Some possible options include the following:
1. Pre-emergent herbicide application (see the current Midwest Commercial Small Fruit and Grape Spray Guide for a listing and discussion of available materials)
2. Cultivation
3. Mulching
If the decision is made to apply pre-emergent herbicides, growers are advised to carefully read and comply with product labels. In particular, many pre-emergent herbicide labels prohibit application after bud swell, as these products might injure vine foliage. If bud swell has not been observed, and weeds are already present in the vineyard, growers should investigate the possibility of tank-mixing a post-emergent herbicide with the selected pre-emergent product. Always read the product labels for compatibility information.
Vigor management: Established vineyards carrying an atypically low yield can become excessively vigorous and create a dense, shaded canopy that may favor fungal disease development, which might depress bud fruitfulness for the following season. Several strategies can be employed to help avoid this situation:
1. Reduce or eliminate nitrogen fertilization. If nitrogen fertilization is needed to build vine size, the rates should often be reduced under low-yield conditions, and growers are encouraged to consider applying nitrogen as 2 to 3 split applications throughout the first half of the season. Doing so will provide the grower the opportunity to influence vine vigor by the frequency and rate of nitrogen application, and to tailor the total amount of nitrogen applied to the estimated crop.
2. Limit irrigation rates, but be certain to maintain a functional leaf canopy throughout the growing season.
3. Allow a light cover of annual weed growth in the vine rows. Doing so will provide some competition with the vines for water and nutrients. Do not allow these weeds to produce seed, however, doing so will favor undesirable weed growth in subsequent seasons. Chemical or mechanical mowing can be employed to keep weed height and competition in check.
4. Even if the crop will not be harvested, allow the crop to remain on the vines. Vine vigor is heavily influenced by crop load; even a light crop will aid in reducing vine vigor.

Although immature vineyards might also have been negatively impacted by the April 4 to 9 freeze, these effects should be temporary. Growers managing young vineyards should follow the recommendations provided above as they pertain to them. In particular, usable "replacement" shoots should be retained as young vines are more likely to have suffered trunk or cane injury than mature vines. Vigorous sucker growth can be an important indicator of freeze injury, as suckers often grow very rapidly on vines that have suffered injury to the trunks and/or cordons. If growers observe this phenomenon in their vineyards, they are urged to retain replacement tissues until the full extent of injury can be assessed.

Throughout the 2007 season, the ICCVE will be placing an emphasis on vine health and recovery in freeze-injured vineyards at the Best Management Practices tailgate meetings. We sincerely hope that growers in the region will consider attending these meetings to become better educated in managing their vines through these difficult circumstances. As the full extent of injury from the April 4-9 freeze becomes apparent, additional updates and information will be posted on the ICCVE Web site as necessary to help regional grape producers aid vine recovery processes. This Web site also offers a complete listing of the Best Management Practices tailgate meeting dates and locations for the 2007 season.

Possible Freeze Damage

Vineyard alert posted April 6, 2007

With grape buds either swollen or already open and recent low temperatures, we are facing probable freeze damage to grapevine buds throughout the region.

Many growers have called expressing their concerns and requesting information regarding the best means of managing the situation and minimizing damage. Although it might be too late to implement many of the active countermeasures to prevent or reduce frost/freeze injury in the current situation, we offer the following information on mitigating cold injury events in vineyards for future reference. Some prevention methods might have limited effects in the current situation, dependent upon your individual situation, vine development, and cultivar.

We will be monitoring the results of the forecasted freeze events this weekend and in the days and weeks to come to determine the extent of damage, and we will be issuing further advisories on how to best deal with the injuries that do occur.

R. Keith Striegler, Ph.D.
Director and Viticulture Program Leader, ICCVE

Andy Allen
Extension Viticulturist, ICCVE

Eli Bergmeier
Senior Lab Technician, ICCVE