Virginia's 'Silent Spring'
What growers can do to recover from cold injury reminiscent of 2004
Silent spring. That was what some grapegrowers experienced this May, when buds should have been bursting with rapid shoot growth. Instead many eastern farmers saw more widespread evidence of winter cold injury than they expected based on estimates made earlier this year. Typical bud injury was evident, but so was injury to trunks and cordons, which spanned a surprising range of varieties.
The 2013-14 winter will be remembered as a cold, changeable and persistent season. On the whole, the low temperatures experienced throughout Virginia were not that low. Many locations reported lows in the 0° to -3° F range, while a few dipped as low as -7° F.
Reports of injury started rolling in following the early January dip in temperature, when a low of -2° F was recorded at the Agricultural Research and Extension Center (AREC) vineyard in Winchester, Va. The vines at the AREC appeared to come through that event unscathed. Reports of damage to cold-tender vines including Tannat and Merlot arrived soon after. Critical temperatures appeared to be at or below about -4° F.
There were at least five oscillations in air temperature between early January and early March, with the event on March 4 following a three-day stretch of highs close to 60° F.
One grower said that the injury at his vineyard during this most recent winter was reminiscent of what he observed a decade earlier, following the 2003-04 winter season. I went back and read what I had observed following the 2003-04 winter in “Viticulture Notes,” a newsletter I write for the Virginia Cooperative Extension, and was struck by some of the similarities in injury following the two winters.
When did the injury or winter kill occur during the 2013-14 winter? I’m certain that some of the injury occurred as early as the freeze events of Jan. 5-6, 2014, because we were seeing bud kill in tender varieties soon after that. Some of the injury may have occurred as late as March 4, however, when temperatures at the AREC in Winchester dipped as low as 3° F. We noted some trunk splitting in a nearby vineyard in late April. Split trunks appeared to be more common on older, larger trunks than on younger trunks with smaller diameters; multiple-trunked vines often have a mix of live and dead trunks (an argument for multiple-trunk training systems).
As with my 2003-04 report, some of this splitting appeared to be due to the drying of trunks and canes of vines injured sometime much earlier (possibly during summer 2013) as the wood was bone dry and completely discolored. We considered that some injury might have occurred as early as Nov. 24-25, 2013, when we had our first real blast of cold air (see temperature chart on page 77). In this case the vines’ health was compromised going into the winter, and those in poorly drained sections of otherwise well-situated vineyards may have had some increased “dieback” of shoots and potentially greater winter injury incidence.
In addition to the more tender varieties mentioned above, we’ve also seen injury in Cabernet Franc and Cabernet Sauvignon as well as more hardy varieties including Riesling, Chardonnay and Pinot Noir. Injury to early budding varieties such as Chardonnay, Pinot Noir and Cabernet Franc is perhaps consistent with the notion that these vines might have deacclimated some by early March, in time to be damaged by the cold temperatures March 4.
Options for dealing with injured vines
I anticipate that we’re going to continue to observe injury and damage from the 2014 winter, including collapse of some vines that have gone through bud break and begun to develop a canopy. This can occur when some buds are alive but the regenerative vascular cambium tissue is damaged. The first real hot weather of the season puts a strain on the damaged vascular system, and the new shoots collapse when their transpirational loss of water exceeds the ability of their impaired vascular system to transport water. We also may see a resurgence of crown gall as a result of the trunk and cordon injury.
In either case, new trunks will need to be developed, and this will require retraining from shoots that originate from uninjured portions of the vine near the graft union (assuming there are live, latent buds in the graft region of the scion). With no appreciable winter injury during the past 10 or more years, many of us have ceased hilling and de-hilling of vines for cold protection. With very tender varieties, there’s a chance that severely damaged vines may need to be replanted.
Renewal trunk shoots that do develop from severely injured vines will likely be extremely vigorous because they are supported by a large, intact root system. Permit these vines to develop as many shoots as is reasonable (five or six if possible), and keep the shoots well exposed and supported upright on the old trunks and trellis system to minimize shading and permit effective disease management. The injury that I observed was not uniform across entire vineyard blocks, and you may find that uninjured vines (or uninjured trunks on multiple-trunked vines) have more shoots than can be trellised. Be prepared to do some shoot (and crop) thinning where needed in June and July.
Where injury is certain and significant, avoid nitrogen fertilization for this season and consider sowing a cereal (or allow weed growth) under the trellis to provide some competition and reduce shoot vigor. Retain moderate ( 0.5 to 0.625 inches in diameter) canes next winter to retrain trunks and cordons. You don’t need to remove the injured trunks immediately; there will be time for that after you’ve gained a fuller picture of how much injury your vines might have sustained. Trunk injury (and splitting) may also be observed during the following winter (2014-15).
The appearance of cold-injured trunks is a reminder that we are growing cold-tender grapes in a region where winter temperatures can be marginally acceptable. Drought, excess rain and defoliating diseases (we saw lots of downy mildew problems in 2013) can reduce vines’ capacity for cold acclimation and mid-winter cold hardiness, further increasing the potential for severe injury. Many growers—particularly those who have entered the industry since 2000 or so—have ignored multiple-trunking recommendations, and some have planted very cold-tender varieties. These can be calculated, acceptable risks with small plantings in excellent sites, but the hazards are real and should be carefully considered.
I’ve talked to a few seasoned growers who are using the significant winter injury from the past several months as motivation to rethink their varietal planting scheme. If, for example, you have considerable injury to a Merlot clone that is less than optimal, perhaps now is a good time to replant the block with a superior clone.
Dr. Tony K. Wolf is a professor of viticulture and director of the AHS Jr. Agricultural Research and Extension Center at Virginia Tech. He provides extension resources, teaches an online viticulture course and was principal author and editor of the Wine Grape Production Guide for Eastern North America (2008).
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