From: Tony Wolf, Viticulture Extension Specialist
Situation: Frost/freeze potential, Tuesday morning
No sense dwelling on the obvious; we have seen an unusually early warming and bud-break this March. For historical contrast, some will recall the Virginia Vineyards Association’s annual technical meeting in Charlottesville the weekend of 12-13 March 1993: temperatures in the twenties and more than 2 feet of snow that persisted well into April (“the blizzard of the century” to be followed by another blizzard of the century less than 3 years later). Chardonnay here at Winchester are at full bud swell to 3-leaf shoot development, Merlot and Petit Manseng more closed buds than open, but other early budding varieties such as Cabernet franc are out as well. Cabernet Sauvignon is hardly showing any bud swell. The low-pressure system of the weekend has passed and a high-pressure, cooler air mass has moved into the region. The NOAA report for the next 48 hours is showing the upper Shenandoah Valley under a freeze warning, and much of the northern Piedmont under a freeze watch for early Tuesday morning (http://www.erh.noaa.gov/er/lwx/). Areas further to the east, closer to the Bay, and further to the south show warmer temperatures Tuesday morning. The NOAA short-term reports, in my experience have been pretty reliable (as have the long-term, but that’s another story). Winchester lows are projected for around 35F on Tuesday morning (27 March). Winds will be light, under clear skies: perfect conditions for radiational cooling. Thus, low-lying areas will be at greatest risk of experiencing sub-freezing temperatures. Temperatures warm again on Tuesday, and Wednesday morning temperatures are forecast to be about 10 degrees warmer, with continued warm (but seasonal) temperatures for the remainder of the week. NOAA expects warmer than average temperatures for the next 30 days, but cautions that frost events can still occur (we have at least the month of April to get through).
What are the risks of frost damage to grapevines? Aside from the obvious site considerations, the extent of bud/shoot development will affect the potential for frost development. Swollen, unopened buds can withstand temperatures into the low twenties. Even opened buds with a leaf or two exposed can occasionally withstand temperatures as low as 28 or 29F under some conditions. With some variance due to wind speed, cloud cover, and the relative dryness of the air, the temperatures (degrees F) that will damage grape buds and shoots are: dormant bud (<20F); dormant swollen (26F); burst bud (28F); one leaf unfolded (28 – 29F); and two or more leaves unfolded (29 – 32F). The forecast dewpoint temperatures for Tuesday morning are in the high teens, surprisingly low, meaning relatively dry air. This means that temperatures can dip quite low before condensation begins to form on tissues, and the drop in temperature will be fairly rapid. Those who are monitoring air temperatures in advance of flying helicopters or activating wind machines should be aware of the low forecast dewpoint temperatures, and will probably start active protection at actual temperatures of 35 to 37F, rather than waiting until 31 or 32F.
Other than site selection, what frost control measures can be used? Most effective approaches are active energy inputs; heaters, helicopters and wind machines during air temperature inversions, and overhead sprinklers. With the possible exception of helicopters, unless you have this infrastructure in place, it won’t help you in the next 24 hours. A detailed description of these and many other measures can be found in Chapter 11 of the NC Wine Grape Growers Guide (http://cals.ncsu.edu/hort_sci/extension/documents/wine_grape/winegrapesada11.pdf).
But many of these systems are expensive and I want to know what I can spray to avoid frost now… If only it were that simple and effective… Products advertised to provide frost protection to sensitive plants fall into several categories: those that affect the freezing point of the tissue or the condensate on the tissue surface; those that reduce the population of ice-nucleating bacteria; those that retard the development of the plant (i.e., delay bud break); or perhaps reduce the point at which condensation of atmospheric moisture occurs on sensitive tissues. If you search the literature, you can find examples where all of these methods have been reported to confer some frost protection under some conditions some of the time. And you can find negative results. Publishing negative results from scientific inquiry is difficult, not because the results contravene accepted doctrine, but because journals are often more interested in advancing knowledge, not simply confirming existing knowledge. That said, yes, oils and other materials that inhibit tissue respiration have been shown to delay bud break to some extent – mid-winter application was superior to a week-before anticipated bud break application. Concentration was critical to avoid killing buds. For these and other reasons, including cost of application, we have not actively pursued this course. Biocides such as copper (which in several forms is registered as a pesticide for use on grapevines), can reduce the population of ice-nucleating bacteria on plant surfaces. At least one commercial product was available (and might still be) that contained a cocktail of bacteria that were antagonists to the Pseudomonas syringae ice+ strain that normally populates tissue surfaces and is considered an ubiquitous biological ice nucleator. Under some conditions, application of these biocides or competitive bacterial strains has led to a small but significant depression in the freezing point of condensate on the tissues (this is unlikely to be a factor with the low dewpoints forecast for Tuesday morning though). Freezing point depressants (salts, sugars, polyols such as sorbitol) are another potential avenue and are of interest because certain cold-adapted (and heat-adapted) plants may show elevated levels of some of these materials in their cells, especially when stressed. But spraying the same compounds on the plant does not necessarily mean that the plants will absorb the materials or that they will confer the same freezing point depression in non-adapted plants. On the other hand, osmotically-active sugars and salts, the latter including some foliar fertilizers, could slightly depress the freezing point of condensate on the tissues. This assumes that the freezing of condensate (“dew”) on tissues quickly causes an ice nucleation (and freezing) of the subtending grape leaf/shoot tissue (a valid assumption with grapes). However, with dewpoints forecast in the high teens tomorrow morning, I would predict tissue injury (mid-twenties) before dew begins to form on the leaf surfaces (high teens). But as of now, the actual temperatures are not forecast to drop that low. I will be watching the forecast weather conditions though….
I do understand the temptation to try any means of frost protection, no matter how faint the potential for success. The sprayable products do not have a very high success rate, but other than their cost of application, probably do not cause harm. If you choose to use something, be sure it’s a legal application (especially if using copper, which is a pesticide) and try to leave a row or two untreated as a check.
Good luck, all.
Dr. Tony K. WolfProfessor of Viticulture Director, AHS Jr. Agricultural Research and Extension Center 595 Laurel Grove Rd. Winchester VA 22602 USA Ph: 540-869-2560 x18 Fax: 540-869-0862 E-mail: firstname.lastname@example.org Web: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/index.html