Cold hardiness is influenced by many different factors, including variety, crop load, harvest time and postharvest conditions, and vineyard weather conditions. There is a genetically determined limit to cold hardiness. However, while this is true for mid-winter hardiness, the ranking might be different at the start or end of the dormant season. Some varieties will acclimate earlier in fall and will be able to withstand colder temperatures earlier in the dormant season than varieties that have otherwise more mid-winter hardiness. Likewise, early bud-breaking varieties tend to lose their hardiness earlier in spring and might be damaged at warmer temperatures than late-breaking varieties, irrespective of their mid-winter hardiness. Also, cultural practices can have a profound influence if the genetic potential of a given variety is achieved. In very general terms, warm temperatures tend to reduce bud hardiness while cold temperatures tend to induce more hardiness (within limits). Hence, the weather conditions at a site will influence the ability of buds to withstand cold temperature, and the values presented in Table 1 are in part affected by the temperature conditions at that site.
The standard cold injury assessment process is the oxidative browning method. After freezing, buds are held at room temperature for 24 h. Following the 24-h incubation, a cross and/or longitudinal sectioning is made on each bud with a single-edged razor blade to confirm cold injury manifested by tissue browning. Buds showing vibrant green tissue are judged to be viable whereas buds showing brown tissue are judged to be dead (see photos below). The brown coloration is the result of oxidation of the phenolic compounds being released in the damaged tissues. Severe damage results in more pronounced, deeper browning of damaged or killed tissues. Less severe damage may produce slightly browned tissues. To assess cold injury following an event, canes from a vineyard should be held for a minimum of 24 hours at 70 oF (room temperature, 21 oC) before cutting to maximize tissue browning.
Photos: Sectioned grape buds showing the compound nature of the ‘latent bud’. All buds are alive in the left photo while they are dead in the right photo (P – Primary bud; S – Secondary bud; T – Tertiary bud).
An alternative technique to assess cold hardiness of plants is Differential Thermal Analysis (DTA). With differential thermal analysis, freezing episodes called exotherms can be identified as change points, local minima or selected inflection points of differential temperature (Gerard and Schucany, 1997). When super cooled water freezes extracellularly, the heat released is referred to as a high-temperature exotherm (HTE); extracellular freezing is considered nonlethal. On the other hand, the freezing of intracellular water creates a similar, low-temperature exotherm (LTE), which is lethal (Burke at al., 1976). As an example, Fig. 1 shows high and low temperature exotherms from ‘Albarino’ grapevine buds from tests in early November and early December, 2017.
Fig. 1: High (HTE) and low temperature exotherms (LTE) for ‘Albarino’ buds from grapevines growing at Colorado State University’s Western Colorado Research Center – Orchard Mesa near Grand Junction, CO, on 7 November 2017 and 5 December 2017. The larger exotherms occurring at temperatures between approximately -5 oC to -9 oC are from high temperature exotherms (HTEs), indicating non-lethal extracellular freezing of extracellular water. The low temperature exotherms (LTEs) show acclimation in bud cold hardiness from early November (LTEs from -15 to -18 oC) to early December (LTEs from -23 to -26 oC).
Dormant buds were collected from mature vines growing at the Western Colorado Research Center – Orchard Mesa site near Grand Junction, CO. Vines are planted at a vine spacing of 5′ to 6′ in rows 8′ to 10′ apart, spur pruned on bilateral cordon, and trained to either a VSP or a high cordon. Buds were taken from canes of moderate vigor that had no obvious sign of damage. Canes were cut so as to leave a 4-bud spur, and eight buds were used from each cane (i.e. bud position 5 to 12). The buds were randomly assigned to 6 to 10 sets of 3 buds per variety and used for the differential thermal analysis (DTA). The samples were placed on three trays; each tray included eleven thermoelectric modules (TEMs) that detect temperature gradients generated by the exotherms according to the methodology described by Mills et al. (2016). Buds were covered in aluminum foil and placed directly on each TEM protected by foam insulation pads. A chamber lid was tightened to the tray and then loaded into a programmable freezer (Tenney Jr Test Chamber, Model TUJR, Thermal Product Solutions). The freezer was programmed for a cooling rate of 4 oC/h decline. Initially, the temperature was held at 4 oC for 1h and then dropped to −40 oC in 11h, then returned to 4 oC in 10 h, and then a DTA analysis was performed. Thirty TEMs were loaded per run (90 buds). The system recorded for each TEM a voltage signal that corresponds to the temperature at which super cooled water presumably in the tissue freezes. The signals are sent to an output directly to an Excel spreadsheet. Exotherms were identified plotting the TEM signals (mV) against the temperature (oC) (Fig. 1). Bud exotherm output from the DTA system was previously compared with tissue browning (as described above) during the 2016/17 dormant season and found to yield similar results.
The data presented here is for information only, and growers should make their own assessment. Information on how to determine bud injury can be found here:
Cold hardiness information for a large number of varieties grown in Washington State, a region with a semi-arid climate with some similarity to the climate of Colorado, can be found at WSU’s viticulture page:
Gerard, P.D., and W.R. Schucany. 1997. Locating exotherms in differential thermal analysis with nonparametric regression. J. Agric. Biol. Environ. Stat. 2:255-268.
Burke, M.J., L.V. Gusta, H.A. Quamme. C.J. Weiser, and P.H. Li. 1976. Freezing injury in plants. Ann. Rev. Plant Physiol. 27:507-528.
Mills, L.J, J.C. Ferguson, and M. Keller. 2006. Cold-hardiness evaluation of grapevine buds and cane tissues. Am. J. Enol. Vitic. 57:194-200.