This spring has been cool, with temperatures in April and May below average. Although bud break hasn’t happened yet in most areas, growers are reporting that early varieties such as Baco noir are starting to show green tissue in some areas (Lake Erie Northshore).
The weather forecast this weekend is calling for cold temperatures with a risk of frost in many areas of the province on Friday and Saturday as a polar vortex moves into the region, so it is a good time to review the principles behind spring frost and freeze damage in grapes, and mitigating frost injury in vineyards.
What is the difference between a frost and a freeze?
Researchers define a frost as “the occurrence of an air temperature of 0 °C or lower, measured at a height of between 1.25 (49.2 in) and 2.0 m (78.7 in) above soil level, inside an appropriate weather shelter”, while freeze “occurs when water within the plant freezes” (Food and Agriculture Organization of the United Nations, 2005). So a frost becomes a freeze event if ice forms within the plant tissues
What causes the damage to vines during a frost?
The ice formation inside the plant is what damages the vine, as opposed to the low temperatures. Ice crystals can form in the spaces between cells (inter-cellular) or within the cells (intra-cellular) causing damage to the vine or cell death. Researchers believe that during spring frosts, a water vapor deficit is created between the interior and exterior of the cell wall. This results in the production of ice crystals in the inter-cellular spaces. As the ice continues to develop, the cells lose their turgor pressure. Freezing can damage the structure of cell membranes and other cellular components resulting in wilting and/or discolouration of tissue.
Does the water inside the vine freeze during a frost event?
Plants have developed avoidance strategies to prevent ice formation in the tissues, for example, by supercooling, and tolerance strategies (e.g., solute content of the cells) to survive inter-cellular ice formation without irreversible damage of the plant tissue. As a result, water within the plant doesn’t always freeze during a frost event.
What impacts the susceptibility of the vine to frost?
There are many different factors that affect the impact of cold temperatures on the vine. Frost damage will not occur unless green tissue is present. Green tissue has a high water content, making the vines more susceptible to the formation of ice at freezing temperatures. Different cultivars vary in the rate at which they acclimate and de-acclimate. Cold-hardy cultivars (e.g., Baco Noir) may acclimate and de-acclimate faster than less cold-hardy cultivars (e.g., Cabernet Sauvignon), which can make them more vulnerable to early spring frosts. Air temperature of –2 to –3°C can permanently damage green tissues. Early spring growth is particularly susceptible to freeze injury. However, vines remain vulnerable to cold temperatures during most of the growing season. There is some variation among cultivars, but the LT50 (lethal temperature of 50% of subjects) of Pinot noir was -3.3º C for green swollen buds, -2.2º C at bud burst, (-1.66º C at two-leaf stage, and -1.1º C at 4-leaf stage (Sugar et al., 2003). Fruit bud susceptibility will vary within a single vine, and between vines based on their growth stages. Each grape vine bud contains primary, secondary and tertiary fruit buds. If the primary shoot dies due to frost, usually a shoot will develop from the secondary bud. Most cultivars do not have good fruit production from secondary shoots, although some grape cultivars, especially French hybrids, may produce 50% of a normal crop or more from secondary buds and non-count basal buds (Hellman, 2019). If primary and secondary shoots are both killed due to frost tertiary buds may survive and keep the vine alive.
Dormant grapevines are able to tolerate freezing temperatures as the cells within the dormant buds are tolerant of lower temperatures through dehydration and accumulation of cryoprotectants. Dew point, surface temperature, and pre-frost environmental conditions also impact the extent of injury caused by cold temperatures.
Symptoms of Frost Damage
Frost damage may not be immediately noticeable and the symptoms may appear after a few days. Young succulent shoots may begin to wilt and brown once the frost thaws, but older more hardened shoots will take a few days to show symptoms.
Types of frost
There are two types of frosts. Radiations frosts occur during calm clear nights, and have a temperature inversion with warm air sitting on top of cooler air. Advective frosts occur when there are moderate to high winds, and there is not an inversion (the temperature is consistent regardless of the height).
Methods of avoiding freeze injury
There are several strategies that may help avoid frost damage, or lessen the injury. None of these have proven to be completely effective, and all have failed to protect from frost in some situations. Each should be assessed for its suitability to your individual site and business. In some cases, it would be wise to implement several strategies to protect your crop from more of the risk.
Frost mitigation strategies to consider:
- Production insurance (PI):Production insurance can be purchased well before bloom, and can give you peace of mind that at least some of your input costs will be covered.
- Selecting sites less susceptible to frost: Avoiding low-lying areas, analyzing the effect of buildings and windbreaks, and seeking sites with good air drainage or located near large bodies of water can help avoid frost. These are not available to all growers, but should be considered when choosing your vineyard site.
- Thinning hedgerows or clearing forested areas: This may reduce the area of a frost pocket, or promote better air drainage. Generally thinning/clearing needs to be done in advance of the season. Prior to clearing forested areas or cutting trees in naturalized areas, landowners should first contact their local Municipal office to determine whether a permit to cut trees is required and to learn the requirements of local tree-cutting bylaws.
- Frost fans/wind machines: Tower fans/wind machines raise temperature of the blossoms by drawing down warm air from an inversion, and mixing it with cold air at ground level. Wind machines can only prevent frost if and when an inversion is present and there is enough temperature difference to raise temperatures above freezing. Unfortunately meteorologists are not predicting that there will be a temperature inversion with the polar vortex that is bringing Ontario cold temperatures this weekend. When an inversion does occur, wind machines can raise the temperature at ground level by about 1-3 °C when warm air is trapped high above the orchard. Frost fans are installed in advance of the expected frost, and only protect about 15 acres per fan. They are expensive to install and require a fuel source (propane, diesel, natural gas, gasoline) that must be replenish frequently (possibly daily) when in use. However given the proper weather conditions, wind machines can raise the temperatures enough to save the crop or significantly reduce the amount of frost damage. They cannot be operated in a wind (even a breeze). Neighbours do not like the oscillating noise, especially on clear, cold mornings, and the motors may be difficult to start in the cold. There are smaller, tractor-PTO mounted versions for much smaller areas, but these require continuous driving while temperatures remain below freezing. Please refer to OMAFRA Factsheet “Wind machines for minimizing injury to Horticultural Crops” at http://www.omafra.gov.on.ca/english/engineer/facts/10-045.htm
- Cold air drains: These machines try to avoid frost by blowing cold air upwards, to keep air moving around blooms and prevent crystal formation. They require a barrier to direct cold air towards them (which may in itself create a frost pocket). Generally, at most they can only provide a few degrees of protection on about 10 acres. The current models are tractor PTO- or engine-driven, and can be moved between orchards. They are less expensive than tower fans. Again, they work best when an inversion is present.
- Frost protection with sprinklers: Applying low rates of water will cause ice formation around the blooms, because water releases small amounts of heat as it freezes, enough to protect the blooms. This technique has proven effectiveness in strawberries for many years. There is a cost for a solid-set irrigation system, including low-flow frost sprinklers. A large source of water is needed, because once started, the irrigation must continue until the sun comes up and melts the ice. If you run out of water, the vines will freeze. The weight of the ice build-up on vines may break some branches. This method of frost protection isn’t commonly used in Ontario vineyards.
- Burning hay or smudge pots: This technique covers the vineyard in smoke, which may prevent frost formation at ground level. It is less expensive than frost fans, but requires a source of hay or fuel, and can be implemented the day of anticipated frost. This is not the most effective method of frost protection, and at most, it may offer only a few degrees of protection. Local burning regulations must be followed, and the smoke may irritate neighbours, workers and cause environmental issues and traffic concerns. This requires labour to ensure the fires burn as intended and do not escape.
- Heaters: These could be placed through the vineyard, but generally only a small amount of heat can be produced, and it would only protect a small area. In some situations, a heater may actually draw cold air down as air currents are affected.
- Delaying budbreak with oils: Researchers in the US have been looking at the use of dormant vegetable-based oils to slow bud de-acclimation and delay grapevine budbreak anywhere from 2 to 20 days. These products are registered in Canada, however they would not be effective at this time of year as buds are already swelling.
- Products with frost protection claims: Scientists have tested numerous products over the years to protect against frost, including foliar zinc, copper. Currently there is minimal research available to support the efficacy of these products.
It is important to develop a frost protection plan for vineyards. The best frost protection plans include preventative strategies (site selection, cultivar choice), and mitigation strategies (wind machines, etc.). Monitoring maximum and minimum temperatures near your vineyard is important in making decisions about the crop. Currently public weather information can be accessed from the Environment Canada website.
Centuri, M. 2016, Understanding and Preventing Spring Frost/Freeze Damage – Spring 2016 Updates (Blog), PSU.
Fraser, H., K. Slingerland, K. Ker, H. Fisher, R. Brewster. Reducing cold injury to grapes through the use of wind machines Factsheet.
Food and Agriculture Organization of the United Nations. 2005. Frost protection: Fundamentals, practice and economics. Vol. 1.
Fraser, H. 2019. Wind machines for minimizing injury to Horticultural Crops Factsheet
Sugar, D., R. Gold, P. Lombard, and A. Gardea. 2003. Strategies for frost protection. In: E.W. Hellman (Ed.) Oregon Viticulture. Oregon State University Press. Corvallis, Oregon.