The current cold and frosty conditions, along with the predicted El Niño spring event (predicting dryer conditions and possibly increased chances of frost events) warrant a discussion on frost management.
This week’s column takes points from GWRDC fact sheets Arming against frost, September 2010 and Cover crops and vineyard floor temperature, May 2012.
Technically a frost occurs when the temperature at ground level falls below 0°C. Most temperate plant species, including vines, tolerate such temperatures, even though surface ice may form. At approximately -2°C, water from within the cells begins to move out as the cell walls rupture, resulting in desiccation and destruction of all or part of the exposed tissue. Lower temperatures over longer periods cause more severe damage.
Methods to control frost aim to reduce damage occurring by introducing heat to the sensitive cordon height of the vine.
Control options include:
Adding heat via irrigation
Using irrigation to cover new foliage with a layer of water or ice provides insulation to protect vines from even colder air temperatures that may freeze and destroy cells. This option requires a spray irrigation system that can apply enough water to the vines foliage to make a difference.
Circulation of air within the vineyard mixes warmer air from an inversion layer into the colder air nearer to the ground.
Maximising heat from the natural day/night cycle
Maximising heat gain during the day to help the vineyard stay warmer overnight is probably the most achievable option for most growers. Keeping vineyard soils moist, compacted and weed free will maximise heat gain during the day. Recent research has shown bare midrows provide the warmest temperature at cordon height in preference to ‘tilled/compacted’ or ‘mown sward’ midrows. However, bare midrows come at a cost. These are generally the hottest vineyards in summer, so careful planning is required to optimise controls throughout the year.