Frost damage to grapevines primarily occurs in spring in September and October during the early growth phase of the vine. Frost at this time can kill the foliage right back to the cordon or partially kill the shoots and inflorescences, resulting in significant crop losses.
In a warming climate, while the number of frosts may decrease, budburst is likely to occur earlier and still be susceptible to frost damage. A warmer, drier climate also provides conditions more suitable for frost and consequently there may be an increased risk of a damaging frost.
Depending on the timing and severity of the frost, growers may wish to consider management options to improve the yield in the current and subsequent season, as well as providing good quality canes and spur positions for pruning in the following winter.
Autumn frosts prior to harvest also occur occasionally but little can be done to resurrect the vine. If the fruit has been frozen it should be harvested immediately. If only the leaves are killed the sugar concentration in the fruit will only increase slowly through dehydration of the berry. Deciding when to harvest needs to be weighed up against the possibility of another frost freezing the fruit and further reducing the quality. On balanced vines, canes should be sufficiently hardened to provide good quality canes and spurs for pruning in winter.
Research results from post frost management trials can be difficult to interpret. Dates of frosts are often mentioned but the stage of vine growth and extent of the damage are less clearly documented. This could be resolved by developing a standard frost injury index integrating grapevine phenology and the degree of damage.
Techniques to prevent frost are also presented on the Horticulture Industry Network website.
Frost damage in spring
- Frost damage may not be immediately noticeable and the symptoms appear more clearly after a period of sunny days.
- Young succulent shoots will wilt once the frost thaws but older more hardened shoots will take a few days to show symptoms.
- Frosting of inflorescences may not be immediately apparent but after several days they begin to dry out and individual flowers start to fall off, particularly when handled.
- There may be a sequence of frosts whereby regrowth from an early frost may get affected by later frosts.
Determine your aims for the remainder of the season
The way the vines are managed after a frost event will depend on what your aims are for the current and subsequent season. The strategy for maximising crop in the current year will differ from that if you wish to focus on good quality canes for pruning in winter.
- Maximising the crop in the year of the frost by taking no action after a frost often leads to lower fruitfulness in the subsequent season.
- Cane pruning requires good quality canes which are better achieved by removing all green shoots after a frost rather than letting lateral growth proliferate.
- If in a high rainfall, disease prone area, removing the dead green growth will reduce the potential for disease during the growing season.
- The anticipated benefit or disadvantage of a particular approach has to be weighed up against the cost of undertaking remedial action.
- Pruning affected shoots with secateurs by hand is more than twice as costly as rubbing off the shoots by hand.
- Most grape varieties have fruitful secondary buds which will produce 50-70% of a full crop. Less fruitful varieties such as Sultana or some varieties grown in cool climates that require cane pruning will produce much less secondary crop.
Severe frost damage
- In the case of a severe frost where all green shoots are killed back to the cordon, no remedial action is justified – just let the vines re-shoot and grow out the season.
- The regrowth will be from dormant secondary buds and the crop will ripen evenly but perhaps later than usual.
- Secondary shoot growth should be adequate to establish spurs and for cane pruning.
- If frost occurs before the shoots are at the five leaf stage (Modified Eichhorn-Lorennz system (EL) 12 – see http://door.uwex.edu/files/2010/10/ModifiedEichhornLorennzsystem.pdf), there is minimal impact on bud fruitfulness or yield in the following season – shoot growth has adequate time to produce fruitful buds during the growing season.
- If frost occurs after the shoots are at the eight leaf stage (EL 15), then reduced bud fruitfulness has been observed in the following season.
- If frost occurs after EL 15 then checking the bud fruitfulness during winter is recommended so adjustments to pruning can be made to maintain desired cropping levels.
Moderate to low frost damage
- If incomplete kill of shoots occurs before the EL 12 stage, then rubbing out the buds to force secondary buds may be considered.
- If damaged shoots are not removed, a proliferation of lateral shoots from the green shoots results. The laterals may be poorly placed for spur positions in the following season and may not provide good quality canes for cane pruning.
- Before stage EL 12, shoots can be readily broken off the spur or cane without damaging the dormant secondary buds at the base of the shoot.
- For damage at and after EL stage 15, no remedial action is suggested. Shoot removal by rubbing out after that stage can damage dormant secondary buds and current season yields are less than with taking no action.
- With the latter, the cost of shoot removal, either by rubbing out or pruning off dead material, is not recovered through any higher yield response.
Options for responding to damage
In general three main options for dealing with frost are considered in research into immediate recovery management options. It is recognised that closer attention to pest and disease control, irrigation and nutrition are required to match the changed vine growth pattern, the depletion of reserves in the vine during early growth and the increased amount of dead tissue within the canopy which is prone to some diseases.
- Once the vines reach a certain stage (EL 15) research has shown that taking no action is the best approach
- Vines yield more than those that have been cut back or shoots removed.
- It is the best option where shoots are fully killed off
- There is increased risk of disease due to the dead material retained in the canopy, particularly botrytis
- Canes and spurs available during winter pruning may not be in optimum positions but pruning times are not increased significantly
- There can be a reduction in fruitfulness of buds retained from pruning. Checking for bud fruitfulness in winter is advised and pruning level adjusted accordingly.
Removing damaged material only
- Research trials have shown this treatment produces no advantage over no action.
- Cutting the tops off the green shoots stimulates bursting of secondary buds lower down on the green shoots which are not very fruitful and produce extra lateral growth that can be an issue with pruning.
- A late ripening, secondary crop can cause issues at harvest.
- Bunch number, yield and growth responses are the same as that produced by no action.
- This approach is not justified due to the cost.
Removing all shoots back to the cordon
- This action may be considered if shoots are less than five leaves (EL 12) and there is a desire to maintain good quality pruning material for the following season.
- Either rub the shoots off using gloves or use some form of mechanised rubbing or brushing.
- Yields in the current season will be less than taking no action, but yields in the following season can be greater than the no action treatment.
- The more advanced the shoot growth is, the more potential there is to damage secondary buds at the base of the shoots being removed.
- The later the shoot removal is conducted, the greater the reduction in bud fruitfulness in the buds for the following season.
- This operation removes dead plant material which decreases the risk of diseases such as botrytis.
- Whilst overall growth is less than taking no action, canes are of better quality and better placed for winter pruning.
- Growers need to decide whether they want to focus on the yield in the season of the frost, the potential quality of wood available for winter pruning or the yield in the following season.
- If the frost damage is early in the growth period (before EL 12) then complete shoot removal is an option providing a cost effective practice to remove shoots is available.
- The shoot removal practice produces less yield in the current season but better quality wood for pruning in the winter and minimal impact on yield in the following season.
- If frost damage occurs later in the growth period (after EL 15) research has shown no advantage in removing the full shoot or in trimming back dead portions of the shoot.
- The no action practice maximises yield in the current season, but produces lower quality pruning material in winter and potentially lower bud fruitfulness in the subsequent season, which can be compensated for by leaving extra buds during pruning.