Location:Home > HEALTH > My grapevines have been frosted - What now?

My grapevines have been frosted - What now?

Time:2020-04-09 14:22wine - Red wine life health Click:

What have Grapevines been frosted

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.

This document provides information on management options for a spring frost. Techniques to prevent frost are presented in other documents elsewhere on this website or 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 ), 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.

Copyright infringement? Click Here!