Adopting Best Practices After Spring Frost in Viticulture

Technical articles - Climate Resilience

Vine pruning and management Frost Vine resilience Precision agriculture

Frost on a bud of a young vine, the shoot on the left did not freeze (green), the future shoot on the right shows the consequences of frost (brown)

Vineyard vines are regularly confronted with episodes of frost in spring that cause significant damage to the vines, thus compromising the harvest. This type of hazard will likely become more frequent in the coming years due to climate change. While there are many techniques to prevent frost in viticulture, it is also important to know the good practices to implement after a frost period to treat the vine and ensure a small harvest.

The Vine's Resilience Capacity

First of all, it is important to recall that the vine has significant resilience capacities thanks to the energy reserves stored in its wood throughout autumn.

The amount of these reserves from photosynthesis depends on various factors :

• Exposure to light
• The amount of leaves relative to fruiting (the load)
• The persistence of foliage after harvest
• The parasitic pressure
• The quality of lignification

If these reserves provide the plant throughout the year with the energy necessary for its growth and flowering, they also ensure the resilience of the vine against various stresses it may face. Indeed, the vine's reserves offer a significant frost resistance capacity thanks to the production of antifreeze sugar but also the energy necessary to trigger defense and regeneration mechanisms needed against external aggressions (pruning, hail, frost). To allow the plant to retain all its resilience capacities, it is important to help it maintain a sufficient energy stock throughout the year.

To do this, one must be aware of sources of energy deficit :

• Reduction of a cane or topping of the vine
• Pruning impacting wood older than 2 years
• Excess load in grape relative to effective leaf area
• Soil fertility problems
• Poor sanitary condition of foliage during the season

These reserves will allow, in case of frost episodes, to improve the plant's ability to bud break again, restart growth and thus increase the flowering capacity of secondary buds. Indeed, the development of secondary buds will in many cases ensure a small harvest despite a frost episode.

Estimating Damage

After a frost episode, the vine is in a state of shock that is difficult to assess visually. It is absolutely necessary to wait at least 48 hours before knowing the real extent of the damage. On partially frozen vines, one must then wait 2 to 3 weeks before estimating frost damage by counting. Indeed, just after the frost wave, it is impossible to know how much the tissues are affected and if the leaves will be able to resume their physiological function and ensure cluster development.

Frost damage can be observed on different parts of the vine :

• on buds : a cross-section made on frozen buds shows a brown or black coloration up to the insertion point on the cane. These damages can sometimes be difficult to observe because secondary buds start immediately and can mask the phenomenon.
• on canes: a cross-section shows a brown or grayish phloem (thin layer usually green located under the bark through which elaborated sap circulates). The tips of more sensitive shoots wilt, brown, and dry out.

In practice, it is recommended to perform counting by variety and collect 10 to 20 canes with at least 10 buds each, randomly and following a U-shaped path in the plot. For cordon de Royat or Goblet pruning, check the survival of the first 3 to 5 buds of each cane from their base, while for long pruning systems like Guyot, observe a larger number of buds depending on how many you want to keep during pruning.

Cutting angle of a bud on vine wood

The diameter of the wood must also be similar to that of the branches that will be kept during pruning.

Once the wood samples are taken, they must be placed in a cool, shaded place and left to rest for 24 to 48 hours before observation. It is during this period that buds damaged by frost will oxidize and turn brown. Using a scalpel or razor blade, buds are then cut and observed with a magnifying glass or binocular microscope. The angle and depth of the cut are important to properly observe the bud.

The company Scanopy now offers to estimate frost impact on vines using drone mapping.

Based on threshold values from experience feedback, one can determine 3 classes of damage with varied consequences :

• Damage up to 40% : there will probably be compensation by other unaffected shoots (provided climatic conditions around flowering are optimal);
• Damage between 40 and 60% : the harvest will be partial, and the objective will also be to ensure pruning wood for this winter;
• Damage above 60% : there will be little or no harvest. The objective will primarily be to produce wood for the following year.

Fertilization

It is not necessary to fertilize vineyard plots immediately after a frost episode. Indeed, damage caused to vines delays the mineral needs of the plants over time. This is explained by vine physiology : buds produce auxin, the hormone stimulating growing root meristems, from 5-6 leaves. In their absence, no growth signal is sent to the root system. Therefore, one must wait for the development of latent buds and the resumption of vine growth and photosynthetic activity before considering nitrogen fertilization.

Strengthening the Vine with Plants

After a frost episode, it is possible to support the vine in its healing process using plants. For this, it is advised to :

1. Spray a mixture of :
   • Valerian (biodynamic preparation no. 507) and Arnica Mother Tincture (20 ml/ha) to destress the vine. Valerian has the ability to capture heat and return it to the plant.
   • Herbal tea of Willow to help circulation of sap flow.
   • Oregano-Thyme-Savory as essential oils to warm the plant.
   • Herbal tea of Yarrow for its overall regenerative action.
   • Rescue – Bach Flowers – 5 drops/ha.
2. For vines where foliage survived and suffered a strong shock, add nettle herbal tea to the previous mixture.
3. As soon as the vine has concretely resumed growth, it is possible to support the plant with a mixture of :
   • Nettle manure (=fermented extract) and comfrey manure for a directly assimilable nitrogen supply for the vine and strong remineralization. However, be careful not to supply too much nitrogen to vines, as it may cause berry shatter on remaining clusters and favor downy mildew.
   • Yarrow herbal tea for its overall regenerative action and willow tea to help sap flow.

Other plants may complement this base depending on weather and disease pressure. Indeed, during vine regrowth, it is important to be very attentive to disease pressure because fertilizer inputs in all forms (including plant-based preparations) can amplify the development of bioaggressors.

Adapting Shoot Thinning

For damage up to 40%, shoot thinning will be done as usual. Beyond that, shoot thinning should aim to obtain pruning wood :

• eliminate shoots to favor bud regrowth on the head;
• to form a spur, keep a shoot in the sap flow on the vine head;
• to form the fruiting arm, keep at least one shoot on one-year-old wood.

Adapting Pruning

Actions to Take Immediately After Frost

Arbre & Paysage 32 organized a crisis workshop following the severe frosts of April 2021, find all their advice in this video :

Frost Preserved the Flowers

If frost preserved the flowers, one must :

• Keep the outermost internode to replace the apex without disturbing hormonal balances
• Remove other internodes to limit vegetation overcrowding
• Support a complete nutrition of the plant
• Favor selective pruning of pruning wood if time is short

Frost Caused Damage to 100% of the Vine

If frost caused significant damage to the entire vine, it is advised to :

• Do nothing until secondary buds break (15 to 20 days)
• Then, select the best-placed regrowths to form pruning wood for the following year

Frost Did Not Impact the Entire Shoot

If frost impacted the clusters but not entire canes, it is advised to :

• Keep living canes and later select one internode to obtain an interesting cane for pruning.
• On the cane, manually remove all frozen shoots (except the last one : the farthest on the cane for Guyot) to maintain hydration. This operation must be done as soon as possible : the day after the frost episode if possible. Shoots are removed by pulling opposite to the secondary buds to avoid damaging them.
• On a spur, rely on secondary buds and crown buds.
• If the bud was in cotton wool and the secondary bud is not visible, do nothing as there is a risk of removing secondary buds.

How Does the Next Pruning Go?

After a severe spring frost episode in the vineyard, vines can have different appearances, so it is important to adopt pruning gestures adapted to field observations. In all cases, care must be taken to limit the surface of pruning wounds.

Whatever the pruning system, if vines are destroyed at the vine head, replanting will be mandatory from a sucker. Mechanical or chemical shoot thinning is therefore to be avoided to preserve wood for selection. Depending on the situation and extent of damage, pruning times can be heavy and should be entrusted to experienced pruners. The harvest the year following frost will be particularly impacted in plots under reformation.

Not all vines react the same way after a frost episode; one can observe, depending on the case and vine care just after the frost episode:

• Regrowth on crown buds or suckers on old wood
• On vigorous vines heavily frosted, few shoots developed with an abnormally large diameter which will be difficult to bend to tie
• On some vines, frost may have affected only the main bud, shoots will then vegetate and remain thin : it will be difficult to select a cane, especially for Guyot;
• On short pruning (cordon de Royat and mechanical pruning) parts of arms may be completely bare leaving neither spur nor sucker for pruning;
• In the most extreme cases, no regrowth occurred.
• Sometimes, splitting of old wood (especially on cordon de Royat) or trunk (in young vines) is observed.

Guyot Pruning

When shoots have grown and are of sufficient diameter, try to choose a crown bud (more fruitful), to give rise to a future cane and a spur below. If no other choice, keep a low spur. In this case, in N+1, the vine head must be re-staged at a maximum of 15 cm below the wire. If the cane is too thick to bend, prune it in “pisse vin” style with 4 to 6 buds. If the wood is too fragile, prune only to spur and review spring fertilization. If a good sucker has developed, one may choose to reform the vine on it.

Guyot pruning on a frost-damaged vine

Goblet Pruning

If possible, prefer crown buds, otherwise prune on possible suckers.

Cordon de Royat Pruning

If suckers have forced, select them to reform spurs but this case is rare on old vines. There may be a sucker forced directly above the vine base with which it will be possible to reform an arm. As much as possible, try to protect the pruning wound of the cut cordon. Leave enough distance so that the drying cone does not compromise the development of the new arm. In the extreme case of no regrowth or split cordon, wait until spring for possible replanting from a sucker.

Cordon de Royat pruning on vine after frost.

Mechanical Pruning in Formation Year

In case of defoliation, reform the cordon starting from the vine base if there is a sufficiently long shoot, otherwise prune to spur below the wire.

Mechanical pruning after frost period

Mechanical Pruning Already Formed

There is no other solution than to continue mechanical pruning. Possibly, on young vines, a shoot in the middle of the arm can be wrapped before machine passage to replace the frozen cordon. Otherwise, reform some arms from available vigorous shoots.

Pruning of Young Vines

• First leaf : Replace plants without any regrowth.
• Second leaf : If there are weak regrowths, choose the most vigorous and prune to 2 buds, otherwise replace the plant.
• Third leaf : Check that the trunk shows no lesions. If cracks are observed, cut the shoot below the necrosis and start again on a new shoot. In the extreme case where frost caused damage up to the graft point, the two possible solutions are either cleft grafting on the rootstock or plant replacement.

Phytosanitary Protection

From a phytosanitary protection point of view, plots frozen up to 60% should be protected normally. For more affected plots, wait for vegetation to restart and protect the vine from the 7-8 leaf unfolded stage to preserve wood and limit inoculum formation of downy mildew and powdery mildew.

Further Reading

• Frost in viticulture: What preventive methods to limit risks and what control means? CA PACA.

• What is the contribution of phytotherapy against spring frost? Vichard, J. (2023) - Le Pacte Végétal.

Appendices

Est complémentaire des leviers

• Protecting vines from frost

S'applique aux cultures suivantes

• Vine