Protecting Perennial Crops from Hail

Technical articles - Climate resilience

Viticulture Viticulture Irrigation in arboriculture Precision viticulture Pruning and training of the vine

Anti-hail nets in viticulture

Hailfalls can cause significant damage in vineyards and orchards. Here is a reminder of the means available to farmers to protect perennial crops.

Impacts of hail on perennial crops

Hail is a violent phenomenon, for which it is important to specify the damage and make comparisons from one event to another.

The ANELFA has published an intensity scale for hail episodes, based on measurements of nearly 4000 hailfalls recorded in France since 1988.

Hail intensity scale

The scale published by ANELFA suggests that damage caused by hail on vines and trees fruit trees begins for diameters between 1 and 2 cm.

However, the damage caused by hail is variable and the intensity of a shower depends on several parameters :

• the diameter of hailstones and their mass, which determines the fall speed,
• the duration of the shower,

• the phenological stage of the crop: a low-intensity shower can have significant consequences for crops close to harvest,
• the presence of wind during the shower can also be an aggravating factor, by precipitating hailstones at a more or less significant angle relative to the vertical and thus increasing the vegetative surface potentially affected by hail.

Hail injuries on vine

In vine, the damage can be varied :

• complete defoliation
• impacts on old wood
• potential losses for the current season's harvest,
• difficulties in pruning future shoots and yield reductions following reserve formation problems, in case of hail from July onwards
• destruction of buds

Hail damage on apple tree

In arboriculture, the damage can cause :

• fruit drop
• wounds on fruits
• breakage of bearing shoots – which can affect the orchard's production potential for future seasons – or damage to the bark.

This damage can be exacerbated by the presence of cryptogamic diseases before or after the hail episode since their proliferation is favored by wounds on fruits and trees.

Methods for detecting a hailstorm

Detecting hailstorms is a significant lever for protecting vineyards and orchards. Early detection allows the implementation of appropriate anti-hail protection systems. Various detection methods are available and can complement and refine forecasts made by Météo France.

X-band weather radar

Operation of an X-band weather radar

In addition to the Météo France surveillance network, the local installation of individual radars with a range of about 50 km, smaller and using so-called X-band radar technology, allows refining forecasts and improving the localization of potential hail risks.

This type of radar allows to be warned if a cumulonimbus likely to contain hail approaches the monitored area. The radar sensitivity is adjustable and allows generating alarms for various meteorological phenomena (from light rain to severe hailstorm).

Cost: Several companies offer the installation of this type of radar for a cost of about €30,000 to €35,000, plus a subscription to a digital interface to exploit the data collected by the radar.

Advantages	Disadvantages
Easy installation	Good accuracy only on short scales (~30km)

Meteorological alert services

Various organizations and websites offer meteorological alert services to warn and alert operators of potential storms with hail risks.

Météo France issues alert bulletins only at the French national level. To refine the accuracy of these alerts, consulting firms also offer forecast bulletins and alert sending by email or SMS :

• French Observatory of Tornadoes and Severe Storms
• ARHYA
• Permanent Observatory of Natural Disasters and Natural Risks

These paid services can be essential to trigger anti-hail protection systems by anticipating risks. Some providers, such as the company SELERYS, also offer to combine decision support tools, meteorological alert solutions, and protection devices.

These services are based on information from Doppler weather radars, very reliable for hail detection but generally not very precise regarding the location, duration, and timing of the phenomenon.

Advantages	Disadvantages
Low cost	Lack of geographic and temporal precision

Visual identification

Types of clouds

Hail generally forms in cumulonimbus clouds recognizable by :

• their shape (anvil shape)
• their thickness (at least 5000 meters)
• electrical activity (presence of many lightning strikes)
• the precipitation level (generally heavy rain)
• the color of the sky: the cloud front of a hailstorm takes on a slightly "green" hue

Cumulonimbus

However, it is risky to rely solely on visual identification to protect an agricultural operation because most of these observation criteria are not applicable at night.

Advantages	Disadvantages
Costs nothing! These methods are based solely on experience and environmental knowledge.	High probability of human error especially in absence or at night.

What protection methods?

Row orientation

When planting vines and fruit trees, it is interesting to orient the rows parallel to the prevailing winds to reduce the risk of hail damage.

Plot dispersion

Dispersing plots around the farm headquarters prevents exposing the entire production to the same hail shower. Since hail phenomena are usually very localized geographically, this technique reduces the risk of damage to all trees and vines.

Anti-hail nets

So-called anti-hail nets are deployed in vineyards and orchards to protect vegetative parts from hailstones. The fabrics can be made from different materials :

• high-density woven polypropylene, which has good mechanical tensile strength
• extruded polypropylene, less resistant
• starch in the case of biodegradable nets, rarer due to their cost and shorter lifespan

Nets are estimated to have a lifespan of about 10 years but this can vary depending on :

• their structure,
• the composition of the threads,
• the external environment (UV resistance, which may involve the use of anti-UV chemical additives during manufacturing)
• hail episodes endured.

A recycling channel for anti-hail nets has been set up by ADIVALOR to recover and even recycle used nets.

Anti-hail nets represent one of the most effective methods and are the only means recognized by insurance companies in France to guard against hail risks. However, like all other protection systems, anti-hail nets are not effective in case of very heavy hailfalls. If a "catastrophic" hailstorm occurs, the consequences can be disastrous: a collapse of the nets on the trees usually requires complete replanting of the plot. Anti-hail nets have other disadvantages:

• installation cost,
• reduction of sunlight,
• impossibility of deployment when trees are in bloom

This system is rarely installed on large agricultural operations due to its very high installation and handling costs for large areas.

Total plot coverage

Anti-hail net chapel system

These devices consist of installing nets horizontally above rows of fruit trees and vines to ensure total coverage of the plot. Suitable for all orchards, this protection system is usable in vineyards with wide row spacing, at least 3 meters, to allow opening for discharging accumulated hail between rows if the weight becomes too great.

Two systems are distinguished :

• in tent (or chapel) style: nets are attached to columns placed at row ends and connected by cables stretched above the rows. Cables are placed between rows to fix the nets and allow hail to evacuate at the inter-row level.

Anti-hail net with elastics

• with elastics (or overlapped): the net is stretched flat above the plot, connected by elastic fasteners to columns at the rows. The elastics allow the net to deform to evacuate hailstones. This system is less expensive than the chapel system.

The presence of nets above the orchard modifies the environment:

• shading effect
• increase in humidity reducing water needs (irrigation demand under white net is 10% lower and under black net 20% lower).
• the increase in wetting durations, due to reduced wind, has shown a greater sensitivity of fruits to certain diseases: scab, black rot on some varieties (Pink Lady), and certain pests (mites in particular).

These changes can affect production quality: slight loss of sugar, coloration, and fruit firmness.

This system allows manual and mechanical interventions in the vineyard and orchards, but requires a heavy infrastructure for installation, with an aesthetic impact.

Cost:

• Material investment:
  • €10,000 to €15,000/ha in orchards,
  • €20,000 to €25,000 per hectare, for 3-meter row spacing, in vineyards.
• Installing the net the first year requires on average 200 h/ha and 100 h/ha in subsequent years

	Advantages	Disadvantages
Arboriculture	No access constraints to trees Devices can be combined with anti-insect nets Reduced irrigation inputs Shading effect limiting sunburn on fruits	Shading effect impacting fruit quality High labor requirement
Viticulture	Facilitates manual and mechanical interventions within the plot.	Device not allowed in production AOC. Suitable only for vineyards with wide inter-rows. Possible impact on cryptogamic disease pressure. Visual impact.

For more information on installing anti-hail nets in orchards, watch this video:

Single-row nets

This device consists of protecting perennial crops from hail row by row.

Alt'carpo net

The use of this type of net as a hail control method has few technical references in orchards, as single-row devices are more often deployed to combat pests primarily the codling moth.

For example, the single-row Alt'Carpo net has a very good level of effectiveness in protecting against the codling moth and offers good protection against hail because the nets do not accumulate load, envelop the entire tree canopy, and cannot be lifted by the wind during a hailstorm.

A trellis can be installed at the row level to support the net above the canopy. The infrastructures required for installation are adapted to these nets and allow the use of complementary control methods at certain times of the season.

Single-row nets are very well suited to protect medium or narrow spaced vineyards:

Single-row net

• This device consists of installing, in winter, a net on each side of the row and vertically, so as to cover the vegetation and clusters over the height of the vine (from 0.5 to 1 m).

• Roll-up models can be rolled up before harvest and unrolled at the beginning of the vegetative cycle, after debudding.

These devices were authorized in 2018 by the INAO following experimentation in the AOCs of the Burgundy region. INAO recommends, to limit shading effects:

• choosing meshes that allow sufficient light transmission
• limiting net application duration to also reduce bioaggressor pressure linked to vegetation crowding

Trials conducted in the Burgundy vineyard showed a 90% reduction in damage on vines protected with nets.

Cost in viticulture:

• Installation is expensive, between €15,000 and €25,000/ha,
• Operating costs around €300/ha/year
• Additional labor for installation and subsequent green work.

Cost in arboriculture:

• Indicative cost for Alt’carpo single-row net: €6,000 to €10,000/ha for material
• Installation time of 120 h/ha the first year and 70 h/ha in subsequent years

	Advantages	Disadvantages
Arboriculture	Protection against certain pests with suitable nets. Easier to install than full-plot nets.	The net can rub and damage fruits. Less effective application of phytosanitary products. Possible development of unwanted fauna (aphids) under the net. Difficult access to trees during manual thinning interventions.
Viticulture	Easier to install than total coverage net. Directs shoot growth upwards, reducing trellising time Authorized by INAO Protects clusters from sun affecting sugar, tannin, and anthocyanin concentration.	Limits mechanization of the vineyard requiring systematic prior lifting before interventions other than phytosanitary treatments (leaf removal, green harvest). Labor time to unfold and roll up the net. Visual impact. Possible effect on grape coloration. Pruning method to be adapted

A more rudimentary and economical installation (€3,000/ha) can also consist of simply placing the net on the trees, which is more suitable for large trees that can support hail accumulation in the net on upper branches.

Climate disturbance

Cloud seeding

Cloud seeding aims to multiply hail embryos to prevent them from reaching a size too large and thus acquiring too much inertia during their fall.

Silver iodide generators

Silver iodide is used because its structure is identical to that of ice, which is why ice crystals can form on silver iodide particles; it therefore serves as an artificial nucleus.

This hail control device consists of artificially introducing silver iodide ice nuclei into clouds, in order to increase the number of ice crystals and reduce the size of hailstones.

These then fall more slowly and melt completely or partially before reaching the ground.

Silver iodide generator

This system has been proposed by ANELFA for more than 60 years in France:

• It is based on a network of vortex generators that allow the diffusion of silver iodide from the ground.
• The network covers about 70,000 km² in metropolitan France.

ANELFA carries out an annual hail prevention campaign that runs from April to October.

It recommends starting the generators 3 to 4 hours before a probable hail event. Each generator is started by volunteers, who receive alerts by SMS about 4 hours before the hail risk, based on forecasts from Météo France, so that the clouds become charged with silver iodide.

In terms of effectiveness, the device allows to halve the intensity of hail.

Cost:

• The installation of the chimney costs about 2,000 euros and 1,000 euros of products each year.
• Various winemakers' associations have been created to set up generators and manage their maintenance. This has allowed to reduce costs to between 5 and 8 €/ha and to cover larger areas.

Advantages	Disadvantages
Well-established network operation Presence of an alert system Easy handling of generators Possible automation of generator ignition via smartphone.	Need to anticipate the storm risk in advance, making the success of the device dependent on forecast reliability. Need to operate several generators, as isolated action has no effect.

Diffusion of hygroscopic salts by balloon

Probe balloon firing system

One of the latest inventions against hail is an inflatable balloon that transports and diffuses hygroscopic salts (calcium and sodium chloride) into a storm cell. This system is coupled with a storm cell detection radar and a hail risk assessment software covering a radius of 30 km.

The balloons, inflated with helium, carry torches loaded with hygroscopic salts. Once the determined altitude is reached, a system triggers the ignition of the torch. The salts are released and act as condensation nuclei promoting droplet formation.

Cost:

Investment in:

• a inflator for about €1200,
• three to six balloons per storm at €350 each
• an 800 €/year subscription to the detection service.

Advantages	Disadvantages
Possible coupling with an alert system (paid) to trigger balloon releases.	Little feedback on the radar + balloon device (recent commercialization). Need to intervene before hail formation and to act collectively. Falling debris on the ground.

Anti-hail cannon

The anti-hail cannon is a shock wave generator. It consists of an explosion chamber and a conical diffuser, the whole measuring 6 meters high. The principle of the anti-hail cannon is to prevent hail embryos from growing by means of shock waves directed towards the cloud that prevent hailstones from agglomerating.

Since the anti-hail cannon is not or little effective on already formed hailstones, it is important to start it early enough to prevent hailstone formation. Manufacturers recommend starting it between 5 and 20 minutes before the first hailstones fall. It is therefore advised to use this protection system with a detection solution (weather radar and/or subscription to a weather alert service).

To limit noise emissions and reduce possible disturbance to neighbors, it is recommended to install the cannon in a concrete building, a container, or a "silencer".

Cost:

• Investment of €30,000 to €40,000 for the installation of a cannon
• Operating cost of €1,000 to €1,300/year per cannon

Advantages	Disadvantages
Device can be used individually or collectively Possible automation of firing by coupling with a storm detection system	Noise nuisance for residents due to repeated explosions (up to 130 dB at the foot of the cannon)

Appendices

S'applique aux cultures suivantes

• Vine
• Apricot
• Almond
• Cherry
• Quince
• Fig
• Pear
• Apple
• Plum
• Peach