Image 1: Vineyard with hail damage
By: Kirk Williams, Lecturer-Texas Tech University
For many grape growers that are East of the Rocky Mountains, hailstorms can occur throughout the growing season from late Spring through the Fall. Hailstorms can reduce shoot growth, reduce yield and occasionally damage the woody parts of the grapevine. The severity of these hail events can be mild to severe. Multiple hailstorms are possible in some years at hail prone vineyard sites. An example of an extremely severe early season hail event is seen in Image 1. Late season hail events can cause fruit damage, increase disease issues and cause leaf area loss which can impact fruit ripening. Leaf area loss can decrease carbohydrate production and storage which can reduce bud hardiness in the dormant season as well as reduce growth and yield the following growing season. An example of a late season hail event can be seen in Image 2:
Image 2: Late Season Hail Event
A strategy that is being adopted for reducing the impact of hailstorms on grapevines is the use of hail netting. Hail Netting is a high-density polyethylene woven fabric that is installed on both sides of the grapevine canopy. The hail netting is flexible and can absorb energy from falling hailstones to prevent damage to the canopy and fruit. The mesh spacing on hail netting is much tighter than on bird netting and is around 4mm by 6 mm (0.16 inches X 0.24 inches). A common width of the hail netting is around 40 inches although other widths are available. Most hail netting has reinforced edges on the top and bottom of the netting.
The hail netting is usually attached to the top catch wire using clips and then the netting is attached below the canopy, sometimes to the drip line or sometimes to an additional wire that is added for that purpose. Hail netting use is limited to vertically shoot positioned trellis systems with catch wires high enough to spread the hail netting vertically. Varieties that have a mostly upright growth habit are preferred but a wide range of varieties are being grown under hail netting. Hail netting does promote upright shoot growth as the shoots are held in place vertically and compressed by the hail nets. See Image 3 for an example of hail netting and its installation. Hail netting is installed as a permanent part of the trellis system and is usually lowered soon after final pruning is completed to prevent damage from early season hailstorms. Hail netting is usually installed after a vineyard has started producing fruit but when installed soon after planting, hail netting can prevent hail damage to the developing woody portions of the grapevine which can prolong the productive life of the grapevine.
Image 3: Hail Netting Installed
While hail netting does prevent damage from hailstorms it does require extra steps to complete work in the vineyard. To do work in the canopy, the hail netting must be raised up prior to the work and then lowered after the task is complete. Having dedicated lower wires that the hail netting is attached to and are loose enough to move up to the cross arms can speed up raising and lowering of the nets. Hailstorms can quickly develop especially in the late Spring and early Summer when canopy management practices are taking place so care should be exercised to raise the hail netting on just a few rows at a time. At harvest time, the hail netting must be raised and rolled up to be out of the way of harvesting activities whether harvesting is done mechanically or by hand. The hail netting will usually stay raised and rolled up until final pruning is completed.
Installing hail netting requires a substantial investment in vineyard infrastructure. The hail netting and clips will cost around $1,600 to $2,000 per acre depending on the density of the vineyard. Installation of the hail netting is estimated to take eight to ten hours of labor per acre. While the cost of the hail netting and installation is expensive, the saving of a crop from a severe hailstorm or reducing crop loss from several small hailstorms will recover the cost of the hail netting. The hail netting has an estimated life of eight to ten years when properly cared for.
In addition to prevention of damage from hail, hail netting can reduce bird damage. If bird pressure is high, birds can still get into the hail netted grapevine canopy from either the bottom or top where there are narrow openings. Tying the bottoms of the two panels of hail netting together can prevent most of the birds from getting into the grapevine canopy. Under heavy bird pressure, the tops of the panels of hail netting may also need to be tied together.
Hail netting reduces hail damage but can the closely woven fabric impact the grapevine canopy? In a study that took place on the Texas High Plains, two varieties of grapevines were studied with and without hail netting and it was found that there were differences. Grapevines grown using hail netting had a change in the vine canopy’s microclimate. The hail netting was found to make the canopy cooler, reduced air and leaf temperatures and altered the moisture level in the canopy. This change was likely due to reduced canopy airflow in the netted vines. These canopy microclimate changes could cause increased disease pressure for grapevines grown under hail netting.
The change in temperature and light due to the netting affects the vine’s leaf gas exchange, a key process for the plant’s health and growth. Differences were also noticed between grape varieties in the study (Malbec and Pinot Gris). For instance, the rate at which the fruit matured was influenced both by the netting and the type of grape.
The study showed vines without netting had more clusters. The ‘Malbec’ variety, in particular, showed lower yields under netting. Additionally, netted vines, especially ‘Pinot Gris’, showed less vegetative growth.
Installing hail netting can prevent crop loss, preserve grape quality and improve grapevine longevity in areas that receive hailstorms during the growing season. While hail-netting is beneficial for protection against hail, it is essential to understand its broader effects on grape growth, disease pressure and yield and adjust your vineyard practices accordingly.
References: Ruland KT, Montague T, Helwi P (2023) Impact of hail-netting on Vitis vinifera L. canopy microclimate, leaf gas exchange, fruit quality, and yield in a semi-arid environment. Viticulture Data Journal 555: e108805.
Kirk Williams is a lecturer in Viticulture at Texas Tech University and teaches the Texas Tech Viticulture Certificate program. He is also a commercial grape grower on the Texas High Plains. He can be contacted at email@example.com