Despite higher than average precipitations in December 2021, since the beginning of January dry weather and high atmospheric pressure have set in over most of France and western Europe. Almost no precipitations have been recorded in southeastern France in January, resulting in a rain deficit of up to 50 mm for this grape producing region (see twitter)

Whereas the growing season has not started yet, this winter drought might have heavy consequences on grapevine growth, fruit set and even on the functionality of drip irrigation systems. These effects are known by growers and documented by scientific studies.

Winter drought causes delayed spring growth

According to an Australian study by CSIRO, reduced precipitations during winter  delay budburst and decrease subsequent yield by up to 40%. Excessively dry soil conditions can also cause growth disorders. For example, in central California, the 2021 winter drought had severe consequences on vineyards . Many growers reported delayed phenology, poor growth, and fruit abortion. Some severely impacted vineyards suffered a substantial yield loss.

The problem is known as « delayed spring growth » and can be caused by lack of soil moisture. It results in bud wilting, stunted growth and, in severe cases, fruit shatter and abortion. It is partly due to vascular impairment due to dry conditions. Dormant buds become dehydrated over winter and have relatively weak vascular connections to the rest of the vine. Spring sap flow (bleeding) helps repair any vascular embolisms that may have formed over the winter and rehydrates the buds. However, when the soil is excessively dry or the carbohydrate reserve too low, this process can be impaired and shoots with faulty connections do not receive adequate carbohydrate supplies at growth resumption, even if soil moisture is subsequently re-established.

Another consequence of soil dryness in winter and spring is the inhibition of fine root formation. Fine roots are non-woody, short-lived roots with very small diameters < 1 mm. They are important for resource acquisition and microbial interactions. Their growth is promoted by carbohydrate demand from the plant (in the spring, for example) and is strongly limited by the lack of soil moisture. For this reason, early season drought can lead to decreased nutrient absorbtion and cause nutrient deficiencies.

These consequences are not homogeneous in vineyards. Only some shoots are affected by unresolved vascular embolisms and dry patches in soils are often distributed hetereogeneously depending on soil variability. This results in desynchronised budburst and phenology within the same vineyard, which makes viticultural operations more complicated.


Climate change, ants, and winter drought: a terrible mix for your drip irrigation system

In addition to its adverse effects on plant growth, winter drought can be accounted for yet another problem in vineyards: the breaking and failure of subsurface drip irrigation systems.

In recent years many vineyards in southern France have reported damage and failure of irrigation systems at the onset of the growing season. This occurred only for subsurface drip systems and after drier and warmer than average winters such as 2019.  Close investigations have revelead that water emitters along the dripperline are damaged by soil-dwelling ants that search for water at the end of winter rest. When the ants cannot find water in the soil profile, they enter the irrigation system and pierce the emitter membranes to enter in the water pipes. The phenomenon has been particularly pronounced in recent years and fueled by climate change.

In most mediterranean and temperate species, ants stop foraging for food and become inactive in winter. Because their activities are greatly reduced, they can survive with their energy reserves for a few months. This slower metabolic state is called « diapause ». The increase in temperature during the early spring months cues ants to re-emerge from their chambers, prompting workers to search for new food sources and water. In the last few years, record winter temperatures have promoted earlier ant activity, inside dry soils that had not yet been irrigated. The lack of moisture has pushed ants into irrigation systems where the damages have been done.

Many different ant species live in mediterranean soils, but only a few are small enough to enter the emitters and wander through the system. In southern France, the culprits have been identified  as belonging to the genus « Solenopsis ».

Solenopsis sp., a male thief ant prepares to depart on a nighttime mating flight. Tucson, Arizona, USA. Photo by Alex Wild.


How to protect your vineyard: the importance of winter irrigation

Given the adverse effects that excessive soil dryness can have in winter, it is important to ensure that soils retain their moisture during winter rest and are ready to support a healthy growth resumption at the time of budburst.

In winter months, growers should assess soil moisture, review the weather forecast, and consider whether winter irrigation may be needed. If the soil is dry and no significant precipitation is in the forecast, then irrigation is recommended. Grapevines use little water over the dormant season, and lower temperatures decrease soil evaporation, so often only one irrigation will be needed in January or February.

Irrigation experts advise to perform one « technical irrigation » to fill up the soil profile to manage any potential potential growth disorders and prevent ants from damaging subsurface drip irrigation systems. To know exactly how much and how long to irrigate, a precise knowledge of soil composition and depth and the evaluation of its water holding capacity are very useful. These information can be provided by agronomic models and irrigation DSS like Vintel.

With drought setting in southern France and no significant precipitation forecasted for the next weeks it is the right time to think about  “watering ”  your vineyard. A strategic irrigation now can avoid many later problems and ensure a good start of the 2022 season.