The effectiveness of phytosanitary treatments depends strongly on the weather conditions during application. Air humidity, temperature, wind speed, and precipitation are important data points that influence the efficiency of a treatment.

Other meteorological parameters such as dew point, wind direction, and gust speed should also be taken into account in order to optimize treatment.

The active ingredient is transported into the ground and surface water through run-off and infiltration. In addition, the outer layer of the plant is covered with a film of water that prevents the product from penetrating.

It is important to keep an eye on the weather forecasts for rain measurements. The plant protection products with contact effects must remain on the plant surface for a longer period of time. Rain can remove this layer of product, causing pollution and reducing the effectiveness of the treatment. Depending on the product, the amount of rain that causes washout varies, but the limit of 20 mm of rain should generally not be exceeded (Davy, Institut Français de la Vigne et du Vin, 2016).

The optimum temperature for treatment depends on the type of pesticide used. Moreover, each product has its own ideal area of application. In order to optimize the treatment, it is important to follow the instructions for the use of the product carefully.

The receptivity of organisms also correlates with temperature. Extreme values slow down the metabolism. High thermal amplitudes can also have an inhibitory effect, especially for the post-emergence root products and leaf herbicides used (Verdier et al., 2012). Amplitudes between day and night of more than 15°C slow down the metabolism of the plants. The active ingredient accumulates in the organism and cannot be extracted, which may increase the risk of phytoxicity (Burgundy Chamber of Agriculture, 2012).

Thus, the temperature has a decisive influence on the quality of the spraying (Chamber of Agriculture, Burgundy, 2012). Hot (and dry) air causes higher evaporation at the sprayer outlet (>25°C) In general, spraying is recommended for a temperature range between 5 and 25°C. However, these recommendations depend on the plant protection product used.

Too low a humidity intensifies evaporation, as some of the product's droplets are too light and evaporate as soon as they leave the atomizer. Too high a humidity level (close to 100%) is also harmful, as the active ingredient of the product appears on the surface in the form of dew and seeps into the environment.

Morning and evening are the best times for spraying, as the humidity level is then usually in a favorable range. The morning hours are particularly suitable when the leaf canopy is still cool from the night and the cuticle of the plants is rehydrated and more receptive to the product.

The aim is to limit, as far as possible, the phenomenon of drift, which has significant environmental consequences. The effectiveness of sputtering is dependent on wind speed because the objective is to ensure that the maximum amount of active ingredient reaches its target. Improved treatment efficiency reduces the number of passes and, thus, the associated input and mechanization costs.

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Excerpt from Farm Advisory Service, Information Note: Pesticide Spray drift 👆

The lowest possible wind speed is most favorable. Above a certain value, there is a high risk of wind drift, and then caution is required. Certain devices, such as the shape of the nozzle and optional equipment on the sprayer, make it possible to limit drift despite slightly higher wind speeds. The use of specific aids also reduces the risk of drift.

The indicator for gusts of wind is an additional parameter to be considered. "Gusts of wind do not allow clear predictions of wind direction and can mean temperature swings" (Deveau, 2009). If the gust speed is high, it is therefore best to postpone spraying until another time.

This indicator also makes it possible to evaluate the adhesion of the product to the plant. If the dew is too strong, the product will run off during spraying, and the treatment will not be effective. The farmer can observe the conditions by tapping on a leaf. If the dew runs off the leaf, one should wait before spraying (Verdier et al., 2012).

In fact, herbicides in particular are 'all the more effective when used in growing weather' (Arvalis, 2014). This means several days in a row with growth-friendly weather conditions: high humidity (over 60%) and cool temperatures (between 4°C and 18°C).

In addition, spraying after severe frost is risky, as the plants may show injuries on the epidermis. There is therefore a high risk of phytotoxicity. Treatment with a systemic foliar herbicide after a drought is also not very effective because the plant cuticula is thickened. It is therefore important to observe the meteorological parameters before and after treatment in order to assess the effectiveness of the treatment.

It is not easy to find a time window for the treatment in which all parameters are favorable for spraying. A compromise has to be found that can be reconciled with the timetable. The weather conditions are best in the morning when humidity is high and the plants are more receptive.

Treatment in the evening is also possible, but one should take into account the weather conditions of the previous day, as the plant may be more sensitive to the product in the evening.