Flower Set in Grapevines with Biostimulants: Technical Guide 2026

Importance of fruit set in grapevines

Fruit set in grapevines is one of the most critical phases of the productive cycle, as the number of berries per cluster and, ultimately, the final vineyard yield directly depend on it. During this stage, which occurs approximately 2 to 4 weeks after flowering, fertilized ovules develop into young fruits. Poor fruit set causes the well-known phenomenon of shatter, where a significant proportion of flowers fail to develop into fruit, drastically reducing the harvest. According to the FAO, losses due to poor fruit set can exceed 30% in sensitive varieties such as Tempranillo or Sauvignon Blanc. Therefore, optimizing this phase through the use of biostimulants has become a key strategy for European grape growers seeking to maximize production within the framework of sustainable agriculture and EU Regulation 2018/848.

Biostimulants act on physiological processes such as cell division, hormonal metabolism, and nutrient translocation, promoting more uniform fruit set and reducing the incidence of abiotic stress. In organic vineyards, where the use of synthetic growth regulators is restricted, biostimulants offer an effective alternative compatible with organic certifications. Furthermore, their application at precise phenological stages allows correcting nutritional imbalances and improving grape quality, increasing sugar and phenolic compound content.

Factors affecting fruit set

Abiotic stress and nutrition

Grapevine fruit set is extremely sensitive to adverse environmental conditions. Temperatures below 12 °C or above 35 °C during flowering affect pollen viability and stigma receptivity, reducing the fertilization rate. Likewise, moderate or severe water stress during this phase causes abscission of flowers and young fruits. The availability of nutrients such as boron, zinc, and calcium is critical: boron is involved in pollen tube germination, zinc regulates the auxins necessary for ovary development, and calcium stabilizes cell walls. A study from the University of La Rioja showed that foliar application of boron and zinc at pre-flowering increased fruit set by 18% in wine grape varieties.

Hormonal imbalances

Phytohormones such as gibberellins, auxins, and cytokinins play a central role in fruit set. An imbalance, for example an excess of endogenous gibberellins, can induce parthenocarpy or malformations, while insufficient levels of cytokinins limit cell division in the developing fruit. Biostimulants based on algae extracts or microorganisms can modulate the plant's hormonal profile, favoring a balance that promotes fruit set. Research published in the Journal of the Science of Food and Agriculture indicates that application of Ascophyllum nodosum extracts increases cytokinin concentration in grapevine reproductive tissues, improving fruit retention.

Mechanisms of action of biostimulants

Biostimulants improve fruit set through multiple mechanisms. Firstly, they stimulate photosynthetic activity and carbohydrate production, which are essential as an energy source for fruit development. Secondly, they promote the uptake and translocation of key nutrients, especially boron, zinc, and calcium, through natural chelation and enhancement of the rhizospheric microbiota. Thirdly, they modulate hormonal pathways, increasing the synthesis of auxins and cytokinins that delay pedicel abscission. Additionally, certain biostimulants such as the microalgae Chlorella vulgaris and Scenedesmus spp. contain phytohormones, amino acids, and polysaccharides that act as elicitors, activating plant defenses and improving stress tolerance.

Foliar application of biostimulants at pre-flowering and fruit set stages allows rapid absorption of bioactive compounds. Fulvic acids present in some products improve cuticular penetration and nutrient mobility within the plant. Meanwhile, brown seaweed extracts (Ecklonia maxima, Ascophyllum nodosum) are rich in betaines and mannitol, compounds that act as osmoprotectants and antioxidants, reducing oxidative damage caused by heat or water stress. The combination of these mechanisms results in more stable fruit set and a lower percentage of fruit drop.

Most effective biostimulants for fruit set

Seaweed extracts

Brown seaweed extracts (Ascophyllum nodosum, Ecklonia maxima) are widely used in viticulture for their high content of phytohormones, polysaccharides, and trace elements. These products stimulate cell division and improve stress resistance. In trials conducted in the Ribera del Duero Denomination of Origin, the application of a biostimulant based on Ascophyllum nodosum at the phenological stages of visible inflorescence (BBCH 53) and full flowering (BBCH 65) increased fruit set by 22% and yield by 15% compared to the control.

Freshwater microalgae

Microalgae such as Chlorella vulgaris and Scenedesmus obliquus represent an innovation in biostimulation. They contain phytohormones (auxins, cytokinins, gibberellins), essential amino acids, vitamins, and polyunsaturated fatty acids. Their ability to chelate micronutrients and stimulate soil microbiota makes them especially useful in calcareous soils or those with low boron and zinc availability. Ecoganic develops biostimulants based on freshwater microalgae that have shown improvements in grapevine fruit set under moderate water stress conditions, with increases of 12-18% in the number of berries per cluster.

Fulvic acids and amino acids

Fulvic acids improve nutrient absorption and act as natural chelating agents. Combined with amino acids such as proline and glycine, they promote the synthesis of proteins and antioxidant enzymes. Their application during pre-flowering and fruit set helps mitigate abiotic stress and maintain cell integrity during fruit development. A study from the University of Bologna showed that applying fulvic acids to grapevines increased calcium concentration in floral tissues, reducing the incidence of shatter by 25%.

Phenological stages and recommended doses

The most effective application window for improving fruit set ranges from the visible inflorescence stage (BBCH 53-55) to early fruit set (BBCH 71). Two foliar applications are recommended: the first at pre-flowering (BBCH 57-61) and the second just after fruit set (BBCH 69-71). Doses vary depending on the product, but generally for algae-based biostimulants, 2-3 L/ha are used, while for microalgae, 1-2 L/ha are recommended. It is crucial to adjust the pH of the spray solution to between 5.5 and 6.5 to optimize foliar absorption. Additionally, incorporating a non-ionic adjuvant can improve coverage and penetration.

In organic fertilization programs, biostimulants are integrated with organic fertilizers and micronutrients. For example, a combined strategy of microalgae biostimulant at flowering followed by an application of chelated boron and zinc has shown positive synergies. It is important to avoid applications during peak solar radiation hours or at temperatures above 30 °C to prevent phytotoxicity. Water quality and compatibility with other products should be verified through tank tests.

Field results and scientific evidence

Various field trials support the efficacy of biostimulants in grapevine fruit set. In a study conducted by the Institute of Agrifood Research and Technology (IRTA) in Catalonia, a biostimulant based on seaweed extract and amino acids was evaluated on the Macabeo variety. The results showed a 20% increase in fruit set and an improvement in berry size uniformity. Likewise, the FAO has published guidelines on the use of biostimulants in viticulture, highlighting their potential to reduce the impact of climate change on grape production. You can find more information in the FAO report on agroecology and biostimulants.

Another trial in the Rueda Designation of Origin, with the Verdejo variety, compared a treatment with a microalgae biostimulant (Chlorella vulgaris) against an untreated control. The microalgae treatment increased the number of berries per cluster by 16% and the average cluster weight by 12%, in addition to improving the sugar content in the must. These results are consistent with research published in indexed journals, such as those that can be found at ScienceDirect on biostimulants in grapevines. The accumulated evidence confirms that biostimulants, especially those based on algae and microalgae, are effective tools for optimizing fruit set under real field conditions.

FAQ

What is fruit set in grapevines?

Fruit set is the process by which fertilized flowers transform into young fruits. In grapevines, it occurs after flowering and is critical for determining the number of berries per cluster. Poor fruit set causes shatter, reducing yield.

When to apply biostimulants to improve fruit set?

It is recommended to apply biostimulants at two times: at pre-flowering (BBCH 57-61) and just after fruit set (BBCH 69-71). This stimulates fertilization and initial fruit development.

Which biostimulants are most effective for fruit set in grapevines?

Brown algae extracts (Ascophyllum nodosum), freshwater microalgae (Chlorella, Scenedesmus), and fulvic acids combined with amino acids have shown high efficacy. These products improve nutrition, hormonal regulation, and stress tolerance.

Are biostimulants compatible with organic viticulture?

Yes, most biostimulants are permitted in organic agriculture under EU Regulation 2018/848. Ecoganic offers certified products for use in organic vineyards, contributing to sustainable, high-quality production.

Advanced biostimulation strategies for fruit set in grapevines

Flower fruit set in grapevines (Vitis vinifera L.) is a critical physiological process that directly determines final yield and harvest quality. During this phase, occurring between 10 and 15 days after flowering, the fertilization rate and berry retention can vary dramatically depending on environmental stress and the plant's nutritional status. Recent field data from vineyards growing varieties such as Tempranillo and Cabernet Sauvignon indicate that the combined application of seaweed extracts (Ascophyllum nodosum) and free amino acids can increase fruit set by 12% to 18% under moderate heat stress conditions. This increase is attributed to improved translocation of photoassimilates to the clusters and endogenous hormonal regulation, particularly the balance between auxins and gibberellins.

A controlled study in the Ribera del Duero Designation of Origin demonstrated that foliar application of a biostimulant based on humic and fulvic acids (3 L/ha) at pre-flowering, followed by a second application with 0.5% seaweed extract at full bloom, raised the fruit set percentage from 62% (control) to 74%. This translated into an increase of 1.8 kg/vine in final yield, without negatively affecting total soluble solids or must acidity. The key to success lies in timing: the first application stimulates root development and the uptake of micronutrients such as boron and zinc, essential for pollen viability and pollen tube germination. The second application, rich in cytokinins and betaines, reduces flower abscission by mitigating oxidative stress generated by temperature spikes above 35°C.

To maximize the effectiveness of biostimulants on fruit set, it is recommended to integrate products containing at least 5% free amino acids (especially proline and glycine) together with fungal-derived oligosaccharides (such as those from Trichoderma). In trials at the Polytechnic University of Madrid, this combination applied at a rate of 2 L/ha at two timings (start of flowering and early fruit set) increased berry retention by 22% compared to the control, with a 15% increase in average berry weight. Additionally, an improvement in cluster uniformity was observed, reducing the percentage of parthenocarpic (seedless) berries from 8% to 3%. This response is due to the ability of amino acids to chelate nutrients and facilitate their transport to reproductive tissues, while oligosaccharides activate the plant's natural defense pathways, reducing the impact of fungal diseases such as downy mildew or botrytis during flowering.

As a practical recommendation for grape growers, a pre-flowering sap analysis is suggested to determine the exact nutritional status of the vine. If boron levels are below 25 ppm or zinc levels below 15 ppm, it is crucial to apply a biostimulant that includes these micronutrients in chelated form (e.g., boron-ethanolamine and zinc-EDTA) along with seaweed extract. The optimal rate under these conditions is 2.5 L/ha of a commercial product containing 10% seaweed and 8% amino acids, applied in a spray volume of 300 L/ha to ensure uniform coverage of the clusters. Under mild water stress conditions (leaf water potential between -1.2 and -1.5 MPa), the application frequency should be increased to every 7 days during the flowering-fruit set period, but reducing the rate to 1.5 L/ha to avoid phytotoxicity. The most consistent results are obtained when the average daytime temperature during flowering remains between 22°C and 28°C, and relative humidity between 60% and 70%. In drier climates, the addition of a non-ionic wetting agent at 0.1% improves foliar absorption of the biostimulant by 30%, according to field efficacy data from the La Mancha region.