Bloom-to-Harvest Nutrition for Wine Grapes: A Science-Driven Approach

Bloom-to-Harvest Nutrition for Wine Grapes: A Science-Driven Approach

The right mix of nutrients supports bloom, higher fruit brix and abundant, profitable bunches of superior-quality grapes.

Throughout the wine grape growing season, there are several opportunities to positively impact the crop. The previous article on pre-bloom nutrition mentioned three crop timings where Science-Driven Nutrition™ could be used to achieve the brix, color and uniform maturity desired at harvest.

As with the pre-bloom period the second half of the grape growing season requires a particular mix of nutrients to meet the demand of the crop. This article breaks down the essential nutrients for the bloom, post-bloom, berry development, Veraison and post-Veraison crop timings.

Wine grape growers that use a science-driven approach to nutrition are more likely to produce a consistently high-quality crop. The foundation of this approach is the Five Rs: The Right nutrient applied at the Right time in the Right form in the Right mix targeting the Right location in the plant.

Proper nutrition at the beginning of the season sets the stage for a high-quality crop. Mid-season, proper nutrition ensures wine grape growers can meet the winemakers’ demands while exceeding their expectations. Read on to learn more about the right mix of nutrients for each of the six crop timings leading up to harvest.


As the pre-bloom period transitions to bloom, growers must turn their attention toward supporting pollination and fruit. This requires boron for pollen viability and to help extend the length of the bloom window.

Additionally, applying calcium at this time will help to foster firm grapes which, due to their firmness, will be less prone to disease pressures later in the season. Calcium—an essential nutrient for building thick, strong cell walls during cell division—synergizes with boron. When applied together, such as in Agro-K’s Cal Sea-Set, both calcium and boron uptake by the vines is greater than if these two nutrients are applied separately.

“Calcium applications are particularly important during this time because calcium can only get into the cell wall structure of the grapes during fruit cell division,” says Rick de Jong, International Business Development Manager for Agro-K. “Once this period has passed, growers can apply maintenance applications to meet the vine’s other needs for calcium but they will no longer be able to strongly influence the calcium levels in the skin and flesh of the fruit where it improves disease tolerance.”

Cal Sea-Set also contains seaweed to help support plant functions. This foliar product should be applied during bloom along with Agrobest® 9-24-3 liquid fertilizer. This NPK formulation not only provides nutritional support during cell division, but it also helps to maximize the grapevine’s canopy size. The phosphorus and nitrogen in the Agrobest 9-24-3 provide the plant with the right nutrition, in the right forms to support cell division during this window of opportunity.

Fruit Set and Differentiation

The fruit set and bud differentiation periods take place as bloom comes to an end. The buds that are fruiting out now were developed during the previous season. As these new shoots develop, the vine repeats the process by simultaneously creating buds for next year’s leaves and fruit while developing this year’s crop.

“This is an ideal time to apply micronutrients that can be assimilated into the tissues and stored in the wood,” says Sean Jacobs, Viticulturalist and Technical Marketing Representative at Agro-K. “By applying these micronutrients now, they will be rapidly available to growing tissues in the spring. Many nutrients from the soil require microbial interaction to become available to plants and those microbes don’t function well when the soil is cold. Combine cold soils with little-to-no leaf area, and vascular flow from the roots to the growing points is very limited, setting up your plants for deficiencies right from the start.”

Three Agro-K products work together to provide maximum nutrition for the wine grape crop at this time: Micro SeaMix, Vigor-Cal™ and Magnesium Dextro-Lac®. The Micro SeaMix, which contains a combination of manganese, magnesium, iron, copper and seaweed supports photosynthesis, and fosters nitrogen metabolism. Next, Vigor-Cal—a sugar-complexed calcium foliar product—continues to provide calcium when the crop can best use it to contribute to healthy cell walls inside the grape.

Berry Development

Three to four weeks post-fruit set, cell division slowly comes to an end and berry bulking begins. At this point, Cal Sea-Set and Agrobest 9-24-3 become relevant again.

Cal Sea-Set is important because this is the time for maintenance calcium applications. Here, calcium won’t be able to enter the fruit cell walls any more but it is a secondary plant messenger involved in many enzymatic activation pathways and is still needed for healthy vine growth.

Beyond this point, berry growth will transition to building sugars and bulking the grapes by expanding overall size of the existing fruit cells. The macronutrients in Agrobest 9-24-3 also support vine growth and development, as well as cluster development and bunch stretch.

Buckshot Berry

When the berries on the vine are pea-sized (buckshot-sized), leaf function becomes incredibly important. This is when the plant goes into over drive building the sugars that will fill the fruit. Micro SeaMix, as a complete foliar micro nutrient mix, is well-suited to manage this development phase.

“We want to make sure that we’ve got optimum leaf function, which means it’s all about photosynthesis at this stage of the game,” de Jong says. “We’re not looking for a whole lot of nitrogen, but we are looking to alleviate stress and we’re also looking at those photosynthetic nutrients: iron, magnesium and manganese. We want to make sure that that those leaves are functioning at a very high level because that’s what’s going to drive those sugars that we’re going to use later into the fruit.”

The buckshot berry crop timing also acts as a last call for seaweed applications. Seaweed beyond this point may delay ripening—something growers want to avoid come veraison.


Veraison is defined as the onset of ripening, or the point at which the grape berries begin to soften and change color. Physiologically at this stage, cell division and expansion within the berries is essentially complete, vascular flow from the xylem ceases, and the berries start accumulating sugars. Prior to this point, the grapes are purposefully acidic and sour. This is an evolutionary trait. The grapevine’s chief purpose is to propagate through seed, and the acidity deters hungry critters from consuming the fruit before the seeds are mature.

“So, we have a lot of acid and all these really astringent strong compounds at this point, and they are basically keeping you from wanting to eat the grape,” Jacobs says. “Well, once you get to Veraison, this is where the berry is beginning to ripen. The seed itself is becoming mature, and the grape is starting to use these acids as a substrate for cellular respiration. Energy is generated through the respiration process and that energy is used to drive sugar pumps that move the sugars against a concentration gradient from phloem companion cells into the berry.”

Because sugar is essential to the wine-making process, growers want to support the development and movement of sugars into the fruit. The grapes are also beginning to take on their green or red color. The rise of sugar and dropping off of acid must be supported.

Potassium aides the vines in pushing the sugars into the fruit by resetting or activating the above mentioned sugar pumps. Agro-K’s KDL® and Ultra Finish DL™ foliar potassium products are designed to effectively move potassium into the berries and surrounding tissues to get this job done.


Around 19 brix, the crop has entered the post-Veraison phase and is entering harvest. Here again, Agro-K’s KDL® and Ultra Finish DL™ support the process by delivering the right amount of potassium to the crop. As the brix continue to rise, the crop needs to support the movement of these sugars and potassium can help with this.

For more information on Science-Driven Nutrition™ for wine grapes, visit

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