Wider window for post-harvest nutrition – Vineyard

In the UK, where grape harvest can run into October, or even November in some sites and seasons, there is often little, if any opportunity to apply foliar nutrients after harvest, before leaves start senescing and the weather closes in. But, the warm, sunny, season thus far has kept many crops well ahead of ‘normal’, with picking on course to start earlier (at the time of writing), presenting a wider post-harvest window.

Some growers may question the logic of applying nutrients after harvest, when much of that material will be pruned out over the winter, but there is sound logic behind doing so.

Vines rely on the carbohydrate and mineral reserves stored in stems and roots from bud burst, well into flowering, when fruit potential is set, so topping-up those essential nutrients before leaves start to change colour means that more can be laid down ahead of the coming season.

As leaves senesce, nutrients are remobilised, moving out of the leaf, through the plant’s vascular system (phloem), into other parts of the vine, where they can be stored until required.

Of course, vines should be adequately fed via both soil and foliar-applied nutrition throughout the year to avoid any potential deficiencies, but with so many factors at play, variations inevitably occur, and now is a good time to critically appraise the nutritional status of vines to help determine requirements and prioritise any foliar treatments.

Consider simple questions, such as:

• What is the nature of the canopy and foliage?
• How many times did you have to trim the canopy? (Twice per year seems to b
• the sweet spot)
• Were the bunches well-formed and resilient or did they become flaccid in the heat?
• Were the vines overcropped?
• Will you have adequate amounts of good quality cane to choose from while pruning?
• Do you suspect rooting issues after the wet winter followed by dry summer, particularly in young vines?

Generally, established vines on reasonable soils have excellent yield potential this year, so the task is simply to replace nutrients removed via crop offtake, or lost through leaching. Vines that are less well established, and those that have struggled to develop strong root systems following recent wet winters and the dry spring/ summer will need some extra support.

Regular soil testing provides a useful benchmark to monitor changes in nutritional status, and may be especially useful for highlighting underlying issues on areas that have underperformed in terms of yield or grape quality this year.

As an aside, some of the best-looking crops this year appear to be where poultry manure pellets have been applied earlier in the season, providing nitrogen, phosphate, potash, and a range of trace elements. The pellets also provide a substrate for soil biology, which may be having wider benefits on nutrient cycling and availability.

As long as leaves are green and photosynthesising, they will be able to take up foliar-applied nutrients, even in dry conditions, and the results will be far better than if products are applied once leaf senesce begins.

The benefits from foliar applications are likely to be greatest for more phloem-mobile nutrients, such as boron, but requirements will vary for different situations. Generally, growers should consider products that provide a broad range of micronutrients and trace elements, including magnesium, boron, and zinc, which are key to supporting strong bud development and flowering in spring.

Calcium can also help boost root development, and wood maturity (winter hardiness), while phosphorus aids root development.

Although a small amount of nitrogen can help build vine reserves before dormancy, avoid applying too much, as this encourages soft, sappy growth that will be more prone to winter damage.

When applying foliar nutrients post-harvest, consider including adjuvants, particularly surfactants, to improve uptake. Plants take in foliar nutrients through the stomatal pores, or via absorption through cells of the outer layer (epidermis), and some research suggests that reducing surface tension leads to better spreading and coverage on the leaf surface, increasing the chances of absorption.

Legislation limits the use of adjuvants once the edible part of the crop has formed, requiring the rates of crop protection products in the tank-mix to be halved. Post-harvest, this rate restriction is lifted, so they can be used to assist uptake of nutrients without compromise. As ever though, discuss the options with your agronomist.