This is our second discussion on managing frost in the vineyard. With this blog post, we wish to generate discussions around the consequences following a frost event and the best practices to apply to mitigate frost negative effects on vineyard productivity. To understand this discussion it is best to have read the previous blog: here
Context: After the 2017 frost episode in France, plants ran low on their reserves which negatively affect vine ability to generate vegetative organs (shoots). For that reason, one should expect that the level of reserve available will be poorer. This means that the vine may lack sufficient “fuel” to sustain an new growth restart. Consequently nutrient deficits are more likely. This rises the question: what do we know about vine needs during organs development ? In our last blog we discussed the timing of biomass accumulation for different organs. Here we discuss a few basic ideas regarding the timing of Nitrogen uptake
What do we know about the timing of nitrogen uptake?
Assessing Nitrogen uptake is not only important for plant growth (because it is part of the chlorophyll, amino acids and proteins). Nitrogen uptake during period 2 and 3 is also important for fruit production during period 4.
The following graph describes the vine growth dynamics:
Figure 1 : Growth dynamics for the main grapevine organs during the season
(adapted from JP Goutouly, in “Vintage report, Bordeaux 2016”)
Based on figure 1, we observe that shoot growth happens mainly during period 2 and precedes root biomass accumulation during period 3. If we focus specifically on active Nitrogen accumulation, scientific literature suggests that it happens mainly during period 2. In fact, Schreiner (2016) reported an estimated 65% of Nitrogen absorption during period 2 and 25% during period 3, while from period 4 on, Nitrogen absorption is lower.
Consequently when a new growth is expected to re-start after a frost even, it is important to account for sufficient nitrogen availability in the soil. As we discussed in our previous blog, following a frost the vine undergoes two successive “periods 2” immediately back to back. Thus, the success of accumulating sufficient biomass during period 2 is not the only challenge. A regrowth after a frost may also require an additional supply of Nitrogen to meet vine nutritional requirements during period 2.
What impact on practical decisions?
Considering vine biomass accumulation periods and vine nutrient accumulation dynamics for different vine organs will help adapting vineyard operations to plant natural cycles. Incorporating such understanding into practices will help winemakers and vineyard managers designing the best pruning practices after a frost. It should contribute to minimizing negative effects of frost onto fruit production.
To help the grapevines overcome the stress of “sustaining 2 successive growths” it is relevant to look at the work of Verdenal et al. (2015) and draw some practical conclusions for shoot length management after a frost
In figure 2, scientists have reported that vines shoots Nitrogen content is lower with longer shoots. This observation reflects a “Nitrogen dilution” effect into a larger biomass, since longer shoots are being produced. As you can see in figure 2, the dilution effect not only decreases Nitrogen content at the shoot levels. Other organs, are directly affected such as roots, fruit, trunks. This result means that imposing shorter shoot length is important to manage Nitrogen distribution not only at the shoot level but a the whole vine scale. This is particularly relevant to optimize vineyard management strategies in situations with low Nitrogen uptake or after a frost.
Figure 2 : Impact of shoot pruning on total vine Nitrogen quantity (adapted from Verdenal et al. 2015)
Practical applications and remediations :
- Figure 2 highlights why it is important to combine the monitoring of shoot elongation with the monitoring of Nitrogen concentration. Diagnosis of Nitrogen concentration is improved if one can combine shoot elongation measurements with leaf Nitrogen uptake at the same time. Combining shoot length with Leaf Nitrogen content measurements is a logical approach to better assess vine Nitrogen needs and avoid over fertilization.
- To avoid important Nitrogen deficit with vines that have suffered 100% frost damage, it is important to limit growth potential by reducing the number of shoots and remove suckers. This strategy will enhance growth of secondary shoots and will reduce dilution effects in a large canopy.
- It is important to keep some suckers for pruning at the end of the season if you are doing cane-pruning or want to keep a kicker cane.
- If the vineyard is cane pruned, it is possible to reduce the size of the cane and reduce the growth area.
- After shoot growth slows down, it will be important to limit the size of the canopy by using a more intense hedging strategy to minimize dilution effects.
- It is important to work the soil. This will increase temperature and water penetration in top soil horizons and should favor nitrogen mineralization as well as root absorption.
Practical corrections and take home :
If Nitrogen deficit happens occurs during the season, using foliar sprays is suggested during the periods when the vine is more receptive to fertilization:
- In period 3: sprays will be useful for canopy development in order to boost photosynthetic activity with Nitrogen absorption.
- In period 4: sprays can be useful a couple weeks before harvest if climatic conditions and disease pressure allow it. At this point, fertilization will have an effect on fruit composition and aims at enhancing yeast available Nitrogen content in the fruit as well as the aromas complexity (Verdenal et al, 2015 ; Helwi et al, 2016).
- In Period 5: a fertilization could be useful after harvest and before leaf fall in order to boost the reserve accumulation. However the rate of Nitrogen uptake is expected to be lower than during period 2 or 3.
Next, we will discuss bud fertility. This discussion will help anticipating best pruning practices in vineyards more or less severely affected by the frost episode.