a) Flowering
This phase runs from May to July in NH and November to Jan in SH. Flowering will typically take place approximately around 8 weeks after bud burst. Type of grape variety and weather conditions can lead to earlier or delayed flowering. Consistent and warmer conditions(minimum 17 degrees) can promote earlier flowering.
Potential Flowering of the vines starts with how the environment has impacted the compound buds during the vine’s initial growth stage. All the genetic coding responsible for the growth of a shoot is contained within these compound buds. The shoot is what ultimately forms the leaves, tendrils, stem, and grapes. So shading, consistently low temperatures, water stress and nutrient deficiency can all impact the compound buds and thereby limit the fruitfulness or the following year’s harvest. Hence, the protection of the compound bud is critical here.
Riesling is one vine that can create fruitful buds at low temperature. It is a winter freeze tolerant vines.
b) Fruit set
Grapevine flowers like most plants have both male and female reproductive parts. Most vines are self-pollinated, which means pollens are transferred from the anther (male part) to the stigma (female part) within the same flower or to the other flowers on the same plant. Once the pollen lands on the stigma, it germinates and grows a pollen tube, which attaches itself into the ovary. The sperm cells within the pollen tube will then fertilise the eggs.
Following fertilisation, the fertilised ovules develop into seeds, skin, and pulps and begins to grow into a grape berry. This is the initial stage of fruit set.
Please note that not all successfully pollinate. Around 30% becomes grapes. Cold, rainy and windy conditions can lead to poor or irregular fruit sets.
Coulure is a condition in which a high proportion of grape flowers fail to develop into fruits where flowers wither and fall off the vine. It is caused by an imbalance of carbohydrate levels normally linked to a low rate of photosynthesis that is triggered by cold, wet and cloudy conditions. Water stress can also trigger this.(diagram can be used). Grenache, Cabernet Sauvignon, Malbec, Merlot are some varieties that are susceptible to Coulure.
Millerandage is a condition where grape bunches contain berries of significantly different size. Seedless, unripe and non-uniform ripening within the bunch.
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