The leaf area exposed to sunlight, known as the grapevine canopy, captures sun energy to perform photosynthesis, regulate water use through transpiration, and modulate the microclimate of ripening grapes.
Determinations about how much canopy is enough, as compared with too much or too little, are critical during the early stages of the growing season. Sure, everything may appear fine at the start of the season, but it is the only time to significantly reduce or boost leaf area in the vineyard.
In absence of conditions limiting leaf area growth (such as nitrogen deficit, sunlight deficit or water deficit), the speed at which leaf area develops is directly and linearly proportional to the amount of heat accumulated as has been demonstrated by numerous authors (Riou, 1995 ; Lebon , 2003; Pallas, 2008, etc..). Cumulating heat is a concept that is described by thermal time. Thermal time, day in day out, accounts for the temperature above a certain threshold experienced by the plant over the course of one day. For that reason, thermal time is expressed in Degrees.day (using Celsius if you want to compare with scientific literature) .
A thermal time comparison of warm and cool sites in Napa’s 2014 growing season shows how quickly thermal time (also expressed as growing degrees days) passes. The window to influence vines during their growth stage is small. in fact, one considers that typically 75 % of the full growth is reached once the vine has experienced around 500 °C days since bud break.
Why is it important to assess vine growth?
Too little canopy and the vine lacks sufficient photosynthesis activity, thus carbohydrates flow to fully ripen grape crops. Too much and the vine becomes overly vegetative; in turn the carbohydrates flows sustain the growth and maintain vegetative organs instead of allocating the flow of carbohydrates towards the fruit.
So, historical context for shoot development and length can offer valuable information about optimal shoot length to mature properly.
As such monitoring shoot length is a good source of information to regulate the conditions of fruit production.
Let’s review some key action areas that vineyard managers have at their disposal to regulate leaf area development.
Trophic competition, or competing growing points, can divert valuable vine resources from the reproductive area of vine growth to other competitive sites. Shoot thinning and desuckering both alleviate this form of competition.
Water use is determined in part by leaf and canopy transpiration. Irrigating the vineyard based on shoot development rate, with the goal in mind that you should reach the top wire by 500 degree days (DD), mitigates water supply concerns.
Cover crops may compete with vines for soil nutrients and water. This competition may slow vine leaf area growth and development. Removal of the cover crop alleviates the cover crops’ use of nutrients and water supply, making those resources available to the vine when they are needed most.
Nutrient supply and vine use of nutrients vary from site to site and week to week. Assessing nutrient deficiency and identifying nutrient uptake patterns in the vineyard allows vineyard managers to improve their fertilization practices, determine if or when to remove cover crops, and develop an intimate picture of their vines nutrient needs.