Fundamentally, the purpose of irrigation is to help the vine regulating its transpiration in order to make the best possible fruit. As such, deciding how to irrigate should not focus on soil moisture content but rather on plant transpiration before, during and after irrigation. Indeed, the relationship between soil moisture and vine transpiration is more complex than we think. Just as an example: did you know that vine roots redistribute water within their root system? This principle is called “hydraulic redistribution” and was quite recently reported with grapevines. It directly affects water availability in spatially heterogeneous soil conditions, particularly if they are dry. There are many other fascinating examples showing how complex is vine adaptation to soil drought… But they will be discussed later.
First of all, let’s keep in mind that immediately after irrigation, a vine should increase its transpiration. If a rise in transpiration is not observed after irrigation, you know for sure that your irrigation, regardless of timing or volume, was useless. With this context in mind, asking when to trigger irrigation can be simply answered if you ask the vine. Irrigation is only needed when vine transpiration is starting to decline.
If you want to adjust irrigation to vine water needs you simply need to maintain a transpiration level that promotes good plant health, optimal fruit composition and avoid fruit dehydration. However, this question can be particularly challenging in situations where water access is limited which is often the case with high quality vineyards.The question on how irrigation best meets vine water needs is really complex if you try “to think“ like a plant. Millions of years of plant evolution probably surpass your own imagination. However, it becomes simpler if you know directly how much water the plant is asking for. Here are a few hints on what we know and learnt from asking the vine about its water needs, year after year in different locations and particularly after irrigation.
Effect of irrigation volume depends on vineyard spacing and rooting depth
Effect of irrigation volume depends on vineyard spacing and rooting depth
- A volume of 6 gallons per plant may be large or small. Vine appreciates the size of water “drinks” differently depending upon its daily water use. In turn, vine water use is mainly a question of vineyard spacing, leaf area exposed and varietal affinity for water. Some vineyards like to drink a lot in order to produce good fruit quality, some like to drink as little as possible.
- For instance, if a plant transpires on average 2 gallons per day, a 6-gallon irrigation covers the vine water needs for 3 days. After day 3, vine transpiration is expected to decline. On the contrary, if a plant uses less than 1 gallon per day, which is often the case with small vines in tight spacing vineyard, a 6-gallon irrigation covers vine water needs for more than a week. After day 7, vine transpiration may show a decline.
- Depending upon the vineyard situation, the same “drink” is perceived as relatively large or relatively small. Consequently, irrigation volumes have to be different to account for vineyard specific properties.
Vines are smart: when soil gets moderately wet, they use less water.
- Imposing and maintaining a period of moderate or even low moisture content is actually a good thing, especially for quality viticulture. As you force the plant to regulate its transpiration to a lower level than it could “potentially” have, you are promoting a better fruit composition. That is why vineyard sites or vintages that impose a lower soil water content rather than a higher soil water content are often sought after by wine lovers.
- Application: should you favor short or large intervals between irrigations? Once you determine the irrigation volume necessary for vine transpiration to rise up to its maximal level – called potential transpiration – just wait as long as possible until the next irrigation. Why? There are numerous benefits that will be discussed later from a winemaker standpoint. From a “vine sustainability” standpoint, the vicious side effect of frequent irrigation is that a constant re-wetting will favor a shallow root system development. The vine ends up taking water from a smaller root reservoir, located just under the dripper and root growth is promoted near the soil surface. Then, because of soil evaporation, shallow soil surface gets always drier first after each wetting a episode. This further precipitates the need for the next irrigation as the root system is concentrated near the surface. Consequently vines use more water than they could under frequent irrigation regime. They simply never reduce their transpiration. Or when they do, it is because you waited too long between two irrigations. When that happens, transpiration decline is brutal and often leads to fruit shriveling.
- It is the same principle as drug addiction: frequent irrigations train the vine to become a junkie, waiting for its next “fix”.
In conclusion vine are smarter than we think when using water. The consequence is that irrigation should adapt to plant response. Using the plant as a sensor enables you to respect it more and to irrigate according to its needs and your production objectives. Additionally, it is the best way to optimize your practices and reveal the best of your terroir.
Thibaut
