It never ceases to amaze me how many people underestimate the extreme resilience of grape vines. Ironically, the “helpful” actions that managers take to encourage vine growth often undermines the vine’s natural ability to grow in a way that improves resilience. Coddling most things, with the exception of maybe an egg, tends to have unintended consequences, so let’s explore what actually causes vines to “stress,” and squash some myths that have encouraged managers globally to coddle their vines.
In my career I’ve crossed paths with some major preconceived notions about vine stress that often arise out lack of or misinformation. I want to state unequivocally for managers worldwide - VINES ARE INCREDIBLY RESILIENT. Don’t get me wrong, young vines, like all young plants, are somewhat fragile. But mature vines are, or at least should not be, as fragile as young vines, and managers have the power to either promote resilience or train “thirsty” vines that tip over when exposed to periods of actual water, wind, or heat stress.
What(er) Stress?
I continue to encounter the view that if vines do not receive water early and regularly, they will lose canopy and wine quality will suffer as a result. This view underestimates vine resilience and growth habits, and tends to overwater at a minimum, and even assume they are stressed when they are not.
I am sure many of these misconceptions about grapevine resilience to water stress comes from anecdotal experiences of managers going away on Friday with everything looking fine, and coming in Monday, seeing yellowing basal leaves, and finding malfunctioning irrigation. What some may not know is that irrigation malfunction happened many days, if not weeks, before visible evidence of water stress was present. Vines, especially older vines, can go for prolonged periods under pretty severe stress before they lose leaves in the New Zealand climate.
There are visible signs as vines approach stress, but they require a close inspection and are not something you can see from a cursory look.
Water stress is mostly invisible
While signs of severe water stress like senescing basal leaves, abscission of tendrils, and termination of shoot tips, are generally easy to spot, signs of the lead up to those are less noticeable. Precision irrigation management necessitates working in this region of nearly invisible stress, because by the time visible signs manifest, the stress is generally too severe to have beneficial effects on wine quality.
Vines respond physiologically to declining soil moisture, and if you know what to look for, you can get some early warning signals of depleting water availability. However, it must be pointed out that these differential growth effects can take days, or even weeks to manifest. There are other, more short term signs of stress like leaves wilting or turning away from the sun and senescing tendrils, but these only happen when stress is reasonably severe. All of this said, I don’t use any visible signs, except for possibly when in the early season to bust out the pressure chamber, my tool of choice for irrigation scheduling, to start measurements. If you’re not familiar with the technology, do yourself a favour and learn about it. It is the “gold standard” scientific method to determine vine water status, has been used in scientific studies for decades, and is the tool I use to precisely schedule irrigation for my clients.
Growth rate and shoot tips
The earliest sign of water limitation is a reduction in growth rate due to shortening of internodes. This starts to occur around -0.6 to -0.7 MPa of midday SWP. Watering before vines get to this water potential has very little, if any, effect on vine growth. Internodes get progressively shorter as water stress increases, and at very severe stress, the shoot tip terminates. Other visual signs to look for as a general metric of vine water status are the appearance of the shoot tips.
· Actively growing, well-watered, shoot tips have tendrils that overarch the shoot tip.
· As water limitation sets in, the leaves extend less, as do the tendrils.
· Stressed shoot tips have very small leaves and tendrils.
Pinot noir (top) and Sauvignon blanc (bottom) shoot tips. Well watered vines are on the left, vines starting growth limitation (SWP -0.6 to -0.8 MPa) are in the centre, and relatively dry vines are on the right.
All of these physiological responses take many days, if not weeks to eventuate, and so are mostly of use in the early season, when the soil is slowly drying down because of small canopies and cooler weather. At full canopy in the heat of summer, vines can come to stress within a matter of days, well before any outward sign of slowed growth can manifest, so these visible signs are of limited usefulness for regular scheduling of irrigation in season.
Backhanding Sun Exposed Leaves?
Leaf temperature can be another “old school” metric of vine stress. As stomata close, water evaporation from the leaf surface slows, and so does evaporative cooling of the leaf. Since it’s still fully exposed to the light of the sun, the leaf surface begins to heat up. Well-watered vines' leaves are substantially cooler than air temperature because the evaporation of water from stomata cools them. As stress rises and stomata close, the temperature of the leaf increases. This can be felt by hand or measured with an IR thermometer. There are real time irrigation monitoring systems built around monitoring vine canopy temperature, because the stomatal closure effect is rapid, and thus a good metric of rising invisible stress before visible signs manifest. I have a porometer to directly measure vines' stomatal conductance, which I sometimes use alongside my pressure chamber.
How long can vines go under severe stress before leaves start to turn? This is context specific, and will depend on the climate, soil, and vine age/history. Most mature vines in New Zealand can sit at severe stress (-1.2 MPa or more negative SWP), with stomata largely/entirely closed, for about a week before there are any yellowing leaves. In a hotter climate, damage to leaves with closed stomata would be accelerated, and thus so would canopy yellowing. Younger vines can show stress a bit earlier, probably due to fewer vine reserves to make up for lost photosynthate, but still take a while to show. However, due to their much smaller root systems, young vines are more likely to come to stress between irrigations. In a rootstock trial we tested last year (one of the driest on record in Marlborough) some vines sat at very severe stress (SWP of -1.4 to -1.6 MPa) for over a month with no yellowing leaves at any point before harvest. However, these vines were over 30 years old, and so would have extensive root systems and carbohydrate storage.
Applying Water
While I have pointed out that there are visual signs of water status in grapevines, I do not recommend using them to schedule irrigation, especially midseason, when stress can come on rapidly. Visible signs in many cases are too slow to manifest to be of immediate value, and taking vines to the degree of stress that visually manifests is generally taking them too far. Deficit irrigation can be a huge benefit to wine quality, but taking it too far certainly degrades quality, therefore more sensitive metrics than the human eye are needed to properly irrigate by this regime. Being a recovering academic, I’m a pressure bomb/porometer guy, but I’m currently testing these tried and true technique versus some new, real-time vine monitoring technologies, so watch this space for updates.
But I need to add one caveat - if you are using soil probes, or other new tech, that has not had the same years of testing and data backing the "stress" points, and these tools are telling you vines are "stressed" but your shoot tips and tendrils are telling you they are happy as, that tends to be a good indication that you should check your tools are calibrated.
What about Heat Stress?
Another common misconception I’ve experienced is that vines must be heat stressed on what we perceive as “hot days.” This is almost never the case in cool climate growing regions, where I’ve heard this one quite a lot. Grapevine photosynthetic rate peaks around 30-35 C. For reference, the average high temperature in Marlborough over the summer is in the mid-twenties. Heat stress, in terms of reduced photosynthetic rates, only comes after about 40 C, temperatures very rarely experienced in growing regions classified as cool climate and never for prolonged periods. Don’t get me wrong, there are plenty of wine regions, with climate change bringing more, that regularly experience prolonged periods of heat stress, and the loss in wine quality associated with it. In New Zealand it’s our absolute blessing that our climate, even in “warmer” regions like Hawke’s Bay and Central Otago, is so benign for grapes in terms of temperature.

Wind Stress?
Finally, a word on wind, a stressor not often talked about, but relevant in all regions of New Zealand to some degree. At low wind speeds, transpiration from the leaf’s surface is increased due to a reduction in the thickness of the boundary layer of still air. Therefore, vines use water more quickly and can come to stress more rapidly. However, as the windspeed increases, vapour pressure deficit can increase so

much so fast that the leaves close their stomata in response. As a result, the vines are actually conserving water, though the closed stomata lead to visible effects on the vine mimic water stress (i.e. reduced canopy growth) if the wind is sustained for long periods. A vine’s natural response, since it evolved trying to grow up competing plants, is to elongate shoots as rapidly as possible. However, like all plants, vines’ growth is altered in response to the environment. Even in the absence of severe wind closing stomata, the physical whipping of the canes by wind slows shoot elongation and encourages laterals so that the vine can still increase sun interception, but without endangering shoots by growing too long to risk physical wind damage.
Another common effect of wind is to cause the tendrils to shrivel up, either entirely, or at the tip. This is also often mistaken for water stress, triggering managers to turn on the tap prematurely. Irrigating doesn’t solve wind issues, and doing so just ensures that once the wind subsides, the vines have potentially excessive water. Here again, measuring water potential will tell the true status of the vine, and whether irrigation will help improve growth or leaf functioning or not.

Wind is largely out of the manager’s control, though not entirely. Windbreaks can be built/planted to protect vines. Keep in mind that living windbreaks can compete with vines near them for water and minerals. Tall winter cover crops can be retained through the spring as a wind break for newly planted vines, encouraging their rapid shoot elongation, getting them to the wire sooner. I have seen this strategy used to great effect by clever managers.
Additional resources
Deloire, A., Carbonneau, A., Wang, Z., & Ojeda, H. (2004). Vine and water: a short review. Oeno One, 38(1), 1-13.
Levin, A. D. (2019). Re-evaluating pressure chamber methods of water status determination in field-grown grapevine (Vitis spp.). Agricultural water management, 221, 422-429.
Nonami, H. (1998). Plant water relations and control of cell elongation at low water potentials. Journal of Plant Research, 111, 373-382.
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