When soil is saturated with water, the soil water content is above field capacity. This is known as waterlogged conditions, as observed during floods or over-irrigation.
In light of the fact that the grapevine can be hydroponically cultivated, it is generally accepted that occasional waterlogged conditions, especially during the dormant stage, should not have a notable effect on vine growth. Waterlogged conditions do however, go hand in hand with the absence of oxygen, as all the soil pores are filled with water. There is sufficient evidence to suggest that the prolonged absence of oxygen due to oversaturated soil conditions can indeed have a negative impact, and this appears to become more evident as the growing season progresses.
During the floods that occurred in the winter and spring of the recent 2023/24 growing season in the Western Cape, several vineyards were partially or totally submerged under water during both their dormant and active growth stages. In some cases, for example in the Olifants River region, a few blocks along the banks were under water at three different stages (post-harvest, dormancy and early growth/spring). As a result of this, various challenges emerged over the course of the season. These ranged from physical damage to vineyard blocks and infrastructure, to limited access which hindered timely crop protection actions in particular. In terms of observations at vine level, uneven budding, poor growth, canopy discolouration (yellow in white cultivars, red in red cultivars) and poor fruit set were often reported.
Access roads and vineyards under water (Worcester, September 2023).
Vineyards under water (Worcester, September 2023).
Dormant vineyards under water
Short periods of waterlogged conditions during the vineyard’s dormant phase can give rise to earlier upliftment of dormancy and earlier bud break. This can occur due to the build-up of hormones such as excessive ethylene in the root zone and synthesis of other hormones in the permanent tissue (trunk and roots). Apart from the fact that buds will begin to swell and burst earlier, unevenness will be markedly visible and symptoms comparable to extreme drought can be expected.
If vineyards are exposed to waterlogged conditions for a prolonged period, the negative impact is heightened. In the absence of oxygen, roots are forced to respirate (ferment) anaerobically for energy, utilising the easily available carbohydrate reserves which will ultimately cause root hairs and feeder roots to die. This diminished and carbohydrate-poor root system will also struggle during the subsequent active root growth period which normally takes place before flowering, due to low oxygen levels and low soil temperatures as result of the high water content.
Bud break and initial shoot growth of vineyards draw on carbohydrate reserves which are stored in the permanent structures of the plant. The vine roots only begin to make an active contribution to above-ground growth around two to three weeks after bud break. If the roots are restricted by waterlogged conditions and/or even cold soils, vegetative growth will be restricted and should these detrimental conditions persist, nutrient deficiencies can also occur due to sub-optimal root function.
During the flowering and set period the demand for carbohydrate reserves is at its highest and if the available carbohydrate reserves cannot meet this demand, both flowering and set for the current season, as well as bunch initiation for the next season, can be negatively influenced.
Yellowed leaves due to wet soil in the spring (Worcester, December 2023).
Active vineyards under water
Hormones
Anaerobic conditions due to water saturation in the root zone slow down the growth rate of roots and/or can cause root tips to die back. Sustained wet and cold soils display yellowed (red in red cultivars) symptoms in the canopy and it is common to observe these symptoms until late in the growing season.
Hormone production and concentration are influenced by waterlogged conditions. Gibberellin is a growth hormone which is synthesised in the root tips and is responsible for internode elongation, bunch stem growth and it also promotes berry set. Waterlogged conditions suppress the synthesis of gibberellin through the presence of excessive ethylene. Similarly, cytokinin is mainly synthesised in the root tips – it stimulates cell division and thus plays a critical role in vine growth. Poor root growth due to waterlogged conditions will lead to a decrease in production of cytokinin and so impair above-ground growth.
Root growth can occur at soil temperatures of > 6°C, and a 10°C jump in soil temperature can double or even triple root growth up to an optimum level of around 30°C. If soils are waterlogged during the root growth stage, the soil temperature remains consistently low (due to the high specific heat of water) and root metabolism, especially of the feeder roots, is impeded.
Wet patches during the early summer due to poor drainage (Worcester, January 2024).
Salts
Salinisation of vineyard soils in some of our dry cultivation regions is a general occurrence. Contrary to the expectation that brackish salts might be reduced/leached out by the water masses caused by a river in flood, the opposite reaction is often observed. These observations are directly linked to the drainage ability of the soils rather than the quality and volume of water during the floods.
Following floods, excessive salts from surrounding saline soils accumulate in poorly drained low-lying areas. The floods allow movement and accumulation of salts which would not occur in normal (dry) conditions, concentrating brackish salts in the waterlogged, poorly drained areas. On a smaller scale, this is also observed where producers effectively force brackish salts out of the root zones on the vine row using strip wetting, and in so doing concentrate these salts in the middle row and lower soil horizons. With full-surface water saturation, as experienced during floods/waterlogged conditions, brackish salts can move back to the root zones, leading to local salinisation. This phenomenon, combined with anaerobic and cool soil conditions, can supress high levels of salts in the feeder roots and in extreme cases toxic salt levels can cause the vine to die.
Saline patches caused by flood waters (Worcester, December 2023).
Vines dying due to high salt levels (Worcester, December 2023).
Practical tips
Although it is not possible to control the occurrence of floods, it is important to drain the excess water from vineyards as soon as possible, keeping the following tips in mind:
- Ensure that drainage design and layout are done correctly for new plantings.
- Ensure that drainage and storm-water infrastructure are sufficiently maintained and regularly cleaned.
- Consider pumping out stagnant water if it is not possible to divert water away from vineyards.
- If access roads are flooded and vineyard rows are inaccessible during the active growth period, alternative methods of applying disease control should be considered, for example using backpack sprayers.
For more information, contact Callie Coetzee, Vinpro regional consultant: Breedekloof and Worcester – callie@vinpro.co.za.
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