By Kathryn Carter, OMAFRA
With hot temperatures continuing and below seasonal precipitation in many areas of the province, we are starting to see some symptoms of water stress in grapevines. For those that haven’t seem symptoms of water stress before, there are some excellent photos of water stress symptoms at How Grapevines respond to Water stress.
If possible extreme drought stress should be avoided as it can have significant negative impacts on vine growth, plant health, and crop quality/yields. In vineyards where irrigation is available, growers should be monitoring to determine the need for irrigation.
Determining When to Irrigate
There are several methods to evaluate the need for irrigation including soil moisture monitoring, monitoring moisture stress in plants, evapotranspiration, historical guidelines and soil moisture/vine stress.
Soil moisture monitoring
Soil moisture sensors are an inexpensive and relatively easy way to evaluate the amount of water available to the plant. Soil moisture sensors can help in determining when to irrigate, and when to turn off the irrigation to avoid wasting water. For more information on using soil moisture probes see Monitoring Soil Moisture to Improve Irrigation Decisions. Soil sensors give an indication of the amount of soil moisture available in the soil, however some soils (ie. clay) may not readily release the water to the plant overestimating the amount of moisture available to the vine.
Monitoring water stress in plants
Evaluating water stress in plants is the most accurate methods of evaluating vine water stress. Sensors that evaluate water stress in plants (ie. pressure bombs, NDVI software for use with drones, or microsensors), provide more accurate information on water stress levels within the vine. Currently these are not practical options for most growers as they can be expensive and are not user friendly, but as technology continues to advance, these may become more viable options in the future.
Evapotranspiration (ET) or weather-based irrigation, calculates irrigation needs on a water budget approach. ET is the total daily loss of soil water by transpiration from plants and evaporation from the soil surface. Each type of soil will hold a certain amount of water reservoir and this reservoir is reduced on a daily basis based on the daily ET and crop maturity level (young plants use less water than fully grown). Precipitation will add to the soil water reservoir. In the spring when your soil is saturated or after irrigating to the full rooting depth your soil water reservoir is full. The daily ET (measured in inches or mm of water) for fully grown plants is reduced from the reservoir total and must be replaced during an irrigation event.
For a detailed explanation of Irrigation Scheduling using Evapotranspiration please see page 39 in the OMAFRA publication Irrigation Management Best Management Practices.
Estimating water requirements for overhead irrigation based on historical data, is not the most accurate method of determining irrigation needs since water status for grapes changes day by day depending on the weather conditions. Historical weather guidelines do not account for these variations and can result in either under or over irrigation.
Soil moisture/ vine stress
The amount of moisture in the soil can be approximated by using the hand feel method. Dig down to a depth of 30 cm, if the soil is still moist and you can readily ball soil up in your hand, there is probably adequate moisture. Vines can also show visual signs of stress with wilting leaves and lack of shoot growth and leaf drop. Physical symptoms of drought stress may not appear until after photosynthesis has been affected, limiting the ability to respond in a timely manner to water stress.
Deciding How Much to Irrigate
The timing for initiating irrigation will depend on the type of irrigation is used. The threshold for initiating drip irrigation is lower than that of overhead irrigation, since drip irrigation can only apply small amounts of water at a time, and it is more difficult to catch up during dry conditions. Drip irrigation often involves frequent irrigation applications of small amounts of water, while overhead irrigation involves few applications with a larger amount of water/application. Drip irrigation is 90% efficient as compared to overhead irrigation which is 60% efficient, so this should be accounted for when calculating your irrigation application rates.
Keep in mind that the majority of moisture is extracted from the upper portion of the root zone. Irrigations that result in water moving down past 24 cm will result in wasted water and can cause issues with soil pathogens.
Tips on Reducing Water Requirements for Vineyards
Whether you are a grower with irrigation and are looking to reduce your water use, or someone that doesn’t have access to irrigation, there are some opportunities to reduce water requirements in orchards.
Canopy size and shape impact the vine demands for water. Avoid canopy management practices that encourage excess vigour, as this increases the demand for moisture. Reducing the crop load on the vine can also decrease the water demand. Leaving wind breaks near the vineyard can help reduce the demand for water by the vines.
Avoid over fertilization as this can result in excess growth, resulting in an increased need for water. Mulch can help increase soil moisture holding capacity, and reduce the soil temperature, resulting in lower water demand. Keep cover crops or sod mowed as they can compete with the crop for soil moisture. Maintain a weed-free strip to minimize competition between weeds and the crop.
Where irrigation is used, target the water to the vines to minimize wasting water. Using drip irrigation as opposed to overhead irrigation can improve water efficiency and reduce unnecessary water use.
Grape vines need access to adequate moisture to ensure adequate vine growth, vine health and fruit quality. When developing an irrigation strategy for your vineyard, ensure that it meets the crop needs. There are many different strategies available to improve water efficiency in vineyards.
Pool and Lakso, 2000.Recognizing and Responding To Drought Stress In Maturing Grapevines
Fiola, 2020. Drought stress, vine performance and grape quality.
Martinson and Lakso, 2016. How grapevines respond to stress.