A heatwave is a run of unusually hot days but what constitutes a heat wave will differ from region to region given the normal temperatures for that area. Inland areas can expect to be normally hotter than areas closer to the coast.
A heat wave will be a run of unusually hot days last from around 3 - 5 days depending on the location (from say 37- 40°C).
An extreme heat event will usually commence with a run of hot days (there are exceptions with a short unseasonal period of heat) with a potential day or two of extreme heat towards the end of a heat wave.
Growers cannot prevent the occurrence of heat waves and extreme heat events but they can be prepared.
Monitor weather forecasts closely and in particular look at the forecast up to 5-7 days in advance. Monitor forecast changes each day and be prepared for changes to forecasts.
The BOM weather site has a range of forecast options that can be used and are regionally based.
There is still much research to be done on the best ways to manage extreme heat events but there are some basic principals to follow.
If crops are close to harvest and the outlook is for an extreme event consider harvesting early before the event occurs. Yield may be reduced slightly with weight and crop/fruit size but this is preferable to the potential significant crop loss that may occur due to heat damage.
Pre-water crops and maintain soil moisture levels but do not saturate soils as this will also result in crop damage due to heat.
If soil moisture levels are not maintained it will be too late to irrigate on the day to prevent heat damage, and irrigation on the day during the heat event may potentially increase crop damage.
Depending on soils and location, some growers irrigate daily in summer as normal practice (e.g. on sandy soils). These crops should be irrigated at night and preferably early morning. Soil moisture levels should be maintained in the days leading up to the extreme event so that the plants have had the opportunity to take up the water and prior to the hot day.
Plants will wilt under extreme heat even if well-irrigated and at optimum water potential. It will be too late to irrigate once plants are wilting as they will be unable to take up the water fast enough with the increased risk of heat damage.
Leafy vegetables are particularly susceptible to heat damage and will wilt and lay down under extreme heat but they can recover once the event has passed if a post-event irrigation is applied immediately.
Site selection – be aware of the limitations imposed by the site selected for your vineyard. Historical records will provide some idea of the extremes likely to be encountered and local knowledge may provide useful information on how your site will perform compared with others around you.
Soil type – lighter coloured, sandy soils reflect heat back under the vines much more than darker, clay soils. The use of mulch and inter-row cover cropping (slashed down) can reduce the reflection of heat onto the grapes under the canopy.
Irrigation capacity –
Rootstocks – different rootstocks provide a range of vigour and canopy sizes, sometimes necessary to protect the fruit from sunburn which can be a common result of extreme heat. Vigorous rootstocks are commonly used with white grapes and more medium vigour is used for red grapes which benefit from some degree of fruit exposure.
Trellis – under extreme heat the fruit requires more protection than under average weather conditions and the design of the trellis can assist. Lifting the foliage wire on vertical canopies will need to be modified to prevent full fruit exposure, particularly on the west side of north-south oriented rows. Rows oriented east-west provide better shading of the fruit during the hotter parts of the day. The absence of a foliage wire (Australian sprawl canopy) may be beneficial in some cases.
Canopy management – reduce the amount of leaf plucking compared with cooler periods. Be more judicious with leaf plucking, for example, retain leaf plucking on east side but minimise it on west side of north south rows. Apply ‘sunscreen’ products to reduce the potential for sunburn.
Extreme heat can be associated with strong winds and wild fires so consider the implications of those events on vineyard operations.
Power outages – extreme heat can produce power outages either through shut downs of sections of the power grid from over use or through damage to the system by strong winds or fire. Consider the impacts of no power for irrigation pumps by pre-planning irrigations prior to the extreme heat event or have a backup power supply or other fuel driven pumps. Power outages will also have implications for wineries such as no refrigeration or power to run crushers, presses and pumps.
Personal protection – ensure you and your staff have adequate drinking water, sunscreen, protective clothing, hats, etc. available for extreme heat days.
Pulsed, short-duration irrigation throughout the day will help cool the orchard and avoid over-wetting the soil.