Weighing up the pros and cons of frost protection strategies.
Controlling frost starts with choosing a good site and layout. Making use of natural wind breaks, sunny positions and good drainage will go a long way to helping buds continue to grow and bloom. But even the best positioned orchard or vineyard is susceptible to harsh Spring & Autumn frosts, and needs additional support to minimise buds dropping below critical temperatures, where the plant tissue can be damaged.
Before looking at any frost protection solution or planting your crop, understanding where cold air drains in and out of a block is really important. Otherwise known as the katabatic drift, it acts like water flowing down a hill. Like water, if there is an obstruction such as a concrete block or row of trees, the cold air will build up and collect behind it like a dam. It’ll rise vertically too, up to 4 or 5 times higher than what it’s being obstructed by.
If crops are planted behind this obstruction, they’ll have a higher likelihood of being impacted when the temperatures drop. If frost protection is placed in the middle of a katabatic drift, it’ll affect how the warmer air circulates and covers the surrounding area (for example, due to the katabatic drift, a wind machine will cover an elliptical rather than a circular area).
Choosing which frost protection solution is right for you isn’t an easy decision. There are many factors to weigh up, and cheaper in the short term could prove costly in the long run. Some of the questions to be asking now are:
Do you have the labour to operate manual equipment?
Can you get equipment in the right position at the right time?
Do you have enough fuel on standby?
Do you have the cash flow to spend on high ticket options?
Do you have the space?
Are there any council or environmental restrictions to consider?
There are several ways fruit trees, vines and crops can be protected from frost.
- Adding heat using water and heaters
- Mixing cold and warm air together and moving it across the planted areas
- Conserving heat and not letting it escape
Each solution will raise the temperature differently, and you can use one or many solutions.
Let’s look at each.
Passive Protection
Before planting, consider all of the current items placed on your property. Buildings, houses, sheds, fencing, processing plants, hills, flood banks, equipment graveyards, mounds of dirt. Can they be easily moved to minimise any cold air pooling behind it?
Choosing late-blooming varieties can help get a jump on frosts if it can’t get to growing buds, but it does need additional work to prune the trees so the fruit grows above where the cold air accumulates.
Pros:
● Inexpensive
● Making use of existing structures to form or reduce a barrier
● No additional outlay or intensive labour resource needed
Cons:
● If placed incorrectly, structures can pool cold air behind them
● No warming aspect to raise air temperatures
● Trees need additional maintenance to prune the height upwards.
Heaters & Candles
Orchard heating is a labour intensive method of warming crops, with around 100-125 heaters or candles needed to be lit and maintained per hectare. Teams stay up throughout the night constantly checking for signs of a frost, and then need to get the heaters lit quickly when the temperature drops, but it’s falling in popularity due to the high cost and use of fuel, air pollution, and they don’t add heat very effectively.
Heaters help prevent frost pockets forming, however only 10 to 15 percent of the heat produced stays in the orchard. Mobile heaters are less effective again as they take the air away from plants as they’re towed throughout the orchard.
However, when used in conjunction with wind machines, their effectiveness is increased as the air that rises is then pulled back down.
Air quality concerns have seen heaters used for frost protection being banned in some parts of the world, and they require guesswork to determine the best time to light them, which can prove costly if you get the timing wrong and light them too early or too late.
Heaters also have high maintenance requirements, needing to be cleaned every 20-30 hours of use.
Pros:
● Increase the effectiveness of wind machines when used in conjunction with frost fans
Cons:
● Require a large workforce to light and maintain
● Costly to run
● Requires guesswork to light them at the right time
● High air pollution, and banned in some areas
● High maintenance requirements
Sprinklers & Irrigation
The biggest increases in temperatures will often come from water-based application, and can be the most economical. Heat from water is released into the environment at low temperatures, and can selectively warm the coldest part of the plants, making it a very efficient solution.
However, whether you’re looking at overhead irrigation to encapsulate the buds with ice or under tree sprinkling, environmental concerns will be a factor with water-based frost protection systems.
Irrigation methods need a continuous high flow of water - 10,000 litres water per ha per hour for every degree of frost (e.g. -2ºC frost would use 20,000 litres per hour per hectare) - so water consumption and impact on the environment is high, and you need large pipes and pumps. If there is drought in the area, water restrictions and policies imposed by local councils, or you have a pump failure, you will need to have a backup solution to avoid disaster.
You may also be looking at disease, saturated soils, runoff, and leaching of nutrients and other agrochemicals, and there is a risk of greater damage with overtree sprinkling if it’s done incorrectly and it could destroy an orchard, rather than protect it.
Undertree sprinkling uses half the amount of water, but the resulting temperature increases are also halved, and most of the heat comes from the water cooling rather than heat released as the water freezes, so warmer water is preferable which would also halve the water consumption.
Pros:
● Economical
● Efficient at raising temperatures and getting warmth where it’s needed
● Can provide some protection in advection frosts
Cons:
● High environmental impact on water use, and related problems it can cause.
● You need a back up if there is a drought or restrictions on water use.
● Pumps can fail
Frost Fans & Wind Machines
During the night, warm air rises into the air, and cold air accumulates at tree and crop level. Wind machines and frost fans pull the warm air down, and blow it across the lower layers. It mixes with the cold air and raises the temperature of the plant and soil to stop frost bedding in and damaging the buds, which means less wastage and increased yield.
Wind machines and frost fans come in a variety of options - from the sleek, quiet and innovative engineering of the FrostBoss® 4 & 5-blade towers (one of the most fuel efficient options available, with a very good coverage area and blades designed by ex America’s Cup yachting specialists), to 2 & 3-blade fans, portable fans which can be towed where they’re needed, and lie-flat machines which help mitigate any visual restrictions.
Wind machines with an automatic start option take the guesswork out of when you need to have protection in place, unlike helicopters you need to book in advance, manual labour on hand to get heaters lit and monitored throughout the night, or irrigation running in time.
Frost fans like FrostBoss® have built-in hardware and sensors, which are compatible with their monitoring apps so you can remotely see in real time whether the fan is armed, how long they’ve been running for, what the ground temperatures are, and allow you to export the data, which comes in handy if your local council requires you to produce usage information to comply with consents and permits.
They work best in a radiation frost where there is warmer air to pull down, however they are less effective in advection frosts where the majority of the air is cooler. Supplementing with heaters helps to keep warm air distributed and temperatures raised.
Blade choice is important. A twisted propeller style with the correct aerodynamic pitch all the way from the tip to the root of the blade keeps a more even distribution of air across a greater area, and the higher number of blades reduce the ‘whack whack’ noise. Turbulence created by fewer blades running at higher rpm can create more movement through the tower, which increases the need for servicing and repair.
Pros:
● Reduces human error
● Saves times
● Excellent coverage and one of the most efficient options for frost protection.
● More affordable than helicopters over the long term.
● Components such as blades and engines can be upgraded over time
● Can be supplemented with heaters to support warm air distribution
● Some have compatible apps to monitor the fans
Cons:
● Wind machines aren’t created equal, with varying quality, output and coverage across manufacturers
● Not as effective in advection frosts
● Need to be sited effectively for greatest coverage and least noise or visual pollution
● Fewer blades can create greater turbulence and noise
Helicopters
Performing a similar role to frost fans, helicopters can be deployed to pull the warm air down and over the crops. They can cover a much larger area than wind machines so can be very effective at getting to the areas needed, but they are expensive to hire by the hour, and may not mix the warm and cold air as efficiently as a frost fan.
Booking helicopters to be in the air at the right time takes guesswork and keeping a good eye on ground temperatures and detailed forecasts. Getting them in the air too early, too late, or not at all if there’s an unexpected frost means not having coverage in the air when the frost hits. This guesswork can prove costly if you get it wrong, paying for helicopters when you don’t need it, or frosts getting the opportunity to settle and do damage to your crops.
As well as runtime, you’ll need to factor time in to have the helicopters on standby, and in colder weather, coverage areas will reduce as temperatures drop and less warm air is available to circulate.
If you have neighbours nearby, noise pollution needs to be a consideration. Helicopters produce more noise and need to hover for several hours for effective frost protection.
Pros:
● Cover a large area
Cons:
● Very expensive to run or if mistakes made
● Requires guesswork to get the helicopters in the air at the right time
● Not as efficient as wind machines
● Very noisy
● Coverage is reduced in colder weather
If you’re interested in learning more about the innovative FrostBoss® range of frost protection solutions, get in touch to see if a frost fan is the right solution to protect your investment.
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