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Overcoming the effects of spring frost

Research & Extension

Author: Steve Spark
+64 3 528 0330

Steve Spark summarises the tools available to growers to reduce the risk of spring frosts and how to use Focus Orchard trial results to reduce the impact of spring frost this year and next.

Last spring many growers across the country were severely affected by spring frost damage, particularly in the Batlow and Orange areas. Frosts are not uncommon in Australia and Tasmania was badly affected in 2006.

Gala apple trees affected by frost.

In our report, ‘Simple pruning rules improve Royal Gala performance’, Kevin Dodds (DPI NSW) and I showed that in ‘Jilba Orchard’, a Focus Orchard in Batlow, NSW, adopting “simple pruning rules” could significantly enhance fruit quality and colour leading to an $18,000 increase in fruit return per hectare. This year it was planned to continue this trial with more simple pruning demostrations, however the orchard suffered severe fruit loss from spring frosts.

In 2007, Craig Hornblow presented a talk and notes on ‘Frost risk monitoring and protection measures’ at the September Future Orchards walk. In these notes he highglighted that the damage caused by frost often depends on the growth stage underway at the time of the frost event and the severity of the frost (Table 1). As the trees move from dormant towards flowering, they become more susceptible to frost damage.

Table 1: Critical temperatures for a 10% kill from frost at various pipfruit growth stages 
Development stage Temperature
Green Tip minus 7.5° C
Tight cluster minus 3.9° C
First pink minus 2.8° C
Full pink minus 2.7° C
First bloom minus 2.3° C
Full bloom minus 3.9° C
Post bloom minus 2.3° C

Fruit are still susceptible to frost damage after flowering. Frost rings are a result of late frost damage.

Frost rings – a result of frost damage after flowering.

Prepare herbicide strip before a frost

There are many ways to reduce frost risk to your orchard. These include keeping the grass sward mown short, especially leading into tight cluster and over fruit set. The next most obvious measure to take is to ensure the strip underneath the trees is weed free. Optimum weed control at this sensitive time of year would be to ensure no green weed matter is visible and preferably the herbicide has been on long enough to ensure only brown clean soil is present. To do this you need to be well organised in early spring and have your weed spray applied in plenty of time for it to act and be at its optimum over this risky period.

At the Batlow orchard walk in November 2013, there was a very graphic example of this at work. Large Royal Gala trees that had not had time for the weed spray to take effect had little or no fruit on them, whereas the trees directly alongside the middle access alleyway, that was literally bare brown earth, were full of apples. The bare earth had trapped the sun’s warmth during the daytime and then radiated it back at night to provide enough protection to the trees to set plenty of fruit. A lesson for everyone that was there at the time.

Well prepared herbicide strip and grass mown to reduce frost risk.

Another important tool is utilising water for frost protection, if you have it. This can be in the form of overhead sprinklers that provide water over the top of the trees during a frost event. One millimetre of water will provide one degree of frost protection for overhead sprinklers. Usually overhead sprinklers can provide protection to -5°C if water application rates are high enough. The principle behind overhead water application is that when water freezes it changes from liquid to a solid giving off heat to the tissue it is freezing around. This is called latent heat of fusion and can provide a considerable amount of heat. In New Zealand, this system is used widely in Otago orchards.

A variation to the overhead sprinkler is to utilise the undertree sprinklers which offer less protection than overhead but can provide some protection for frosts up to -1.5°C. Unlike overhead sprinklers, which coat the senstive tissue in ice, the under tree sprinkler can provide moderate temperature lift from heat released by the cooling of water droplets.

Check frost protection systems work properly

Before spring, frost protection systems should be run to ensure they are working properly and pressure flow rates measured. Flush main lines and sub-laterals to get rid of any debris that might be in them from the last time they were turned on. It is often desirable to leave the system running long enough to check all the sprinklers are working properly and none are blocked.

If your frost protection system suffers from low pressure there are several methods that can be employed to get around this situation. One involves adding an additional pump to the line to boost pressure. Not always easy to do but I have seen it done effectively. The second involves pulsing the frost protection system by turning on one block of orchard sprinklers for a short time and running it for anywhere between 2 to 5 minutes. Then turn this block off and start the next block, alternating between these blocks. Repeat this cycle until the frost risk has gone. The danger with pulsing water frost protection systems is that if the temperatures are too severe, or the pulsing intervals too long, sprinklers can freeze and the problem has now got a whole lot worse. Be wary of pulsing under very cold conditions. Sometimes it is better to provide water to fight frost in only the most profitable orchard blocks if your frost system doesn’t have sufficient capacity or water to protect all the blocks at one time.

The forming ice should always remain clear, which indicates it is still making heat. Once the ice becomes cloudy, this is often associated with no more heat being produced and then freezing of the sensitive tissue occurs. This situation should be avoided as more damage can be done. Turn overhead frost protection systems on early to avoid freezing and blockages. Always ensure plenty of water is available to maintain the heat exchange process for the duration of the frost event. Once temperatures rise above freezing and the ice starts melting from the warming morning, the frost protection system can then be turned off. However this may not be the end of the risk and temperatures throughout the morning still require monitoring as temperatures can warm up, then freeze again.

If using water for overhead frost protection, ensure the trees have a suitable and sturdy support structure because once the water turns into ice, this can place a lot of additional weight on the trees and sometimes tree breakages will occur.

Another method that offers limited protection from frost damage is to apply a foliar spray of Low Biuret Urea at 0.5kg/100 litres of water at 1,000 litres per hectare immediately before a frost event. Other products may also provide protection, but trialling would be benefical beforehand.

Other frost mitigation strategies could include the removal of shelter belts, boundary trees or trees obstructing airflow across the block, particularly on the downward side of the orchard. This can improve frost drainage. The installation of frost fans can also provide good protection but are often expensive. It is common in the viticulture industry to use helicopters which, although expensive, can provide a high level of protection, but only if an inversion layer is present on the night of the frost. Be organised early.

Finally, once you have prepared your frost protection strategy, make sure you have a good forecast and follow it closely. Frosts are often readily forecastable.

Save your crop following severe frost damage

Frost damage can often lead to the start of a biennial bearing problem. Again, there have been some really good Focus Orchard trials on how to overcome this problem. The main points are to set as much of the remaining fruit as possible and slow tree vigour down. There are a number of tools available.

To increase fruit set immediately after a frost event, reduce the use of chemical thinners as much as possible, and increasing pollination and fruit set of the remaining flowers. Sometimes bringing in supplementary bee hives may help.

There are many options to slow tree vigour down. The ‘Root pruning’ report on the trial carried out in Batlow by Kevin Dodds and his Batlow community group highlighted the benefits of root pruning either on one side of the tree (single) or both sides of the tree compared to the control (nothing).

Table 2: Red Fuji root pruning trial results, Batlow Focus Orchard, December 2012.
Red Fuji root pruning Batlow FO trial Shoot numbers compared to control Total shoot length (mm) compared to control Average shoot length (mm) reduction
One side root prune -19% -13% -2%
Both sides root prune -26% -26% -22%

Root pruning on one side of the Red Fuji trees resulted in a 19% reduction in shoot numbers and a 13% reduction in shoot length compared to the control. Whereas root pruning to both sides of the tree resulted in a 26% reduction in shoot numbers and 22% reduction in total shoot length compared to the control trees.

This trial highlighted that root pruning on both sides of the tree was more effective at reducing vegetative tree growth compared to both the control and single side root pruning. This trial is being repeated again this year to determine how the trees respond to two years of root pruning strategies.

In some areas of Australia, root pruning is not a viable option due to soil borne diseases that may enter the tree through the root wounds, therefore other tools are required. One option is to apply Regalis® early to reduce ‘Gala’ shoot growth, which gives better control than late applications, as reported by Stephen Tancred (Orchard Services), in ‘How critical is timing of the first Regalis?’.

Another treatment is to limit irrigation to reduce vigour in an off year.

Getting back to normal the next year

The first winter after a major spring frost event, growers have more tools available to break the biennial cycle. The best place to start is to be aggressive with winter pruning. Sometimes this can be difficult, especially when a block had no fruit the previous season and you are keen to get as much fruit this year as possible. This is made easier if you have already put in place remedies to reduce the risk of frost this spring. This can be the start of biennial bearing if pruning is too light, so a strong conviction to prune well is required.

Start with simple pruning rules. The Batlow Focus Orchard trial showed simple pruning rules can improve fruit quality and marketable yields. Remove surplus bud numbers as some varieties require no more than 1.1 or 1.2 winter buds per fruit (‘Gala’). Other varieties that suffer more from biennial bearing, such as ‘Braeburn’, require as many as three buds per fruit. In an on-year, now is the time to prune hard by removing unwanted branches causing shading and branches with excessive vigour. Aim to leave enough prominent buds that meet your target. Remove buds that are shaded and growing on the underside of branches (6 o’clock) and also remove buds likely to promote strong vigour such as buds located on the top side of branches (12 o’clock). Be selective when pruning and space buds. Counting buds is important to ensure the trees haven’t been over pruned or under pruned. Aim for a calm canopy with wood slightly pendant.

In the June 2014 edition of Australian Fruitgrower, John Wilton wrote ‘Pruning: setting up the tree canopy for future crops’, which is worth another read.

Results of another Focus Orchard trial in the Adelaide Hills was reported on by Paul James in ‘Do summer applications of NAA and root ripping assist in reducing biennial bearing in MM106 Fuji trees’. After the previous year’s yield of 50 tonnes/ha, the untreated control trees returned 36.9 tonnes/ha compared to the treated block producing 53.8 tonnes/ha. Overall an excellent result showing attention to detail will pay dividends.

The November 2013 Orchard Walk report ‘Solving biennial bearing’ by AgFirst shows how the strategic use of chemical thinners, particularly in the on-year, can be helpful. Excessive crop loads, particularly when carried well into the on-growing season tend to cement biennial bearing tendency in place and will override most of the husbandry practices known to reduce the intensity of the biennial bearing problem. Aim to bring the crop load down to within 15-20% of the final crop load within 6-8 weeks, preferably 6 weeks or less of full bloom in the on crop year.

Aggressive chemical thinners include blossom thinning using ATS, ethephon or NAA, which may also be combined. A BA-based post-bloom thinner, depending on the cultivar, can have enhanced thinning effect if used in conjunction with another suitable thinner such as NAA.

Once the chemical thinning response has become obvious, start hand thinning in earnest, particularly on the strongly biennial bearing varieties, aiming to get the crop load close to 20% above the final crop load target. This leaves room for thinning out russet and other defects later on if required. The key message is to use winter pruning and aggressive chemical thinning strategies to get within 15-20% of the final crop loads as soon as possible.

Nutrition has an important part to play in reducing biennial bearing. Ensure in the on-year the trees have adequate nutrition as a bigger than normal crop load will remove much of the nutrients stored in the tree unless these have been replenished. Low nutrition levels can compound biennial bearing the following spring.

There are many tools available to growers from the Focus Orchard archives that can be used to overcome the affects of severe spring frosts. These articles can be found in the archives of the Future Orchards Library on the APAL website.

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