What is frost?
There is a lot of science behind frost, but in basic terms, frost is the formation of ice crystals on plants which more importantly can lead to the freezing of internal sap. Frost is the result of climatic conditions that lead to sub-zero temperatures. There are two main types of conditions that can lead to a frost:
- advective conditions — when a cold mass of air moves over an area and is often associated with wind and cloud cover
- radiation conditions — when heat is lost through radiation and an inversion layer of cold air near the ground surface with warmer air higher up forms and is often associated with still and cloudless nights.
In the South West of Western Australia, the main frosts we have to contend with are radiation frosts.
When does frost occur?
In Western Australia’s main avocado production regions in the South West, frosts most commonly occur in spring. These are usually only mild to moderate events with temperatures usually getting just below 0°C and only lasting for a short period of time around dawn.
In higher frost risk areas, they are more frequent, get to lower temperatures and last longer. The earlier in the evening a frost starts, the longer the conditions will last, the lower the temperature will get and the more damage that is likely to occur to plants.
Frosts can also occur in late autumn (rare) and winter. In the main production regions, mild spring frosts are reasonably common. However, stronger frosts do occur from time to time. In June 2006, the South West had a severe frost with temperatures recorded below -5°C in some areas and lasted for several hours.
Certain climatic condition combinations can lead to frost. Cloud cover helps to trap warm air closer to the ground while wind helps to mix the cold air with warmer air thus minimising the drop in temperature near the ground.
Expected conditions of no wind and clear night skies during cool periods are indicators of likely frosts, particularly if they follow a few days of cold cloudy days. As a result, ‘radiation frost conditions’ can often be predicted.
The avocado tree and frost
Surprisingly, for a subtropical tree, the avocado tree can handle mild frost conditions — conditions that just get to 0°C and only last about an hour. Anything much more than this and damage starts to increase. As such, avocado trees are rated as being frost-sensitive and measures should be taken to either choose locations that are frost-free, or take measures to reduce the risk or severity of frost events.
Frost freezes the sap in avocado trees, which results in cell damage. A mild frost might only result in minor damage to the outer leaves of the tree, with little impact on production. A severe frost may damage leaves, fruit and branches a metre or more deep into the canopy of larger trees and kill younger trees, resulting in significant production losses.
Such a significant frost event can lead to severe biennial bearing in avocado trees. In June 2006, the South West of Western Australia had a severe frost with temperatures recorded below -5°C in some areas and lasting for several hours. This resulted in severe damage to leaves, fruit, branches, flower buds and death of young trees.
As this was a widespread event, the industry has suffered from severe biennial production fluctuations since. Western Australian avocados – industry overview provides information on avocado production in Western Australia.
The result from a frost in the winter of 2006 suggests that developing avocado flower buds might be sensitive to cold. Trees that appeared to have little obvious damage from the frost had a very poor flowering. This was complicated by the trees carrying a heavy crop into the 2006 flowering period which might have contributed to reduced flowering. However, the trees generally had put on good growth and showed early signs of high flowering potential before the frost.
How to reduce risk of frost
The obvious first and best way to reduce the risk of frost is to choose a location that has no history of frost. Unfortunately, the combination of suitable soils, climate and water are not always on offer. If you are forced to consider a region/block that is known to suffer from occasional frosts, ensure that it has some slope to it and avoid planting in the low points.
There are two relatively simple and inexpensive options available when planning an orchard to help reduce the severity of frost events, design of the orchard rows and under-tree irrigation.
Design of orchard rows
Radiation frost events occur as a gradual build-up of cold air due to the loss of heat through radiation. Cold air is heavier than warm air and as such will ‘flow’ down slopes and settle in the lowest areas, progressively ‘creeping’ up the slopes as more cold air accumulates.
Anything that prevents the flow of the cold air will cause build ups of cold air at that point. For example, a solid row of trees across a slope will restrict the flow of cold air and result in a build-up of the cold air and a likely localised frost event.
When planning your orchard, run your rows up and down the slope to allow for maximum air drainage if you are in a frost risk area and do not plant in the lowest areas. Should this require rows to run in an east-west direction and you are concerned about poor light interception on the south side of the trees, you can compromise the direction to benefit both needs.
But there must be some downward slope to the rows, they should never run directly across a slope and should not trap air in pockets along rows. Gently undulating slopes can be tricky as they can lead to localised pockets of cold air if row design is not carefully planned.
In places where frost is a regular occurrence, such as some regions in New Zealand, growers have used this ‘cold air flow’ knowledge to their advantage. They build solid windbreaks around their avocado blocks to reduce the amount of cold air that can flow into the block, and in some cases use heaters internally to help offset temperature loss within the block. (The windbreaks also help to trap this warmth in.)
This set-up can be used with under-tree sprinklers that do not overly wet the plant foliage. Radiation frosts are the result of gradual loss of heat. Water by nature is more than 0°C, so by distributing this under trees you are providing warmth to the surface.
As water freezes it gives off heat, so by providing water to freeze, you are generating heat. However, water cannot flow when it is frozen, so any under-tree irrigation systems must be started before the temperature gets to a point when the water freezes in the pipes or emitters.
Most growers have these automated with temperature sensors and operate short bursts of irrigation through the orchard until the temperature rises again.
Over-tree sprinklers can also be used, but must operate continuously until all ice has melted. This can use a lot of water and is normally only used on young trees in Western Australia.
Essentially, you need to carefully look at the potential income losses to a frost event, the likely occurrence of such events and the cost of frost prevention measures. Some measures to reduce frost severity are reasonably low cost, while others have quite significant up-front and operating costs. Some frost reducing measures will only be effective against mild frosts.
So if you are planning to try and protect against severe frost events you will need to investigate the anticipated effect of the control measure to determine if it is likely to be effective to the level you want.
It may be that you have to use a more elaborate and expensive measure, or you may be able to combine two or more methods to achieve the control measures you would like to achieve. The cost to totally remove the risk of frost damage, if indeed measures exist that can provide this, may just be too high to be economical. So in the end, you may have to put up with an ‘acceptable’ level of risk.
A range of measures can reduce the severity of frost events. Many of these only offer protection against mild to medium severity frosts, say -2ºC lasting for only a couple of hours.
As the potential frost events get worse, the measures to control generally become more elaborate and more expensive to set up and operate. Some measures available need to be planned for at orchard design stage, while others can be added later.
Measures to help reduce severity of frost damage
- Orchard site selection — choose a frost-free site with good air drainage.
- Orchard design — align your rows to run up and down slopes rather than across slopes to allow effective air drainage.
- Under-tree irrigation — irrigation pulses to try and keep the temperature above zero for as long as possible, essentially relying on the heat of the water and heat generated from freezing.
- Maximise tree health — a healthy, leafy tree will generate some internal heat and reduce the depth of frost burn into the tree.
- Weed control — a strip of bare, moist earth under and around the trees will absorb and release heat more than a weedy or dry site.
- Irrigating the day prior to a suspected frost event — moist soil will absorb and release more heat than dry soil.
- Frost covers for young trees — side and top cover is required to protect from frost, trunk guards can help prevent total tree death by protecting the main trunk.
- Frost reducing chemicals — sprays of copper, Seasol®, Envy®, Wilt not® and similar all have claimed abilities to reduce the severity of frost damage.
- Air drain frost wind machines — suck the cold air at ground level and force it up into the stratification layer.
- Frost wind machines — pull warmer air from the inversion layer down and distribute it about the trees to raise the temperature around the trees.
- Orchard heaters — artificially heat the air about the trees within the orchard.
- Overhead irrigation — relies on the fact that heat is released when water turns to ice, must have ice continually forming for true effect.
What to do after frost damage
Immediately after a frost event the main thing to do is carefully monitor your soil moisture. As the tree may have been significantly damaged, the amount of water it will be using will be reduced. The last thing you need is to damage the root system as well through waterlogged conditions.
Initially all you can do is to let the trees start to recover naturally. Once they have started to re-shoot in spring, then you can begin the task of removing dead wood and shaping/balancing the tree.
If your trees have been opened up a lot resulting in exposed framework branches and early spring is looking to be hot, then it would be a good idea to consider sun protection. This can be achieved by applying a 50:50 mix of water and water-based white paint to the upper side of the main framework branches.
As for any fruit that the tree is carrying, you need to wait and evaluate the level of damage. Any fruit that is significantly damaged will likely drop over a week or two. This fruit should not be gathered for sale as it will likely have suffered chilling injury and present to your customers with grey pulp or blackened vascular strands. Fruit that hangs on should be checked for quality before sending off to market. Moderately damaged fruit might have a damaged seed coat and result in reduced fruit growth.