Be prepared for frost
Note Number: AG1424
Published: September 2011
Reviewed: August 2013
Warm winters may result in an earlier spring budburst and this can lead to a higher risk of frost and damage to vines and fruit crops. Growers need to be prepared to manage the risk of frost and monitor the weather to assess the likelihood. Frost damage can severely reduce the current crop with direct impacts on flowering and vegetative growth. Frost damage may also have an affect on production in the subsequent year.
This note does not provide recommendations on frost protection systems but highlights the issues and actions to consider in managing the risk of frost.
How can I be prepared?
- Watch the weather and understand the risk
- Prepare the orchard or vineyard by ensuring the soil is moist and applying an irrigation if necessary
- Ensure that there is good air drainage and well compacted bare ground between the rows
- Control and slash cover crops and spring weeds
- Monitor temperature and use frost control methods if available
- This coming season has the potential to be a high disease pressure year for grapes if damage does occur consideration should be given to removing any frost-damaged tissue to keep the canopy open and remove any source of inoculum.
How does frost occur?
A frost occurs when the temperature at ground level falls to 0°C, however most temperate plant species will tolerate this and it is not until temperature falls below -2°C when water moves out of the plant cells resulting in death and desiccation of the tissue. The lower the temperature and the longer the longer the period below the critical temperature, the worse the damage to the plant. Plants do vary in susceptibility and this is associated with the growth stage of the plant.
Soil in the orchard or vineyard goes through a cyclic change. During the day the soil heats up and becomes warmer and at night heat is lost from the soil and to a lesser extent the trees and vines. Whether the soil surface and surrounding air falls to0°C or lower depends on the:
- amount of heat stored in the soil during the day
- amount of heat lost by radiation at night
- flow of heat form the soil to surface or plant surface (if heat flow cannot keep pace with radiation loss then temperature drops)
- moisture content of the air.
Heat loss by radiation is highest on a clear still night, while clouds have a blanketing effect and wind will mix the air layers bringing warmer air down. The change from water vapour to water (dew) gives off heat and as the temperature cools at night the temperature of the air in contact with soil and the plants may fall below the "dew point' causing moisture to condense and form dew. This gives off heat and retards the temperature drop. If the temperature keeps falling at 0°C it freezes and heat is again released as dew changes to frost. If the temperature continues to fall water in the plant cells freezes and ruptures the cell walls killing the tissue resulting in the characteristic burnt appearance of the plant. A combination of dry soils and more clear spring days and nights will significantly increase the risk of frost.
Patterns of frosty nights
On a calm, clear night, the coldest air is in contact with the soil surface because colder air is denser than warmer air and temperature increases with height. The height at which the temperature stops increasing and begins to decrease is the inversion level.
In hilly or undulating terrain the cold air near the ground flows down the hill pooling in hollows or depressions. These topographic influences are consistent and when planting it is important to avoid them or use the drainage of the slope to advantage.
Understand the risks
It is important to recognise the conditions that may lead to frosts and where the effect is most likely to be the worst in the vineyard or orchard.
- Frosts will typically occur from a weather pattern of a cold, cloudy day followed by a still and cloudless night, for the soil will cool quickly after sunset and has had little opportunity to heat up during the day.
- Be aware of the likely frost pockets on your property and avoid planting in them or take action to protect the plants.
- Dry soils will not compact as well, will not absorb and hold as much heat during the day and it is more difficult for heat to flow out of the soil at night.
- Standing cover crops, weeds or mulch will act as insulating layers preventing heat getting to the soil during the day and preventing heat flow out of the soil at night.
What can I do?
There are a number of management practices, which can reduce the risk of frost.
- On frost-prone sites remove any barriers which may impede airflow down a hill and cause cold air to pool e.g., shelterbelt, long grass along fence lines.
- Monitor low-lying areas by checking where fog patches lie.
- Late pruning can assist for vines (particularly in frost prone areas) and leave additional canes, which will provide additional options for shoot selection in the advent of a frost.
- A moist compact weed free soft surface is key to effective control as this will maximise the heat absorption during the day.
- If there is a cover crop, it should be mown close to the soil exposing a large area of moist soil.
- Rolling may be beneficial to help compact the soil but it must be moist as there will be little benefit of compaction if it is too dry.
- If necessary apply an irrigation well before a likely frost event.
- Place temperature loggers in the vineyard/orchard and check the temperature at around 3.00 pm. This will help in the assessment of likely frost potential.
- Loggers can also provide warnings of critical temperature point and be used to activate irrigation as frost management.
- Consider insurance for frost prone areas but costs needs to be balanced with the risk.
Also, remember the activities of cultivation and rolling need to be balanced with the need to avoid excessive cultivation and soil compaction. Also not every frost event is the same.
Spring weed control
If spring weed growth is vigorous it will add to the frost hazard and may compete with vine or tree growth.
- Rotary slash as close as possible to ground level preferably when the weeds are still small and the soil is moist; this should be completed by midday to maximise absorption and storage of afternoon heat and timing with the likelihood of rain will improve compaction or
- Very lightly rotary hoe the mat of weeds when still small and immediately roll or
- Spray the regrowth when small with a knockdown herbicide, which will expose bare soil.
Frost control methods
There are a range of frost control methods that can be applied including:
- Irrigating the crop before and during the frost event so as the water freezes heat is released and the temperature will not drop below 0°C but it must be continuous throughout the event,
- Frost pots,
- Wind machines or helicopters to mix the warmer air down to the crop.
All these methods have some positives and negatives. For more information see the references below.
Dealing with damage
There are no specific guidelines for pruning damaged crops for it will depend on the amount of damage and the cost. Pruning will open up the canopy and it may be important to remove damaged tissue which can be a source of inoculum, this can be important if disease pressure is likely to be high.
The options for post frost management include:
- Taking no action
- Removing only the visibly damaged material
Frost Fact Sheet, 11 Nov 2010, Dr Joanna Jones & Dr Steve Wilson.
Arming against frost Fact Sheet, September 2010 on the Grape and Wine Research website.
Review of frost mitigation techniques and effectiveness – Available on the Horticulture Industry Network website and enter frost in search. This will also provide some other information.
This Agriculture note was developed by Farm Services in September, 2011. Reviewed August 2013.