The science of frost and frequently asked questions

Clear night skies are important to the development of frost events because they allow thermal radiation from the soil surface to escape freely to space. The rate of cooling and final temperature of the plant canopy is determined in part by the balance between thermal radiation emitted to space and radiation absorbed from the soil. A darker, heavy soil with some moisture will transmit more energy to the canopy than a lighter, low density soil. This is why frosts tend to be more frequent on sandier soils, assuming there are no other landscape influences.

Local topography is also important, as cold air tends to run down slopes and drainage lines, and will pool in flats and basins. Barriers such as tree or fence lines can impede flow and allow cold air to accumulate higher in the landscape.

The severity of the frost and hence the extent of the subsequent damage is therefore variable across the landscape. Topographic and managment practices aimed at mitigating the severity of frost concentrate on influencing the heat balance or drainage of cold air. While these practices may help for lighter frosts, severe frost events of long duration are likely to overwhelm such measures.

Frost: frequently asked questions

Note: This information is also available as a document to download - please refer to Frost: frequently asked questions pdf in the links section of this page.

At what temperature does frost occur in a cereal crop?

This can depend on crop type, stage of development and if canopy is wet. It also depends on how low the temperature gets and for how long. Frost is a three stage response, with damage increasing for each stage.

1. Cold damage: occurs when plants are exposed to temperature less than 5°C down to -2°C. If this occurs during pollen development (Z39 - 45) it can cause spikelet damage.
2. Desiccation damage: occurs when ice forms on the outside of the leaves at temperatures from 0°C to -2°C. Moisture is drawn from the leaves leaving them dry and brittle, subsequently dying at the tips.
3. Freezing damage: usually occurs at temperatures below -2°C when there is rapid ice nucleation and ice crystals form within the tissue. The ice crystals physically rupture cell walls and membranes within the cells causing physical damage. Damage can be seen once thawed as dark green water soaked areas. Ten days after a frost event bleached leaves, stems, heads and reproductive tissue might be evident. 

Dr Ben Biddulph: What is frost and contributing factors?

Why does rain make it worse?

A canopy that is wet from a light shower of rain is often more prone to frost damage. This is because rain contains ice nucleators predominately bacteria and dust. These predominately Pseudomonas sp. bacteria are present in the WA cropping landscape on our decaying stubbles and in the light showers of rain which often fall before a frost (Bekuma et al. 2021; Biddulph et al. 2021). With these very active ice nucleators, water and plant tissue can freeze at plant temperatures around -4°C, well above what they would normally freeze at, (~<-8°C) causing significant yield loss. Without these ice nucleating agents, a plant may remain supercooled with no damage to the reproductive and vegetative parts. Even frost-sensitive plants grown under a glasshouse environment can supercool to between -8°C and -10°C with mild injury (Lindow 1983).

Past frost and stubble management trials have found an increase in frost severity, duration and damage with stubble retention in the WA farming system (Jenkinson et al. 2014; Smith et al. 2017 and Biddulph et al. 2019). The current recommendation is to manage and reduce stubble loads so at seeding they are at around the yield potential of the environment. For example in a 2 t/ha environment reduce the stubble loads at seeding to ~2t/ha.

Preliminary results from more recent work show rainfall before frost events and retained stubble appeared to increase frost damage due to greater activity of biological ice nucleation activity. This causes freezing damage to occur earlier in the night resulting in more terminal damage. In-field thermography after head emergence indicates stubble retention causes ice to form first on the stubble and crop residue in the interrow, before moving to the older senesced leaves and up the plant to the head. Simulated rainfall containing Pseudomonas ice nucleating proteins applied before frost events increases, but a wet crop by itself did not increase frost damage. Spring rainfall has biological ice nucleation activity from Pseudomonas sp, with the highest activity in light rainfall events <10mm which fall in the late afternoon/ early evening. For more information on this please see Bekuma et al (2021) and Biddulph et al (2021). Research is ongoing to determine if we can manage and reduce frost damage by managing the ice nucleating bacterial populations inseason. 

When is the crop most susceptible?

Cereal crops are most susceptible to frost damage during and after flowering but are also susceptible from stem elongation throughout grain filling.

Wheat susceptibility to frost

Pulses and canola are particularly susceptible during pod filling where affected pods have absent, mushy or shrivelled and distorted seeds. 

What does wheat frost damage look like?

The nature of frost damage depends upon the plant development stage at which the frost occurs.

Two guides has been published to assist growers to identify frost damage and consider crop management decisions. Download from here or the link in the right menu

• Frost Identification Guide for Canola and Pulses
• Frost Identification Guide for Cereals

Grain

Frosted grain at the milk stage is white eventually turning brown, with a crimped appearance. It is usually spongy when squeezed and does not release a milky dough substance. Frosted grain at the dough stage is shrivelled and creased along the long axis, like a pair of pliers has crimped the grain in the middle.

Flowering

Frosted anthers are white turning a dull brown colour, ovaries turn a dull brown and are spongy when squeezed. They begin to shrivel as no grain is developed. Also the head will be underdeveloped and/or have bleached florets

Stem

Pale green to white ring on the peduncle (the stem below the head), or between the internodes which can lead to a crimped, cracked/blistered appearance with a rough texture. The damaged area may turn white/brown and the head/stem may bend over.

Refer to the following MyCrop diagnostic pages for more detail and images;

• Diagnosing stem and head frost damage in cereals
• Diagnosing frost damage in canola
• Diagnosing frost in field peas
• Diagnosing frost in narrow-leafed lupins 

How do I check for frost damage?

When

Inspect crops when they are between ear-emergence and grain-fill, after the temperature drops below 2°C (screen temperature). Damage is usually most evident 7–10 days after a suspected frost event.

Where

Examine the crop in more susceptible lower parts of the landscape first and if the crop is damaged proceed to higher ground.

How

Walk through the crop and examine a whole plant every 10–20 paces.

If the head has not emerged from the boot, check that the developing head has not been damaged. You will need to carefully dissect the plant from the top down to find the head of the plant inside the leaf sheaths.

If the crop has flowered, open the florets to check if the grain is developing.

After a frost event, tag a few heads with tape and note the stage of development. Return a week later to determine if head/grain development/ grain filling is continuing.

To check for stem frost, remove the leaf sheath from around the stem from the flag leaf to the roots checking for a pale to white ring, shrunken or wrinkled/blistered appearance. 

Is stem frost damage terminal or as bad a flowering frost damage?

Stem damage may not be as bad as flowering frost damage, provided there are viable grains and mild weather conditions during grain filling. The stem structure is similar to a bunch of straws where water and nutrients travel to the head/grains. Not all straws may be affected, allowing water and nutrients to still reach the head/grains. Dye can be used as an indicator of how much damage has occurred.  To demonstrate this, cut a plant at the base and place outside in a mixture of water and food dye for 24 hours. The ease of which the dye travels up the stem will give you an indication of damage.

Normally it's the xylem (capillary tubes which transport water, the phloem which transports the sugars can rebuild) that are the issue, particularly if it’s hot or water stressed. The heads can lose the ability to maintain evaporative cooling because of restricted water flow and can over heat. This can result in white bleached heads after a hot day. Lodging of cereals can also be an issue if windy conditions occur during late grain filling stages. 

Stem frost damage late in grain filling can be confused with root diseases such as crown rot, rhizoctonia or septoria. To determine the cause of damage, remove the leaf sheath from around the stem from crown to peduncle to check for a pale to white ring, shrunken or wrinkled/blistered appearance. 

Do all crops respond the same to frost?

All winter grain and oilseed crops are susceptible to frost. It is therefore important to consider less susceptible crop species for frost-prone paddocks. The order of susceptibility for cereals is (most to least); durum, triticale, wheat, barley, cereal rye and oats. Wheat is more susceptible then barley at flowering, but barley is as or more susceptible during grain fill. Field peas are the most frost susceptible pulse crop followed by faba beans and lupins. Canola is susceptible to frost, with the most sensitive time from late flowering (90%) to the clear watery stage (about 60% moisture). However, due to its indeterminate nature, canola has a good capacity to recover from frost, given a favourable finish. Canola is an expensive crop so careful consideration needs to be made for frost prone paddocks. Grain and oaten hay crops are the least susceptible to frost. Oats are less susceptible to frost during the reproductive stage than other cereals and as hay production requires biomass, reproductive frost damage will not reduce yield and may in fact improve the quality of the product through the mobilisation of sugars. Pasture rotations are also lower risk enterprises.

Why do crops with high yield potential get frosted more?

Position in the landscape influences temperature variations, frost damage and yield more than management practices. High production areas in a paddock are often lower in the landscape with the increased moisture and better soil types promoting higher yields. Current frost research indicates that crops sown with high seeding rates, high nitrogen and higher yield potentials may be more susceptible to frost. It is thought that high inputs creates denser canopies which shades the soil, minimising soil heat retention and the ability of the soil heat bank to buffer the frosts. There is also more synchronisation of the canopy development so a greater portion of the canopy may be exposed. As a result these crops can experience greater frost severity, duration and damage compared to crops grown with more conservative approaches.  Despite this conservative nitrogen strategies are only recommend for the more severe frost prone parts of the landscape as the opportunity cost of these strategies often out ways the direct cost from frost damage.

Do all wheat varieties respond the same under frost?

All wheat varieties are susceptible to frost however their risk profile during flowering can differ. The frost performance values provided on the National Variety Trial website gives an indication of a variety's risk to frost damage during flowering. Variety choice and time of sowing is a major driver of variation in yield and is still the most reliable way of reducing yield losses from frost. To minimise the impact of frost, first select varieties adapted to your region and then match to the appropriate sowing time to ensure an optimum flowering period. Sowing the correct variety early can lengthen the growing season and deliver increased yields. However, when sowing early, it is critical to choose a variety that flowers during the optimum flowering window (refer to Flower Power tool in WA or local agronomic material).

Consider using multiple varieties (with different flowering times) to target flowering throughout the optimal flowering period for your location to minimise the impact of frost. This can decrease the impact of sporadic frosts that occur within the optimal flowering window in some years.

How do you use the frost performance values on NVT?

All wheat and barley varieties are susceptible to frost. The Frost Values (FV) provided on the National Variety Trial website is an indication of a varieties risk to frost damage during flowering. Using these values enables the direct comparison of the relative flowering frost susceptibility between multiple varieties. The lower the frost performance value, the lower the frost susceptibility of that variety. A new variety should be managed in terms of position in landscape, paddock selection etc, based on a growers experience with a known variety with similar FV. However, there is no economic benefit in selecting less susceptible varieties for the whole cropping program in the absence of frost.  

What is the soil heat bank?

The soil heat bank refers to the amount of heat absorbed and retained by the soil during the day. This heat is then radiated back into the crop canopy overnight to warm flowering heads, minimising frost damage. The amount of heat stored depends on a number of factors such as row spacing which affects canopy closure, soil colour, stubble loads and soil moisture. A moist soil profile will store more heat than a dry soil.

Sarah Jackson: The soil heat bank

How does stubble loads affect frost severity?

Research has shown that by reducing stubble loads, there is a reduction in the severity and duration of frost events, resulting in less frost damage and better yields under frost conditions. No differences were observed between stubble height, orientation or composition. Data to date suggest it’s mainly a load issue.

Stubble reduction can be achieved by various management strategies including: cutting low, windrow burning/chaff carting, stubble mulching, raking and burning, strategic blanket burning and summer grazing. In terms of minimising frost risk, a good rule of thumb is that stubble loads should match target grain yield. For example, in a low production environment, 2 t/ha grain yield potential equates to 4t/ha of stubble remaining after harvest. The stubble load needs to be halved prior to seeding, back to 2 t/ha to minimise frost risk. In a medium production environment with 3t/ha grain potential equals 6t/ha of stubble remaining after harvest. This can be reduced to 3 t/ha to manage frost risk. This calculation assumes a harvest index of 0.5. Current research shows that without frost, once off stubble reductions after opening rains at or just prior to seeding did not reduce yield and in the absence of frost often give a slight improvement in yield due to reduced disease and less nitrogen tie up, depending on site, season and variety choice. With multiple severe frost events, stubble reduction does not increase yield in the most frost prone parts of the landscape, but may in the moderately prone areas (Smith et al. 2017).  

Does sowing direction impact frost severity and duration?

It is recommended that growers use current best practice for weed control and operational management when sowing paddocks. However, in relation to reducing frost risk the theory is that more sunlight, therefore more heat, reaches the soil surface in north south sown paddocks compared to those sown east west. From our research, changing sowing direction from east-west to north-south did not change the severity or duration of frost events. There was no advantage in yield or grain quality from either treatment. East west sowing is currently used as an integrated weed management tool.

What is the effect of nutrition; nitrogen, potassium, copper?

The relationship of crop nitrogen (N) status and frost severity and duration is complex. This is the focus of ongoing research. To date there is no strong evidence that N softens wheat to frost. The 2016 WA frost trials concluded that managing wheat varieties had a greater impact on frost risk than varying nitrogen and seed rates. 

However, it is suspected that high N rates promotes increased synchronisation of canopy development, head emergence and flowering. If a greater proportion of the canopy is flowering all at the same time, this will potentially increase the frost risk, as the whole crop will be vulnerable at the same time. Nitrogen is also a key financial input and there is often no yield response to nitrogen in frosted crops, so the financial risk of nitrogen application needs to be managed in high frost risk/prone parts of the landscape.

Adequate potassium (K) fertiliser application is important for reducing the effects of crop stress on grain yield. Potassium deficiency results in poor water use and uptake of other nutrients, making crops more susceptible to drought, waterlogging, frost and leaf diseases. Based on 21 K experiments since 2011, it has been shown that K fertiliser provides added protection for cereal crops against crop stresses such as drought and frost especially if on marginal light sands (low in K <50ppm) (Ma et al. 2019) and there is still a yield response to K fertilisers with frosts. Current recommendations are to maintain adequate potash levels based on soil and tissue tests, particularly on marginal frost prone sandy soil types.  Luxury levels have not been shown to reduce damage economically; however work is ongoing in this field.

If a crop is deficient in copper (Cu), correcting with a foliar spray at booting is economical with and without frost. Luxury or normal rates of Cu have not been shown to reduce damage in Cu adequate crops (based on tissue testing). The symptoms of copper deficiency are similar to frost, often leading to misdiagnosis as frost damage. Whole-top plant test provides a rough guide if paired good/poor samples are taken, but this should be confirmed with a youngest emerged blade (YEB) test. YEB levels below 1.5 milligrams per kilogram (mg/kg) indicate Cu deficiency.

What is meant by synchronising of the canopy? Why does this matter?

When a canopy experiences different environmental conditions, plant development will vary slightly between tillers. When a canopy grows multiple tillers, the greater the density of these the more plant development throughout the canopy is aligned, potentially as a result of limited environmental variation/canopy closure i.e., light, moisture and competition for nutrients. So, if a greater proportion of the canopy is developing all at the same time, this will potentially increase the frost risk as the whole canopy will be vulnerable at the same time. This is particularly relevant for head and stem frost which generally occur most at head emergence.

Does soil amelioration reduce frost severity on light textured soils?

In WA, there has been a lot of anecdotal evidence on the effect of frost on ameliorated soil. Some growers reported reduced frost damage in paddocks that have been mouldboarded, spaded, delved or clayed. However, there are many confounding factors occurring as a result of the soil amelioration making it difficult to narrow down impacts to a reduction in frost damage alone.

Amelioration of water repellent sands may reduce frost damage by improving the capacity of the soil to absorb and then release heat. Increased heat absorption by the soil can occur as a result of improved wetting and changes to the colour of the soil surface. However, research is continuing to further understand the possible impacts of soil amelioration on frost severity and the impact of soil types and amelioration methods. Soil amelioration is unlikely to be an economically feasible practice for frost damage alone. Potential reduction in frost damage might be an additional benefit in years when frost is an issue. 

The decision to undertake soil amelioration should be made primarily in regard to weed control, soil water repellence, subsoil constraints and profitability. 

Harvesting a frosted cereal crop?

Frosted crops are difficult to thresh due to higher residual sugars in the straw and chaff, lower grain volume and high screenings. Despite lower tonnages, daily harvest maintenance and regular machinery clean down is vital to minimise machinery fatigue and fire risk in these difficult harvesting conditions. Frosted crops generate more dust and the crop residue builds up on the machine when harvested, contributing to increased fire risk. This is due to the tough nature of frosted stems, shattering of frosted grains and  increased fungal growth on the crop. If practical to do so, harvest frosted paddocks last.

Grain quality may also be compromised depending on the timing of the frost event. Frost affected grains usually have a lower hectolitre weight and higher screenings. Adjusting header settings and/or grading can be beneficial but check the feasibility first.

Frosted stubble can also rot off at ground level and be difficult to seed into. To minimise trash flow problems in subsequent seasons, frosted stubbles may have to be cut low at or in a separate operation after harvest.

What management options do I have to minimise frost?

A comprehensive frost management strategy needs to be part of annual farm planning. It should include pre-season, in-season and post frost event management tactics.

1. Identify frost prone paddocks - with topographic, electromagnetic, yield maps and paddock history 
2. Consider enterprise in a zone – cropping/sheep balance
3. Review nutrient management – targeted nitrogen, potassium and copper inputs
4. Modify soil heat bank – stubble levels, crop canopy
5. Select appropriate crops – oats, barley, wheat, canola
6. Manipulate flowering times – stage sowing time, mix long and short season varieties
7. Fine tune cultivar selection – wheat, barley susceptibility during flowering

More information refer to Frost tips and tactics.

Why was 2016 such a bad year for frost?

A chain of climate conditions seems to have led to the very cold conditions over southern WA from July–October 2016. Refer to    

• The WA south-west had the coldest average minimum temperature for spring since 1969 and the coldest minimum temperatures on record since 1910.
• Combined with an ideal break to the season and early planting of Hindmarsh/Latrobe barley (late March through to early May) and Mace wheat (late April through to late May) a significant portion of the central, eastern and great southern agricultural regions, Lakes district and Salmon Gums district incurred significant losses, upwards of 20% across the cereal program.
• There were also impacts on canola yield and quality with lower oil content (<40% oil) and increase screening and low hectolitre weight in barley and oats.
• The extent of the frost damage ranged from paddock to paddock but as there was rainfall prior to several of the events on 23 August and 23 September, there was less topographic variation and a wider sensitivity window than usual. For example, crops were stem and grain frosted in addition to flowering frost and throughout the landscape, not just the low lying areas.
• In addition there were also areas in the northern and southern wheatbelt which were affected, primarily with flowering frost damage in barley and wheat and this is more confined to frost prone parts of the landscape.
• Due to the wide window and severe frosts many of the current best management practices for minimise frost damage have had little effect on reducing damage; crop type (barley over wheat), delayed sowing, mixing varieties, soil amelioration and stubble management for example in the more frost prone parts of the landscape have had little effect.

Frost terminology