Frost Damage: Is temperature the only factor that matters?

As crops continue to mature this season, the risk of frost will have a larger impact on crop harvest timing. Most often when temperatures start to dip into the single digits, it is habit to start looking solely at the thermometer to determine the impact. However, it is a good idea to pay close attention to a few other factors when determining the impact of those cooler or sometimes freezing temperatures on your crops. Questions that you may want to ask when assessing cooler temperatures are:

- What stage is my crop at?

- What temperatures were recorded?

- At what time and how long did the low temperatures occur for?

- Was there any dew or fog?

- Are the plants in the fields affected stressed?

The answers to these questions help to determine what type of impact the weather conditions may have had on your crop. It will likely take three to four days for the frost effects to start to show up in plants and can take up to 10 to 14 days to fully assess the impact, particularly if cooler growing conditions are present.

Crop stage is an important factor to consider as frost damage occurs when moisture in the plant crystallizes and expands, resulting in the watery substance that leaks out. As a plant matures it loses moisture and there is less to freeze which is why less damage can be seen in a ripe canola crop with less than 20 per cent seed moisture or cereals that are in the hard dough stage. Shrivelling of cereal crop seeds in milk stage can be impacted by temperatures at 0 C while in the mid dough stage kernels can tolerate down to -4 C before bran frost occurs and kernels are affected. The earlier the frost occurs relative to crop maturity, the more the germination and seedling vigour can be affected. Kernels affected by frost will appear soft and watery and will ooze water when squeezed in the milk and early-dough stages followed by kernels shrinking. At later stages, mid to late dough, the damage will occur as bran frost and some shrinkage. Wheat is more tolerant to frost than barley followed by oats.

Earlier stages of canola with immature seeds that contain 50 to 60 per cent moisture can be severely damaged by temperatures greater than -2 C while more mature seeds with 35 per cent moisture or less result in very little or no damage. To determine whether frost damage has occurred in canola, you can assess the pods and immature seeds. Pods that have had light frost will have white speckling while severe damage will result in large parts of fully white pods. Immature seeds will become water masses that shrivel away or look like pepper once dry. More mature seeds will remain hard but may retain their green colour resulting in higher green seed counts when crop is harvested.

Often the assumption is that lower temperatures cause worse damage to a crop. This may not always be the case. If the temperature drops quickly and rises quickly then the damage can sometimes be less than a longer duration of a milder negative temperature. For example, a temperature of -2 C for five hours may have the same or greater impact than -3 C for less than one hour on a crop. If lower temperatures occur earlier in the night, they can have a larger impact than a lower temperature seen just before sunrise when warming will occur.

Relative humidity and dew point are other factors that can influence how low temperatures will affect a plant. Relative humidity (RH) refers to how much moisture is in the air compared to how much it could hold at that temperature. When RH is 100 per cent the dew point is reached and condensation or fog appears. If dew is present on plants, then the process of freezing that condensation can give off heat that can help keep plant tissue above a temperature where freezing damage can occur. While water continues to freeze on the outside of the plant it remains at 0 C until all the water is in a frozen state. However, once all the condensation around the plant has frozen the temperature of the tissues can start to drop and the plant is no longer protected. This why there is so much variability in weather conditions and the resulting damage that can occur. Sometimes shorter durations at low temperature where dew or fog is present can cause less damage compared to a night with no dew or fog where -1 C or -2 C frost occurs for a longer period. Along with weather conditions the overall field conditions and crop health can affect the ability of plants to recover from frost. If drier soil conditions occur, then it can reduce the ability of a plant to recover from the injury.

If a severe frost has occurred in your crop and you have made the decision to use it as a feed source for animals, it is important to consider nitrate levels. Nitrate accumulation can become a problem when crops experience frost for even a few hours. Nitrate levels will peak at three to seven days after frost and will then decline to a normal level after 14 days if no other frost events have occurred during that time period. Nitrate levels can be managed if feed is tested and then used in a feeding program with other sources of feed. It is also important to note that fungicide and herbicide products that have been sprayed on a crop may have feeding or cutting restrictions. More information can be found on product labels or in the Guide to Crop Protection.

For more information, contact your local Regional Crops Extension Specialist, call the Agriculture Knowledge Centre at 1-866-457-2377.