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Risk of Frost - Assessing the Damage

By: Ministry of Agriculture Staff

Forecast of overnight lows for Thursday
May 12 / morning of Friday May 13.
With a mild frost in some areas of Saskatchewan last week (lows of 0 to -6 C and prolonged durations of below-zero temperatures), now is a good time to review the risk and impact of frost damage on field crops in the province. 

Seeding has been progressing well this spring. With 35 per cent in the ground as of May 9, seeding is well ahead of average. However, these early seeded crops may have been affected by the May 12/13 temperatures. There are many benefits to seeding early, but one of the risks is a spring frost. Normal dates for the last spring frost extend into mid- to late May, so it’s good to brush up on assessing frost damage each spring.

When the mercury dips below zero in the spring, you should scout fields to assess the extent of any damage to the crop. In most cases, it will take a few days to get a proper assessment of damage – patience is a virtue. Frost can be localized too, such as in low areas, so include those. Even if the temperature is close to zero, keep an eye open for damage.

Frost damage occurs as moisture within the plant crystallizes and expands. This causes cell walls to rupture and fluid to leak out, hence the watery appearance of plant tissue after a damaging frost. The extent of damage caused by frost depends on the crop, temperature, length of exposure time, humidity levels and how long the crop takes to reach freezing temperature.

Frost damage on
canola seedling
Canola: Newly emerged canola at the cotyledon stage can be very susceptible to spring frosts. The growing point is above ground, between the cotyledons. Plants at the three- to four-leaf stage are much more tolerant and can withstand a couple more degrees of frost. Typically, canola can tolerate temperatures down to -4 C. Hardened plants can tolerate temperatures down to -7 C and possibly colder.

A light frost on the leaves may not injure the growing point. If there is regrowth or green material at the growing point, then the plants could recover. It will take a few days to really assess the damage. Within the field, there can be damaged and undamaged plants close together. To determine the viability of the damaged seedlings, the growing point needs to be green and viable and the stem healthy. Severely damaged plants will pinch off at the top of the stem, and the whole seedling will brown off.

Cereals: The growing point of cereals is below ground until approximately the five-leaf or jointing stage. This protects the plant from severe frost injury in the spring. The plants may lose above-ground leaf matter but will regrow from below ground. Partial injury can be visible when the tips of leaves or leaf edges become damaged and yellow, and then turn brown and become brittle. Severe injury to cereals where all above-ground matter is damaged can result in a delay in maturity due to the plant having to regrow. Cereals have good frost tolerance and will tolerate frosts down to -4 C; if hardened, they can withstand -6 C. Even recently planted cereals where the wheat has just sprouted can withstand up to -6 C.

Winter wheat is at a more advanced stage than other crops. Although winter wheat survives the cold all winter, once it is green and growing it is susceptible to frost; however, it still has some tolerance. Winter wheat is quite tolerant at tillering, tolerating -11 C for up to two hours. Tolerance decreases at stem elongation, handling -4 C for up to two hours. It is most sensitive at boot stage, tolerating -2 C for up to two hours.

Flax: Flax is quite sensitive to frost when it is coming out of the ground. Temperatures of -2 C or colder can injure flax up to the two-leaf stage. Frost canker occurs when plants become girdled at or near the soil line and topple over. Damage is most severe in thin stands on light soils and in low spots, and, during early stages of growth, it can reduce stands by as much as 50 per cent. As flax grows it becomes more tolerant to frost. After the two-leaf stage, flax can withstand temperatures down to -7 C and even slightly lower if the plants have been hardened.

Peas/lentils: Peas and lentils have good frost tolerance. They have growing points (bracts) that remain below ground during early development. The above-ground material may be severely injured by frost, but new growth will resume from the nodes within a few days.

Alfalfa: Older plants are more susceptible to damage than young stands. Wilting may be visible within 24 hours at temperatures down to -3 C, and yellowish or brownish discolouration may appear three or four days later. Top stems may bend in the shape of a “shepherd’s hook”. Plants will generally recover, but development may be delayed and yield reduced. At temperatures lower than -4 C for more than four hours, stems will die and growing points may be damaged. In most cases, plants will survive but the stand can be weakened if it is cut before root reserves have had time to replenish.

Spraying after frost: As herbicides work best when crops and weeds are actively growing, you should avoid spraying immediately after a frost. Wait at least 24 hours, or preferably 48 hours, after heavy frost to allow the weeds and crop to recover. The crop needs to be actively growing to prevent injury from the herbicide, and the weeds need to be growing so the herbicide can work.

Weed tolerance to frost will also determine how soon herbicides can be applied. The more tolerant the weeds are to frost, the sooner they can be sprayed. Winter annuals and dandelions have good frost tolerance. Other perennials such as quackgrass and foxtail barley are less tolerant, while Canada thistle and perennial sow thistle are the most sensitive perennial weeds.

Regardless of the product being used or the weed spectrum to control, it is always important to read the label carefully before spraying.

Historical last spring frost
dates for Saskatchewan.
Frost damage is very hard to predict, as there are many factors that affect the crop’s tolerance. Evaluating the damage is difficult and should be done approximately 24 to 48 hours after the frost for initial symptoms, and up to a week to 10 days for full extent of damage. Heavily damaged crops will quickly show signs of frost injury, including discoloration, darkening and a water-soaked appearance of fleshy tissue.

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