Spring frost can injure newly emerged crops, but in all but the most severe frost, the injury will not require replanting for these three crops. The potential for cold injury to seedlings is influenced by growth stage, air temperature, soil moisture, soil texture, and tillage system.

Corn

What temperature may result in injury?

As the growing point for corn remains below ground until corn reaches about V5 (5 visible leaf collars) growth stage it is less susceptible to frost injury, in the earlier growth stages. Freezing temperatures above -2 °C may damage seedling leaf tissue without injuring the growing point, but temperatures below -2 °C may injure or kill the growing point even if it is below the soil surface. It is important to wait 3 to 5 days after a freeze to accurately assess plant damage. Within the first 24 hours after freezing temperatures, the leaves may turn yellow, silver, or brown and may become wilted or water-soaked (Figure 1).

After several days, the leaves may die. The stem may also turn brown; however, if the growing point is not damaged, the seedling can recover, particularly if there is new green tissue in the whorl (Figure 2).¹

A firm, white or cream-colored growing point is a good indication that the seedling is alive and the probability for recovery is very good. Seedlings with soft and dark-colored growing points are likely to die. Under favorable weather, a new leaf should develop and appear 3 to 4 days after the frost. Occasionally, decaying leaf tissue may inhibit the growth of new leaves from the whorl, giving the seedling a twisted appearance.

Will the frost injury result in yield loss?

After regrowth begins in 3 to 5 days, the injury to the plant can be assessed. While scouting, look for evidence of new leaf growth from the whorl and split stems to evaluate the condition of the growing point. Cool days following a cold temperature event may delay recovery and the diagnosis of the extent of injury. Remember, that a determining factor in corn yield is number of plants, so if plant stand is not reduced, yield potential should not be impacted. However, since the frost injury may not be uniform across the field, the injury may result in varying growth stages across the field. By taking several stand counts across the field, while assessing the condition of the plants, will allow the determination of the potential of the number of surviving plants. Compare the estimated yield potential of the existing stand to the estimated yield potential of a replanted stand to help determine if replanting would be economical.

Soybeans

Soybean are more susceptible to frost injury than corn, as the growing point in dicot plants is exposed upon emergence. At the same token, if the main growing point (apical meristem) is damaged, soybeans have a greater ability to recover than corn. Soybean plants can produce new growth by the auxiliary buds found at each node.

What temperature will result in injury?

Frost damage to soybean plants can occur when temperatures range between -2 and 0 °C. Temperatures up to -2.8 °C may be tolerated for short periods of time when soybeans are in the VE (emergence) to VC (unrolled unifoliate leaves) growth stages. Complete death (buds, stems, and leaves) is not expected until temperatures remain at -2 °C for an extended period of time for sensitive plants. Soybeans in the VC stage are slightly more frost tolerant compared to soybeans in the V1(first-trifoliate) and V2 (second trifoliate) growth stages. Soybeans with emerged trifoliate leaves (V1 and V2 growth stages) become more susceptible to temperatures below 0 °C for any extended time.

Will I need to replant my soybean fields?

As with corn, patience is the key when determining if an individual soybean plant is likely to survive a frost, so wait 3 to 5 days before evaluating the potential for new growth at the auxiliary buds. Replanting a field of frost-damaged soybeans demands more consideration since soybeans are more susceptible than corn to frost and cold temperatures. However, soybeans can tolerate stand reductions and still produce similar yields to the original stand because of the ability of each plant to compensate under lower plant densities. Replanting is not recommended unless populations are less than 297,000 plants per/ha (120,000 plants per acre). In Canada, full yield potential is achieved with a final plant stand of 309,000 - 370,000 plants/ha (125,000 - 150,000 plants/acre)². As the season progresses, vigilant scouting should continue for seedling blights and environmental damage in soybean fields, especially those damaged by frost.

Canola

Canola, like soybeans, is a dicot, so the growing point is exposed upon emergence. However, canola is more cold-tolerant than soybeans.

What temperature will result in injury?

Canola that has been exposed to several days of near freezing temperatures, will be “hardened” and allow the plants to withstand freezing temperatures without serious injury. Studies at the University of Manitoba and at the Agriculture & Agri-Food Canada Research Station in Beaverlodge have shown that early seeded canola that had undergone hardening could withstand -8 to -9 °C, while later sown canola which did not undergo hardening was killed by -3 to -4 °C.

Will I need to replant my canola fields?³

Patience is again recommended prior to assessing the extent of the injury. After 3 to 5 days, if the growing point is green, firm and does not appeared pinched off, the plant will recover. Under cool and dry growing conditions, it take up to 10 days before the plant begins to regrow. To evaluate the field, walk in a diagonal path across the field stopping every 20 paces and count the number of plants in a square foot section (make two ‘L’s with your foot to approximate the area). Consider the percentage of plants killed, recovered, and the weed population. A minimal plant population of 40 plants per square metre (4 plants/ft²) throughout the majority should produce an acceptable yield, as long as weeds are controlled. Weed control in any field with lower plant stands is crucial to maintaining yields. Canola can compensate for lower plant stands as surviving plants will take advantage of reduced competition for light, moisture and nutrients. Plants will grow larger, produce more branches, pods and seeds per pod, but will require longer to mature. However, they will still be earlier maturing than if the entire field is re-seeded. Also consider flea beetle populations, if warm and sunny conditions return (greater than 15 °C) flea beetle population can build quickly. Seed treatments should provide protection, but if feeding is severe, greater than 25% leaf injury, an application of insecticide may be warranted.