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News Agronomy
Cold temperatures may cause damage, slow weed growth

May 15, 2020  By OMAFRA Field Crop Team

Temperatures were well below seasonal for much of the past week due to the presence of an arctic air mass. Low night-time temperature records were set in many areas of the province with low temperatures between 0 C to -7 C, and snow fell overnight more than once.

The widespread risk of the cold weather impacts is decreasing with the warm temperatures and precipitation predicted for next week. Overall, planting progress across the province is ahead of normal.


Grain corn planting is between 50-90 per cent complete in the province. Generally, southwestern and eastern Ontario are closer to 90 per cent, while central Ontario is in the 40-60 per cent range. Areas with heavy clay soil, such as Niagara and Haldimand, have not begun planting corn yet. Other regions, such as Peterborough, have had slower progress due to more regular rainfall throughout the first half of May.


Very early planted corn is just starting to emerge. Otherwise, corn is not yet out of the ground, in part due to cool temperatures. If soil froze to seed depth, the first sign of injury or death would be the coleoptile turning brown or grey. It will be important to scout any at-risk fields in the coming days.


Winter wheat growth stage remains largely unchanged since last week due to the cool conditions. The most advanced fields are approaching flag leaf emergence in southern Ontario; later planted wheat in cooler regions is approaching the jointing stage.

It is possible that the recent cold weather may have injured some winter wheat. The crop’s tolerance to cold temperatures varies based on maturity: the more advanced the wheat, the more susceptible it is. Based on research from Kansas State University, two hours at -4 C for winter wheat at the jointing stage can kill the growing point and have a moderate to severe impact on yield. If the wheat is only at the tillering stage (up to GS29), however, it can withstand temperatures as low as -11 C.

For those with more advanced wheat who experienced very cold nighttime temperatures, it will be important to assess your stand in the coming days. Wait for at least three days of warmer temperatures. Evaluate low-lying parts of the field first. For wheat that has jointed, damage to the growing point can be evaluated by splitting open the stem and assessing the growing point.

Sulphur deficiency is being reported on sandier soils where sulphur was not applied.  Additional applications of sulphur are being made to those fields to alleviate the deficiency.

Herbicide applications on winter wheat have slowed over the past week due to cool temperatures, but so too has weed growth. Ragweed and lamb’s quarter in wheat are reported to be at the cotyledon stage.

Many spring cereals have emerged and are at the seedling stage. Weed control will be important in the coming weeks. The critical weed-free period is from crop emergence until the three-leaf stage.


Planting progress for soybeans ranges from 10 to 30 per cent complete in southwestern Ontario, and 20 to 50 per cent complete in eastern Ontario. Many regions are reporting approximately 10 per cent of soybean acres planted.

Pre-plant burndown applications in soybean fields were delayed in the first half of the week in order to wait for suitable spraying conditions and sufficient weed growth. For residual herbicides, duration of weed control will not be shortened by below-normal temperatures since cool temperatures will slow degradation of the herbicide. While weed presence at this time is generally low, applying residual herbicides for control of weeds – such as glyphosate-resistant fleabane – is critical.


Cool conditions have hampered growth in hay fields and pastures. Frost over the past week may have caused damage in some areas.

For production year stands, a hard frost can kill stems. In such cases, the stems will have to restart growth, which will delay first cut. It is important to check the roots for damage. If the roots are healthy but the tops look dead, it’s probably frost damage and just needs heat and time. Mushy, ropey or discoloured roots, however, indicate that winterkill has occurred.

Alfalfa seedlings are most sensitive to cold temperatures between second trifoliate and the end of contractile growth (crown formation), which typically takes eight to 10 weeks. A hard frost (below -4 C for more than four hours) can kill those seedlings; however, much of the province’s new alfalfa seeding has likely not yet reached the second trifoliate stage.

Grasses initiate flowering based on day length, so grass hay and mixed stands will reach maturity at a normal time for first cut, but at lower yields. Delaying harvest on a mixed stand will allow the alfalfa component to improve yield, but grass quality will decline quickly, affecting the quality of the forage.


Canola seeding is reported to be between 75 and 100 per cent complete in the province.

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