Warm spring weather called Minnesota farmers to the fields and prompted many to finish or nearly finish planting in record time by early May.

However, weather patterns changed by May 8, threatening cooler temps as low as the high 20s in parts of the state.

In a recent blog post by Seth Naeve, University of Minnesota Extension soybean agronomist, David Nicolai, U-M Extension crops educator and Tom Peters, U-M Extension sugarbeet agronomist, offered the following insight into low-temp impact on three cropping scenarios:

Emerged crop

Generally, temperatures usually need to be at or below 28 to 30 degrees F for several hours. Attempting to point to specific temperatures is impractical given the many mitigating and compounding factors at play:

Corn. With corn, the seed and growing point remain below the soil surface for several weeks after emergence. Because the growing point is below the surface, extreme hard freezes are required for plant death. Frost damage to exposed leaves has little impact on potential corn yields, especially at very early dates. “Lethally cold” temperatures for emerged young corn plants are those that dip to 28 degrees or lower for one to two hours.

Soybeans. In soybeans, epigeal emergence occurs when cotyledons push up through the soil surface. This exposes the growing point to the atmosphere and puts the crop at a greater risk of frost damage.

Sugarbeets. Sugarbeet seedlings possess more ability to cold-harden and may tolerate freezing temperatures better than soybean. Sugarbeets at the cotyledon stage seem to tolerate cold temperatures better than two-leaf sugarbeets or seedlings just beginning to emerge from soil. The stem is particularly susceptible to ice damage at this time and a soil temperature (not air temperature) at 31 degrees at the half-inch depth may be lethal.

Sprouting seeds that contain only a small amount of water and ice crystals will not form because of the relative high content of dissolved materials.

Type of tissue exposed. As soybeans emerge through the soil surface, their thick, high-moisture tissues are highly resistant to frost damage. The water helps maintain internal temperatures during short frost events. As the hypocotyl extends over days and the cotyledons stretch upward, they will open and expose the developing unifoliolate leaves.

This fragile tissue is the functional growing point of the soybean and can be easily damaged by periods of sub-freezing temperatures. Fortunately, it is common for soybean to produce new growing points from the axils of the cotyledons if the unifoliolate is damaged.

Boundary layer. The soil surface is heated during the day and will retain this heat through the night under normal nighttime conditions. Increased winds reduce the boundary layer and allow the soil surface to chill which increases the risk of frost damage.

Surface roughness also affects the boundary layer. Heavy residue, such as standing cornstalks in no-till, greatly increases the boundary layer and can help protect the crop in marginal frost conditions.

Soil moisture. Water has a very high thermal mass, meaning that it efficiently absorbs, stores and releases heat. This is an incredibly helpful physical property for defending against frost as the soil moisture will allow the soil surface to remain far above freezing even when ambient temperatures — that are measured above the soil surface — plunge. Fields that have received recent rainfall and have enough soil moisture will be more frost resistant than those where soil moisture levels have been drawn down with tillage and warm sunny weather conditions.

Topography. Low-lying areas of fields tend to be affected by frost damage the most simply due to lower temperatures experienced in these portions of the landscape.

Recently planted crops

Recently planted corn and soybean crops should perform fine through this cool snap. As mentioned above, emergence will slow, thus reducing the risk of frost damage to the emerged crop.

Cool temperatures will provide a longer window for farmers to make their pre-emergence herbicide applications.

Seed in the bag

Farmers with corn and soybean in the bag simply need to look for good soil conditions to get rolling. One should not be concerned about cool ambient temperatures. It’s time to get the crop planted, so do not miss opportunities to get to the field when working conditions are suitable.

For more information, the Extension specialists suggest visiting the U-M Extension soybean spring frost page and the corn spring frost page.

Go online to view the interactive weather and temperature forecast maps.

Source: University of Minnesota Extension, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all of its subsidiaries are not responsible for any of the content contained in this information asset.