A hard freeze in May isn’t ideal when 38 percent of soybeans (seven percent emerged) and 67 percent of corn (24 percent emerged) are in the ground, but that’s what a number of growers experienced earlier this week. From as far south as Missouri and Kentucky to the Midwest and Northeast, a late-season cold snap moved temperatures below freezing. We’ve had a number of questions this week regarding freeze damage on newly planted and newly emerged corn and soybeans. Here’s what we’re telling growers to look for and how to tell if there’s cause for concern.
Corn
In most cases, corn may only have cosmetic damage. Although it may look bad, it should still fare the cold temps just fine. Because the growing point in corn remains below ground, it is still protected by soil temps that are likely still above 28 degrees Fahrenheit. However, if the temperatures reach this point and remain that way for several hours, the growing point can be injured or even killed.
The same timeframe applies with a caveat for corn (five to seven days) with warmer temperatures. Corn requires growing degree units or heat units to grow. Colder than normal temperatures that result in negative or a loss of heat units will delay the normal growth and development process.
A quick reference on growing degree days is to take the daily high temperature, add it to the daily low temperature and then divide the sum by two. Then, subtract 50. For example, if today’s forecasted high is 53 degrees Fahrenheit and the forecasted low is 50 degrees F, this will result in 1.5 growing degree days (53 + 50 = 103 / 2 = 51.5 - 50 = 1.5). A corn plant’s normal processes won’t function until you reach 10 growing degree units. Therefore, it may take longer than five to seven days to see new growth if this trend were to continue.
Soybeans
Deciding how to proceed following a long duration cold snap on soybeans can be tricky. Not all cold temperatures are created equally. For instance, a spell of lethal cold temperatures will be more damaging than a brief frost of a few hours. Making informed decisions will have the greatest impact on yield and profitability.
Lethal cold temperatures for soybeans are defined as 28 degrees F, but merely reaching that temperature does not guarantee death of tissue. The ground can act as a heating barrier if days prior to the freeze were warm. Residue such as cover crops and ice can also act as insulators to protect the seed.
It’s important to remember that visible leaf tissue damage on soybeans does not guarantee plant death. Tissue will often turn a dark greenish black after exposure to lethal cold temperatures. However, if the axillary buds are unaffected, the plant may continue to grow without suffering permanent damage or yield loss.
Newly emerged soybeans are most susceptible because the exposed tissue from ground level to cotyledons represents the only live-growing tissue and growing points of the young plant. Damage from the soil surface to the cotyledons will likely permanently damage the young soybean plant.
Time is your friend in making these decisions. If the weather remains cool, plant growth and development will be slow, and it may take five to seven days to assess the true nature of the damage, if any. If after five to seven days there is no new emerging tissue, intervention and replant will be necessary to maintain productivity. If there is new tissue growth in five to seven days following a freeze with remaining cool temperatures, the soybean plant has survived and no intervention will be necessary. However, caution should be exercised. A freeze is a traumatic event for plant tissue and allowing adequate time for recovery is warranted. Allow the growth of two to three true leaflets prior to applying herbicides.
Regardless of what some people say, applying a fungicide, insecticide or any growth-promoting products is not helpful to frost damaged corn or soybeans. If you have questions related to your crops and the recent freeze, contact your local Stine sales agronomist.