Much of the state experienced four consecutive record low temperatures between May 18 and 21 with much frost damage reported. Some farmers are concerned about the effect of the cool temperatures on corn. Others are concerned about lack of seedling emergence and are wondering how long can seed remain below ground.

How will cool temperatures affect corn?

Corn plants will not be killed by frost unless temperatures get cold enough to kill the growing point that is 3/4 of an inch below the soil surface. So corn that has not emerged typically is well insulated from frost damage. During these record low temperatures, soil temperature at two inches was 34 to 49 F at the Arlington and Hancock ARS even though air temperatures ranged from 27 to 36 F (Bland, AWON).

Symptoms of frost damage will start to show up about 1 to 2 days after a frost. Symptoms are water soaked leaves that eventually turn brown. After 3 to 4 days watch for new green leaves emerging n the whorl. If new leaves are not emerging check the growing point for discoloration. Any deviation from a white, cream or light yellow color indicates that the growing point is killed.

On May 21, I observed water soaked leaf symptoms in a field that had been planted April 15. Other field symptoms included uneven emergence and plant development with large gaps in stand. We decided to rotary hoe the field and try to break a slight crust that had formed hoping we could improve plant density.

Why isn't my corn emerging?

Generally, when planting date is April or early May, I'm not concerned about corn not emerging until 30 days have passed. The seed treatments typically used on corn should protect the seed for 6 to 8 weeks. The length of time seed treatments are active depends upon its water solubility characteristics, degradability (1/2 life, microbial and plant breakdown), and dilution due to plant growth. As long as the tissue is white and turgid, the seedling is doing what it is supposed to do. It just needs some warm weather to help it emerge. Watch the seed piece for signs of rot (discoloration). The seedling is dependent upon the seed for food and reserves until after emergence. However, this metabolic process is driven by temperature.

All reporting stations in the ASIG network (Bland, UW Soils) reported on May 21 that only 48 to 116 GDUs had accumulated since May 1 statewide. Corn needs 125 GDUs after planting to emerge. So far this year, most of our GDUs came before April 20 when very little corn had been planted. Typically Wisconsin accumulates about 10 GDUs per day during the month of May so it normally takes about 12.5 days for corn to emerge.

I think what we need to do now is be somewhat patient. Watch what the warmer weather spell will do for the corn this week and reevaluate our stands next week.