Late-season corn development and frost probabilities
Figure 1 shows the most recent 30-year dates for median first fall 28⁰F frost across the Midwest. The median date for portions of NW and NE Iowa ranges from October 1 to 10; that for SE IA range from October 21 to 30. The median first fall 28⁰F frost for rest of the state ranges between October 11th and 20th (from MRCC).
Mesonet provides tables of probabilities by specific locations for fall frost events with different temperature thresholds. These data are averages since 1951. Figures 2, 3, and 4, display probabilities of temperatures less than 29⁰F for Iowa’s nine crop reporting districts. For example: for SW Iowa(fig 2) the average date of the first hard freeze is Oct. 21. In addition, one year in five the freeze may be later than Oct. 28, and one year in 10 it may be Nov. 4 or later. On the other hand, note that one year in 10 the hard freeze is on or before Oct. 5.
click image to zoomFigure 3. Probability of temperatures less than 29⁰F for NC, Central & SC Iowa. The horizontal yellow line marks 50 percent probability. The vertical light purple lines point to the dates of the 50 percent probability of temperatures less than 29⁰F temeprtures for each of the three Crop Reporting Districts. Data from Mesonet. The date with 50 percent probability of less than 29⁰F temperatures ranges with northern CRD’s occurring earlier than the southern CRD’s – a range of 9 to 12 days earlier in the north in the three central and eastern-most CRDs. The northwest CRD has later frost dates then the west cenral CRD.
Fifty percent probability dates of temperatures below 29⁰F for the western parts of the central and southern CRD’s arrive six days earlier than the eastern parts of those regions. Those dates vary little across northern Iowa CRDs, October 14 to 18.
click image to zoomFigure 4. Probability of temperatures less than 29⁰F for NE, EC & SE Iowa. The horizontal yellow line marks 50 percent probability. The vertical light purple lines point to the dates of the 50 percent probability of temperatures less than 29⁰F temperatures for each of the three Crop Reporting Districts. Data from Mesonet. Warmer temperatures in the current short-term forecast may help accumulate GDDs faster. However, much of the state remains dry (see drought monitor). Warmer temperatures with dry conditions will stress the crop even more.
The critical issue of this whole season is the timing of the first 28⁰ F frost this fall. A later than normal frost encourages longer seed-fill period and higher yields. An early frost … well let’s hope it doesn’t happen!