Are we out of the risk of frost or freeze in Indiana? No, not statistically. Are we beyond the point where it's likely that a freeze severe enough to kill soybeans is likely? Yes. Severity includes both how low the temperature goes and how long it stays there.
Tests over the past two decades by independent sources both indicated that soybean varieties at that time could withstand temperatures as low as 24 degrees F before losing a significant portion of the stand. Even at that level, not all plants would be killed. The exact amount of damage depends upon when the coolest temperatures occur during the night, at what level they are above the ground, and how long they persist. Predicting frost and freeze injury is difficult because so many factors affect it, including elevation of the field and location within the state. In general, eastern Indiana tends to be more vulnerable to late-season spring frosts and early-season fall frosts than some areas farther north in the state, simply because of the geography of the region.
Based upon needing at least two hours of 24 degrees F to begin to take out a significant portion of a soybean stand, Ken Scheeringa, assistant state climatologist in Indiana, researched when various parts of the state can expect readings that low late in the spring.
Here is the data he and his student uncovered, based upon a 30-yewr reporting period, 1981-2010, for Goshen, Indianapolis and Evansville. The average last date of a 24-degree reading in the spring for the three cities, respectively, are March 26, March 26 and March 13.
The chance that a 24-degree freeze will occur earlier than these three specific dates is 10%, or one time out of 10. Obviously, this wasn't a year which featured low temperatures even at night in late February and March. The dates are March 13, March 11 and February 25, respectively.
Here's the most important point in their report. How late can you go and still have a 10%, or one in 10 chance, of having 24 degrees F or lower reading in the spring? For Goshen, it's April 7. For Indianapolis, it's April 10. Remember the lake effect. For Evansville, it's March 20.
Draw your own conclusions. The data would seem to say that the odds of a freeze hard enough to significantly affect a soybean stand from this point forward is extremely slim in Indiana.