The Midwestern Regional Climate Center recently released a freeze date tool. It makes it easy to find historical temperature information that may help growers plan. This article provides a quick introduction to the tool.
Many readers are familiar with the average first fall frost and last spring frost dates, and the frost-free growing season between them; information that has been readily available for many years (e.g. see https://ag.purdue.edu/indiana-state-climate/freeze-frost-probability-growing-season-length/). This new tool makes it easy to find that information for your particular county in the Midwest or Northeast, as well as across the entire region. The default map view provides several options for customizing the information presented. In Figure 1, the map shows the 10th percentile date for first fall 32°F freeze. In Marion County, Indiana, 1 in 10 years the first 32°F occurs before Oct. 3.
Figure 1. Early first fall 32°F freeze date (1 in 10 years). Source: Midwestern Regional Climate Center Freeze Date Tool, https://mrcc.purdue.edu/freeze/freezedatetool.html.
One feature of this tool that could make it especially useful for fall crop and high tunnel production is that you can choose temperatures below 28°F. First/last freeze dates have in the past typically been reported for 36°F, 32°F, and 28°F, which is useful for warm season vegetable crops in the field. For cool season and high tunnel crops, dates of colder temperatures are relevant. For example, row covers are often applied over winter crops in unheated tunnels when external temperatures drop below 25°F, and some producers add a second cover when temperatures go below 20°F. The need for supplemental heat in a high tunnel also depends on outside temperature. For example, row covers should be enough to protect a new tomato crop planted early in spring in an unheated high tunnel when the temperature drops below 32°F outside, but if it gets to 20°F supplemental heat would be needed. Of course, day-to-day decisions will be made based on the current conditions, but the historical trends help in planning. Knowing typical dates for colder temperatures can also help in predicting when less hardy cool season crops (e.g. lettuce, swiss chard) are likely to be killed or become unmarketable. As an example of a lower temperature threshold, Figure 2 shows the average date for the first fall 20°F freeze. In Marion County, that date is Nov. 24.
Figure 2. Average first fall 20°F freeze date. Source: Midwestern Regional Climate Center Freeze Date Tool, https://mrcc.purdue.edu/freeze/freezedatetool.html.
The information in the tool is based on data from 1950-2021. You can find dates for each decade separately by choosing the ‘Trend Map’ option, clicking on the county of interest, and choosing ‘Freezes by Decade.’ Figure 3 shows the resulting graphs for each decade for the First Fall 20°F Freeze in Marion County, with the average date indicated by a dashed line. For the 2010’s the average date in Nov. 26. The ‘box and whiskers’ plots provide additional information about the range of dates in each decade. If you prefer looking at tables, click on the ‘Summary Stats Table’ to see a summary of data for 1950-2021 for the location you have selected (Figure 4).
Figure 3. First fall 20°F freeze dates by decade in Marion County, Indiana. Source: Midwestern Regional Climate Center Freeze Date Tool, https://mrcc.purdue.edu/freeze/freezedatetool.html.
This just scratches the surface of information accessible using this tool. I expect it to prove useful for vegetable growers in a variety of ways. Some uses will probably be immediate; others will take more time, research, and communication among growers, Extension, and researchers. If this article inspires you to explore the tool, I would be interested to hear how it, or it combined with additional information, could help in your vegetable operation.
Figure 4. Frost-freeze climatology in Marion County, Indiana, 1950-2021. Source: Midwestern Regional Climate Center Freeze Date Tool, https://mrcc.purdue.edu/freeze/freezedatetool.html.