While fall officially began on September 22, much of Ohio has yet to experience an official "frost". Frost develops when water vapor molecules freeze on the surface of an object that reaches 32°F. This happens in the fall on surfaces that cool more quickly than the surrounding air such as grass blades, garden plants, cars, and roofs.
Interestingly, official air temperatures reported by the National Weather Service are collected five feet above the ground. As most plant enthusiasts and gardeners know, the air temperature at this five-foot height can be several degrees warmer than the actual air temperature at ground level. For this reason, frost is often predicted when the observed air temperature is 33-36°F provided there are relatively clear skies and little wind.
Frost and freezing temperatures can injure or kill sensitive plants unless they are covered and protected or moved indoors. Tender, nonhardy plants in the garden or landscape can be damaged by a light frost (33-36°F) or completely killed by a freeze (32°F) or heavy freeze (28°F). Plant injury and/or death occurs when water inside the plant freezes and expands, rupturing plant cells. Therefore, the last day of 32°F freezing temps in the spring and the first day of 32°F temps in the fall effectively determine the length of our growing season - which can vary significantly from year to year.
Predictions for the last and first frosts for a given area have been generated by evaluating historical weather data, though this is somewhat of a moving target. The development of several savvy tools can also help us better understand and predict frost dates. A few of these tools are described below:
Purdue's MRCC Freeze Date Tool. Current predictions and trends in average first and last freeze dates (can be calculated using temperatures from 20 to 40°F) and length of the growing season are based on a large weather data set collected from 1950 to 2021. This data can be found for each county in Ohio and across the upper Midwest and New England states at https://mrcc.purdue.edu/freeze/freezedatetool.html (Figure 1).
This MRCC tool estimates the first fall freeze, the last spring freeze, and growing season length for any given county, and also illustrates temperature trends for each of these data sets over time. Users simply click on one of the top six tabs to view the county data. Since I live and work in Putnam County, I will use Putnam County as our example:
- The average first fall freeze in Putnam County is October 15th but has occurred anywhere from September 16 (1959) to November 18 (1985). The average date of the first fall freeze has occurred 3.5 days later than the average from the previous decade.
- The last spring freeze in Putnam County generally occurs around April 25 of each year but has occurred anywhere from March 31 (2019) to May 25 (1961). The average date of the last spring freeze has occurred an average of 1.8 days earlier than that of the previous decade.
- Since 1950, Putnam County’s growing season averages 172 days above 32°F. The shortest growing season of 124 days occurred in 1961. The longest growing season of 222 days occurred in 1985. The average length of the growing season in Putnam County has increased by five days every ten years since 1950.
- Putnam County has only experienced seven growing seasons with more than 200 frost free days – 1984, 1985, 1994, 2010, 2011, 2016, and 2019.
Sometimes, smaller subsets of weather data are used to generate frost dates, such as in the table below using data from 1990 to 2020 by the National Weather Service's (NWS) National Forecast Office in Wilmington, OH. The regional weather offices seem to vary in what weather data is explored and shared in more detail. I encourage you to check out your regional NWS office (weather.gov) and see what tools and information are unique to your locale.
Advisories, Warnings, and Watches Oh My!
You've probably seen and been alerted to hazardous weather conditions where you live. Do you know what each of those alerts, advisories, warnings, and watches mean? Here is a quick overview:
Hazardous weather alerts in the form of advisories, watches, and warnings are issued by regional National Weather Service Forecast Offices (weather.gov) across the US. Advisories and warnings are issued when a hazardous weather condition is occurring, imminent, or likely. Watches can be issued a few days prior to the expected hazardous weather conditions.
Weather alerts are issued for frost and freeze events from May through October/November in Ohio. Frost advisories occur when weather conditions could result in frost - which would include temperatures between 33 and 36°F, little to no wind, and mostly clear skies. Freeze alerts occur with expected low temperatures of 29-32°F (freeze) or 28°F and below (hard freeze) for a given location. Frost and freeze alerts are issued in the fall in Ohio until the first widespread freeze in an area occurs.
Below is an example of the Experimental Graphical Hazardous Weather Outlook tool that helps to predict hazardous weather events for a specific region over time. The tool is showing a frost/freeze risk for West Central and Southwest Ohio for Tuesday through Wednesday of next week.
What Can You Do?
A light frost can kill warm season garden plants like tomatoes, peppers, sweet corn, watermelon, cucumbers, and many tropical plants. Hardier plants can withstand temperatures that dip at or below freezing and include carrots, celery, lettuce, parsnips, broccoli, cabbage, kale, peas, spinach, and turnips. Annuals and perennials also vary in their ability to withstand frosty temperatures.
As our 2022 growing season comes to an end, plan to cover any remaining garden plants you wish to continue harvesting from this fall if a frost or freeze alert is issued. Appropriate coverings include many lightweight fabrics like frost cloth, blankets, sheets, towels, burlap, or drop cloths. Cardboard, baskets, totes, and many other items can also be used.
In all likelihood, the aboveground portion of many plants may freeze and die back to the ground. This provides a perfect opportunity to dig up plants like dahlias, cannas, and other bulbous-type plants that won't survive the winter outdoors - but can survive in a more protected location to be planted again next year.
Houseplants and tender plants should be moved indoors at this time to protect from frost and possible freezing temperatures. A more permanent winter spot should be prepared for those plants you wish to enjoy through the last spring frost in 2023!