As daylight lengthens and milder days become more frequent, a lot of us start thinking about tending to our gardens or revving up the tractor for planting. When it comes to springtime planting in the garden and field, however, we must be careful not to start too early. A warm spell in March, similar to the one we experienced last year, does not guarantee the end of freezing temperatures for the rest of the season. If you know when the average date of the last spring frost is for your area, and you have a good idea of how topography can affect temperature, you can use that knowledge in deciding when may be a safe time to plant and avoid freeze injury.
Map 1 displays the average date of the last spring frost (=32 degrees Fahrenheit) in Missouri. These are dates in the spring after which there is a 50 percent chance of a light killing frost. Typically, the last spring frost occurs over northern and central Missouri by April 20 and April 10, respectively. Frosts are more likely to be experienced later in the spring over the Ozarks when compared to central Missouri. The reason for this is due to the higher elevation of the Ozark Plateau which causes cooler temperatures in the Ozark region than would be expected. In fact, some counties in the Ozarks normally do not experience their last frost until the end of April. As the Ozark Plateau transitions to the southeastern lowlands, the average last spring frost generally occurs as early as the first week of April in the Bootheel.
Another item to consider is local terrain. Temperatures can be highly variable within small distances due to topography and this map is only a generalized view of expected last spring frost dates. Minimum temperatures can vary as much as 10 degrees Fahrenheit over a short distance, say, from the bottom of a valley to a nearby hilltop. Cool air is denser than warm air and will move down the slopes of hills, accumulating in the valleys. This is why low lying areas, such as river bottoms, will likely be colder than their surroundings on clear, calm nights. Therefore, while referring to this map, consider your local landscape when making your planting decisions.
As always, when planting time arrives, pay close attention to local forecasts and 1 to 2 week temperature outlooks. Information on 1-2 week temperature trends is updated daily and can be accessed at: http://www.cpc.ncep.noaa.gov/.
To access more detailed growing season summaries for specific locations in Missouri, including median dates for base temperatures of 30, 28, 24, 20 and 16 degrees, please go to the following link: http://mrcc.sws.uiuc.edu/climate_midwest/mwclimate_data_summaries.htm#.
REVISED: October 2, 2015