I see the term “frost-free date” used this time of year. What is the frost-free date for southeastern Michigan and what exactly does that mean?
Frost-free dates are a scientific best guess to determine when you will be able to plant out annuals or any other tender plant without the likelihood of those plants being killed by a frost. Michigan State University has published a table of dates based on a thirty-year average (1951 to 1980) that gardeners can use to determine these dates in their local community in Michigan. To use an example, the published dates for Pontiac’s 30-year average date of the last 32-degree temperature is May 24. This means that it is statistically safe to plant on or after May 24 in the Pontiac area and not experience a low temperature under 32 degrees.
This date is not a guarantee, only a statistical calculation based on historical data. Many gardeners choose to plant earlier and are able to protect their tender plants with coverings on any night when the weather is predicted to drop to near freezing or below. However, planting early is often unnecessary work, as plants placed into warmer soil with warmer night temperatures will grow with enough vigor to match the growth of the earlier transplants very quickly. The old adage, better safe than sorry, can also mean less work to produce the same vigorous plants.
If you would like the exact statistics for your community, visit the website at http://climate.geo.msu.edu/climate_maps.html and click the map on the spot nearest to your Michigan gardening location. Frost-free dates in southeastern Michigan vary by location. On the website you will also find a myriad of weather and climate information that may make your gardening easier, or at least more scientific.