What Does Last Frost Date Really Mean?
What exactly is meant by last freeze or last frost dates?
These dates are published by the National Gardening Association for the College Park ZIP Code 20740. I'll explain what they mean below.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
| Last 16°F | Mar 12 | Mar 5 | Feb 28 | Feb 24 | Feb 20 | Feb 16 | Feb 12 | Feb 7 | Jan 31 |
| Last 20°F | Mar 27 | Mar 22 | Mar 18 | Mar 15 | Mar 12 | Mar 9 | Mar 5 | Mar 2 | Feb 25 |
| Last 24°F | Apr 7 | Apr 2 | Mar 29 | Mar 26 | Mar 23 | Mar 20 | Mar 16 | Mar 13 | Mar 7 |
| Last 28°F | Apr 19 | Apr 14 | Apr 11 | Apr 8 | Apr 5 | Apr 2 | Mar 30 | Mar 27 | Mar 22 |
| Last 32°F | May 1 | Apr 26 | Apr 23 | Apr 21 | Apr 18 | Apr 16 | Apr 13 | Apr 10 | Apr 5 |
| Last 36°F | May 16 | May 12 | May 9 | May 6 | May 4 | May 1 | Apr 29 | Apr 26 | Apr 22 |
Nobody actually measures the last frost dates because there are so many variables. Ever notice that on some chilly mornings that you have frost on the windows on one side of your car but not the other? Wind, humidity, and temperature all influence frost production. Estimated last freeze and frost dates are exactly that -- Estimates. They are based on low temperature readings at an official reporting location. For 20740, these temperatures are usually those recorded at the Central Maryland Research and Education Center (CMREC) - Beltsville Facility located on Beaver Dam Road in Glenn Dale.
The 28°F readings are usually associated with a hard freeze, the temperature at which soft foliage will be killed by freezing temperatures. The 32°F readings are considered soft freezes where plant cells can freeze and annuals and other non-hardy plants can be killed. The 36°F readings are considered the temperature at which frost formation is likely and plant cell damage can also occur. Note that frost can occur at 38°F or even at 40°F if conditions are exactly right. The percentage columns reflect the previous statistical likelihood that that low temperature will occur. So 90% means that the temperature is still very likely to occur up to that date while 10% means that the temperature is not very likely to occur after that date. The percentages literally reflect the number of occurrences for that temperature/date pair over the previous 10 years.
Since frost formation isn't directly measured because it varies with microclimate, 36°F temperatures are typically used as a good estimate. I've highlighted four settings in the table above that I find interesting. Since these dates are based on past measurements and weather is highly unpredictable, making freeze and frost date predictions is a bit of a gamble. Do you feel lucky? Well, do you? The 50% column reflects a 50/50 chance that a particular low temperature was observed (at the recording station!) in the past. So April 5th has been the date where there was a 50/50 chance of a hard freeze occurring. April 18th has been the date where there was a 50/50 chance of a soft freeze occurring, and May 4th has been the date where there was a 50/50 chance of a likely frost-forming (36°F) temperature occurring. So May 4th is often quoted as the average last frost date for ZIP code 20740. I highlighted the Last 36°F 10% cell because this indicates that there was still a possible frost date as late as May 16th. Quite unlikely, but still possible.
In conclusion, April 18th has a 50/50 chance of being the last freeze date for us, while May 4th has a 50/50 chance of being the last frost date for us. By May 16th, the likelihood of frost is almost gone which is why Mothers Day is often used as a good date to start transplanting many things into our gardens. Another thing to consider is soil temperature, but that's the subject for a future blog post.
And now you know more than you thought possible about our last freeze and frost dates.