Please let me know if I should be checking with someone else about this - but I noticed on the Spring and Fall fact sheets covering the frost/freeze dates, Montgomery County sites are few and far between. In the Spring, only Clarksville is listed; in the fall, only Rockville is listed.
I tried to find the sources listed for these fact sheets and could not find any charts. The climates for Rockville and Clarksville are rather different - is it possible to find the Spring frost dates for Rockville or the Fall frost dates for Clarksville?
If you can just point me in the direction of a link, that would be great.
All the best
The historical records of frost free and first frost dates are not maintained by standard sensors. The dates in our handbook are based on anecdotal records and are not 'etched in stone'. In fact, any given landscape in central and western Maryland could have one or more areas where micro-climates could result in a frost as late as mid-May. It falls upon the shoulders of MGs and other horticultural instructors to encourage homeowners to exercise patience in the spring and caution in the fall, and to simply listen to weather reports and monitor their plants. You will find several sites on the internet that suggest such dates, but they are no more authoritative than our own information.