To be honest I'm not even sure if I should tally my first frost. 31.5F was the low (below-freezing being corroborated by other stations in the neighborhood), so unambiguously the first freeze. Even here, though, actual frost was tenuous and confined to well-insulated rooftops. Most crucially, there was no damage to tender vegetation... my petunias and impatiens are no worse for wear. Can you first freeze without first frost? If not, how do you frost without killing the impatiens?