Frost is a tricky thing. Generally, the temperature needs to reach 32 degrees - the freezing point of water. But, sometimes you can have frost when the official reading is 35. Or ... no frost when it’s 28. One reason for this is that official temperature readings are taken 5 feet above ground level. And, cold air drops - just as hot air rises. So, the air can be colder at ground level than at 5 feet up. That’s also why low-lying areas get frost before higher areas do. Another reason is the air’s dew point. You don’t get ANY kind of condensation - including frost - until the air temperature reaches the dew point level. A third reason are heat traps, such as a driveway, stone wall or building. Their stored heat can keep nearby flowers blooming and create those interesting half-and-half frosts on parked cars.
Mary Knapp, Weather Data Library