This was a classic radiational freeze. Calm, clear night, with lows in the mid-30s. Some find it strange that you can have frost when the air temp is above 32 degrees. It happens because some objects (e.g. grass) lose heat to the surrounding atmosphere through radiation. Not the Chernobyl kind. The kind that you feel when you’re near a fire and the glowing coals emit their heat. Only it’s far more subtle than that, thankfully. The result is that the grass actually becomes cooler than the atmosphere, and dew freezes to form ice crystals on the grass. That’s frost. If you notice, under trees, shrubs or just about any object, frost often doesn’t occur on nights like these. That’s because the heat radiated by the grass is reflected back down, keeping things just a few degrees warmer.
The cold grass often refrigerates a thin layer of air at the surface (which tends to pool in low spots) and so you get what’s called an inversion layer. Air is colder at the surface, and gets a bit warmer as you rise. That’s why, as you see in this photo, you can have frosty grass, but everything more than a few inches off the ground is unfrozen. This photo was taken two weeks ago, and those marigolds are still bloomin’ their heads off. If it’s a windy night with a cold front that’s bringing in sub-freezing air, the atmosphere is mixed thoroughly and everything freezes equally. That’s a very different thing—and usually does a better job killing off the whole garden.
To be sure, some radiational freezes are cold enough to take out the garden. But I don’t mind a light frost like this, especially when it’s followed by a long, lovely Indian Summer, with plants ablaze with the reds, yellows and russets that make autumn landscapes so gorgeous.