Radiative cooling lets exposed surfaces drop below the surrounding air temperature. When the surface reaches the dew point and that point is below freezing, water vapour deposits directly as ice crystals — skipping the liquid stage entirely.
Hoar frost grows from vapour on still, clear nights and forms delicate feathery crystals. Rime is different: supercooled fog droplets freeze on contact, building rough, opaque ice on the windward side of objects.
A light cover traps the ground's stored heat and slows radiative loss. Watering soil in the afternoon helps too — moist earth holds warmth longer than dry, and releases it slowly overnight.