]> Horticulture is the art and science of the cultivation of plants The occurrence of air temperature below 0 C (32F) that kills annual vegetation without formation of frost crystals on surfaces. See freeze, dry freeze, hard freeze, light freeze. The occurrence of air temperature below 0 C (32F) that kills some, but not all, annual vegetation. This often occurs in the 0 to -1 C (32 30 F) range. A thin and more or less patchy deposit of hoarfrost on surface objects and vegetation.