The Ice Inside the Blade
Grass blades are roughly 85% water. When the temperature drops below freezing, the water inside the cells of the grass blade physically turns into sharp ice crystals.
The Shattering Effect
When grass is frozen, the blades lose all their natural flexibility. If you walk across the lawn when it is white and crunchy, the weight of your boots physically shatters the frozen cells inside the plant. The grass blade breaks internally.
The Black Footprint
You won't notice the damage while the grass is still frozen. However, as soon as the sun comes out and the lawn thaws, the areas you walked on will turn black or dark brown. The grass has been killed by the structural damage. While established grass will eventually recover in the spring, newly laid turf may not have the energy reserves to bounce back, leaving permanent dead footprints across your new lawn. Stay off the grass until the frost melts!