In the north and transition zones, it is not uncommon for a lawn to be covered in frost on a cold fall morning. Whatever you do, stay off of it!
What Causes Frost?
Frost occurs on cold, clear nights when atmospheric conditions cause objects, like grass, to become colder than the surrounding air. Depending on the conditions, frost can be light and scattered, or heavy enough to kill flowers and bring on dormancy in plants.
On an even more scientific level, grass blades move water through them for nourishment, but this moisture freezes inside the grass when you observe a light frost across the outside surface. Water molecules naturally expand during the freezing process, so plant cells are damaged as the moisture ruptures cell walls. However, a light frost does not freeze the ground; your lawn's roots remain above the freezing point. As a result, you may have some blade damage after the cold weather, but the roots have a chance at healing the damage as the grass continues to grow.
A light frost, especially over several nights, shows visible signs of damage in the daylight, depending on the grass species. For example, Bermuda grass acquires a brown, patchy appearance and dies relatively quickly if warmer temperatures do not return. In contrast, St. Augustine grass develops a brown, yellow or even purple hue in the damaged area. Typically, you have a chance at reseeding the damaged area once spring arrives; the dead grass eventually becomes thatch that nourishes the soil below as the new seeds germinate and grow tall.
Why Stay off It?
When a lawn is frosted, the grass blades are literally frozen. Any activity on the lawn while it is frosty, like walking on it or mowing it, will "break" the grass blade and damage the lawn. It will recover eventually, but it may not fully bounce back until the following spring.
Scientifically, when a heavy object presses down onto the blades, the expanded water molecules slice through the grass and cause significant cellular damage. You notice impact damage after passing across the surface—the blades often produce a white or beige hue from being crushed along with the icy frost.
On a frosty morning at a golf course, nobody moves until the superintendent determines the course frost free. The frost dissipates as the sun rises and the air warms up. Sprinklers are sometimes turned on for 30 seconds or so to speed up the process, especially in the shade.
For a homeowner, deliveries, mail-persons, or children are usually responsible for unwittingly damaging the lawn.
One of the most practical ways to prevent light frost effects across your grass is watering the turf the night before. A deep watering allows the moisture to evaporate slowly overnight. This evaporation creates friction and heat around the grass blades. As the night air temperature drops below freezing, your grass flush with the ground has a slightly higher temperature from the evaporative heat processes. Your turf cannot reach the freezing temperature needed for water molecule expansion that causes so much widespread turf dieback.