Daniel Barton said:

Be careful using the plastic sheeting. It usually does more harm
than good unless you're out there before sun hits it.

Why, what happens?

Because it cooks the plants underneath it. I once put out
a piece of clear plastic sheeting on my lawn to dry. While it
dried it managed to barbeque the grass underneath it.

What you did was create a little greenhouse with no ventilation, the grass
was cooked and smothered. When plastic is used to protect plants from
frost, it should be raised up so it is not touching and it should be
removed the next morning.
--
McQualude