damp off
[for seedlings] to die from too much water. All the new plants damped off, and we had to buy some from the nursery. The little seedlings damped off and withered away.
damp something down
1. Lit. to make something damp. Damp the clothes down before you iron them, Please damp down the clothes first.
2. Fig. to reduce the intensity of a flame, usually by cutting down on the air supply, as with a damper. Please damp the woodstove down. Damp down the air supply or you are going to end up with a raging inferno.