Don't let the grass grow under your feet.
(Not let the grass grow under someone's feet)
Meaning:
Act now without any delay.
If you say "Don't let the grass grow under your feet" to someone, you tell him/her not to waste time and to do something immediately.
If someone doesn't let the grass grow under his/her feet, he/she does something right now without waiting.
Example:
Now that you know what to do, act right away. Don't let the grass grow under your feet.
A: Do you think the manager's already started to do something to solve the problem?
B: I believe so. He's the kind of guy who doesn't let the grass grow under his feet.