The wild flowers and herbs get cut before they have a chance to flower and propagate.
I think the sentence is in the passive voice. Can we use get instead of are to form a passive form? When we have to use get not to be?
Yes, that is a passive construction. "They get cut" can be phrased in the active voice: "Something cuts them".
Yes, there is a difference. The verb to be expresses a state. The verb to get expresses an action.
The window was closed yesterday.
The window got closed yesterday.
With to get, the action is explicitly placed in the recent past. This suggests that the window was open the day before yesterday. With to be, the state is placed in the recent past, but there is no mention of the time of the action. We are free to assume that the window has been closed since some time in the distant past. For all we know, before yesterday the window might never have been open.
However, to be also leaves us free to assume that the times of state and action coincide. Under that interpretation, was closed and got closed are rough synonyms.