fire effect - enable/disable?

hello,

i got hitfilm ultimate some weeks ago and my progress with the program is great so far.

today i wanted to add some fire in the middle of a composite shot (lets say at 8 sec).

i cannot find a enable/disable checkbox to enable or disable the fire
isnt it possible to start the fire delayed where i want it with keyframes? (like the enable/disable checkbox in the particle simulator)

i know i can speed up the initial fire grow but how do i set a keyframe to start the fire in the scene where i want it to?

as my effect is nearly finished i didnt want to cut/slice the track to make the fire start in the second part.

What's the disadvantage of adding a slice in the layer? That's what I would do.

Alternately, you could add a black plane to the project that starts when you want the fire to start, and put the fire on it.