How do leaves know when it's time to drop off of trees?

Question

I know, I know, but it is just semantics!!

Answer 1

The amount of daylight  when it gets to a certain point of daylight versus night.

Answer 2

Bravo...very deep question. I think the leaves do "know" when it's time, At least Washington leaves do. Californian "trees" just don't have a clue.  

Answer 3

Leaves don't know anything. When there's not enough sunlight out, the cells in the leaves begin to shut down. Because they can't undergo photosynthesis, they just die.

Answer 4

When all the leaves are brown leaves are brown and the sky is grey

Answer 5

I tell them its time, yes I know lame and uninformative, lol