Reducing render time

After talking to my lecturer about my rendering, it was suggested I really need to reduce my render time because it is far too long for the scene.

In order to do this it is important to separate 'problem' objects into their own render layers and try rendering them separately, reducing the amount of effects on them also makes a difference. this can be done in the Render settings under the render tab so that you can edit the objects separately and remove them from the main scene (problem bushes have been separated):

My class mate also suggested watching this video for more information: