Why does the fence need to be kept free of vegetation?
Overgrown vegetation, grasses or bushes that come into contact with the fence will create a connection to the ground and thus complete a circuit in the electric fence system. This will short circuit the fence.
The current that now flows reduces the voltage on the fence. Your electric fence is no longer as effective. It is for this reason that you should keep your electric fence system free from vegetation.
See also: Disturbances to the electric fence.