Tim Bowen sheds some light on the origins and definition of the phrase I heard it through the grapevine.
The expression through the grapevine (or sometimes on the grapevine) is commonly used to mean ‘unofficially’ rather than through an official announcement, for example ‘I heard it on the grapevine that they’re planning to make some people redundant’. Rumours and gossip are spread on the grapevine but why ‘the grapevine’?
The term originated in the USA and comes from the telegraph system invented in the 19th century by Samuel Morse. The system required thousands of kilometres of telegraph wire to be installed, held in place several meters above the ground by telegraph poles placed at regular intervals along the telegraph route. People thought the wires and poles looked like the strings used to train vines so the telegraph lines became known as ‘the grapevine’. During the American Civil War rumours were often spread via the telegraph lines. When people were asked whether a particular story was true, they would often reply ‘I heard it through the grapevine’.