Answer and explanation:
A grapevine is the informal communication that takes place among members of an organization. It is informal in the sense that it does not occur through formal, controlled channels. It is spontaneous, usually taking place horizontally - that is, among members that belong to the same hierarchy level. That is not a rule, though. Grapevine can happen vertically as well. But since this sort of communication is more personal, it is much more common for it to happen when there is no hierarchy involved.
Through grapevine, news travels fast. Someone can hear something about a promotion or a massive layoff and quickly spread the information to his friends, who will most likely spread it even further. There are benefits to this informal communication since interaction between employees help them get a sense of belonging, of being part of the organizational family. However, gossip, rumors, and fake news are not filtered and discarded. They are passed along and can undermine efforts and security.
The best way to counteract this is to find channels to provide news and clarifications. A question and answer session with employees, for instance, may allow them to address the rumors and hear what is really going on for themselves. Another way of beating the grapevine is spreading the necessary news in a quicker manner, through a fast and reliable channel, in order to avoid the informality altering the news' content. Having frequent group meetings can also help combat excessive informality. If employees have access to information straight from the source on a frequent basis, they are less likely to seek and believe gossip.