Aboveground Mechanical Stimuli Affect Belowground Plant Communication

Maize plants can detect the presence of their neighbours and modify
their growth behaviour. Above ground interactions between neighboring
maize plants by brief light touches affect on below-ground root-root
communication. This study urges re-interpretation of results in
experiments in which plants are touched during the experiment. The
ecological significance of the observed responses still needs to be
further explored.

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0195646

https://www.theguardian.com/science/2018/may/02/plants-talk-to-each-other-through-their-roots