In the past few years we’ve learned a lot about how plants communicate with each other. The first such communication to be discovered was through volatile organic compounds that plants secrete in order to notify each other of predators. This is known as hormonal sentience^.

More recently, scientists have discovered that plants communicate even underneath the surface, using fungi:

http://www.the-scientist.com/?articles.view/articleNo/38727/title/Plant-Talk/^

http://www.bbc.com/earth/story/20141111-plants-have-a-hidden-internet^

Things get even more interesting. It appears that trees have a complex social life, being aware to recognize family members and different species. It’s as mind-blowing as it is expected (humanity is slowly but surely waking up to the fact that intelligence comes in many flavors):

https://www.brainpickings.org/2016/09/26/the-hidden-life-of-trees-peter-wohlleben/^

Edit 2018-08-30: check this short video^ about the wood wide web.