The Plant domain provides no method of communicating with plants
Some domains include mechanical benefits in their granted powers sections (like the Travel domain), some domains mandate behaviors or attitudes in their granted powers sections (like the Law domain), and some domains make wild, unsupported claims in their granted powers sections (like that pesky Plant domain). It's terribly inconsistent, and you're not the first to notice this particular discrepancy.
So, much like picking the Law domain doesn't actually mean instantly attaining enlightenment, picking the Plant domain doesn't mean actually being able to speak with plants. This happens in Pathfinder, where purely descriptive text if taken literally sometimes has no mechanics to support it (like in this question). Therefore, while it's odd that a cleric with the Plant domain can't speak with plants (and, in fact, typically can't ever cast the spell speak with plants), unfortunately, that really is the case here.
(Game balance would likely be unaffected were a house rule made allowing a cleric with the Plant domain to use 1/day as a spell-like ability an effect like the spell speak with plants. I can say this having played a character in a D&D 3.5 game with the spell-like ability speak with plants 3/day from levels 1 to 9; the campaign remains unbroken by my character's ability to chat with shrubbery. Of course, the campaign doesn't hinge on plant monsters invading the world or some similar plant-themed apocalypse; in that case, such an ability might have more value.)