Not speaking for
@Umbran and not speaking about anything other than 5E, but that's the way it's phrased in the 5E PHB (which I happened to have next to me for entirely other reasons).
I don't have my books with me to verify, but I think this is the 1e text:
"When cast, a Speak With Plants spell enables the cleric to converse, in very rudimentary terms, with all sorts of living vegetables. Thus, the cleric can question plants as to whether or not creatures have passed through them, cause thickets to part to enable easy passage, require vines to entangle pursuers, and similar things. The spell does not enable the cleric to animate non-ambulatory vegetation. The power of the spell lasts for 1 melee round for each level of experience of the cleric who cast it. All vegetation within the area of effect are under command of the spell. The material components for this spell are a drop of water, a pinch of dung, and a flame."
And this is the 3e text:
"You can comprehend and communicate with plants, including both normal plants and plant creatures. You are able to ask questions of and receive answers from plants. A regular plant’s sense of its surroundings is limited, so it won’t be able to give (or recognize) detailed descriptions of creatures or answer questions about events outside its immediate vicinity.
The spell doesn’t make plant creatures any more friendly or cooperative than normal. Furthermore, wary and cunning plant creatures are likely to be terse and evasive, while the more stupid ones may make inane comments. If a plant creature is friendly toward you, it may do some favor or service for you."
So yes, you are right in that I'm describing how I run my own game, but I had thought I was running my own game based on what the game rules suggested per the above. And those rules suggest that plants are thinking slow plant thoughts about things plants would be concerned about even when a cleric or druid isn't speaking to them. And that suggests an animistic universe inspired by beliefs that anything that moves must be propelled by some intelligence. Since plants grow, they are in some sense alive.
When I grew up in the Caribbean, these beliefs weren't in fact just part of the ancient past, but many older people still held them in part or whole. For example, some laborers asked to cut down a particular large and ancient tree, would, before laying on the ax or saw, say something like, "Please do not be angry with me Mister Tree. I have to cut you down, because the white man has ordered it." And so forth.
I think some idea of how pervasive animism is to my campaign can be perceived through this post:
I've been toying with writing up my monster house rules in a MM format for at least my own use. This would be a portion of the Dryad write up. I thought I'd post it as inspiration. Even if you aren't using 3.X rules much less my house rules, the concepts here might be useful to you. Dryad...
www.enworld.org
However, it's not like dryads are unique to my campaign. They are a standard monster in D&D that has been around a long time.