I encourage my players to break my game as much as possible, because it leads to the best results, so I don't mind this. But hell, I've had to rewrite SO MUCH because the ranger decided to talk to a tree or something.
A list of names but one is smashed apart??
Talk to a tree and figure out what happened AND the name you weren't supposed to know till a few months later!
Murder took place and you have to find out the killer????
Hm...Did you say there were flowers on the table ? :)

There are plants EVERYWHERE! Animals where there aren't plants! How am I supposed to avoid spoiling my players when they do this? I don't want to just tell them no

The information that plants and animals have is up to you. They won't differentiate between different humanoids in the same way an intelligent creature would, they probably can't read, and what is a table? The corpse of my family? They aren't guaranteed to be cooperative and the information they have isn't guaranteed to be reliable. Maybe they provide vague clues, maybe the details most fascinating to them are irrelevant. Let your imagination run wild and focus on what makes it fun for everyone, you and your players.

Animals and plants have one big limiting factor when it comes to info: low intelligence. I usually DM where plants and animals don't learn names of people or places, but they can give descriptions ("a big, scary man" or "it barely touches my branches...so graceful and dextrous"). Think of it like this: a plant/animal is like a toddler. It can give you broad descriptions but the info may be vague or even incorrect.

You're not telling them "no". They just decide to get all their info from dumb organisms!

Flowers on table "PAIN...PAAAIIIINNNN" or "SADNESS....so SAD....Starving...." or "What evil man?! He gave us Water!"

Would cut flowers on the table still be "alive" to be able to respond to the ranger?

Quote from woodja >>

Would cut flowers on the table still be "alive" to be able to respond to the ranger?

Depends. Most still draw water but eventually die. Some cuttings like from gardenias will root in a jar of water and will grow if you plant them.

Quote from Wysperra >>

Quote from woodja >>

Would cut flowers on the table still be "alive" to be able to respond to the ranger?

Depends. Most still draw water but eventually die. Some cuttings like from gardenias will root in a jar of water and will grow if you plant them.

 Isn't that just capillary action in effect?  The flowers no longer grow so it is just the plant sucking up the water and the flower not drying out.

Quote from woodja >>

Quote from Wysperra >>

Quote from woodja >>

Would cut flowers on the table still be "alive" to be able to respond to the ranger?

Depends. Most still draw water but eventually die. Some cuttings like from gardenias will root in a jar of water and will grow if you plant them.

 Isn't that just capillary action in effect?  The flowers no longer grow so it is just the plant sucking up the water and the flower not drying out.

 Again, it depends. In some ways, they're undead. However, the plant will still perform bio functions until it gives out. It just dies faster than it can grow. But like I said, some plants continue to grow and even root in plain water.

You can probably be super vague with time, also... My toddler thinks everything is either "tomorrow" or "last year."

So, your Ranger could ask a question about "who passed by here," and get a story about something that happened 37 years ago, as it was just a sapling...

Also, what kind of sensory input do the plants get and how do they get it? If they don't have anything that equates to vision or hearing, that limits what they can pass on to your players. Maybe they only have touch (which might include limited hearing if you think about sound waves, but the plants might not speak common).

I think talking to plants and animals presents some fabulous opportunities for humor and mis-direction

That was the exact reasoning I gave to get out of that situation omg

Quote from pocketmouse >>

 If they don't have anything that equates to vision or hearing, that limits what they can pass on to your players. Maybe they only have touch (which might include limited hearing if you think about sound waves, but the plants might not speak common).

 They can definitely detect light and color, not sure about details. Plants are also very sound sensitive. They tend to grow better when exposed to classical music (both classical classical and classic heavy metal).

I'd say this ability is as powerful as you make it. I also think that's some commendable creativity on the part of the players to think of using it that way.

Plants and animals are great for clues and hints, but are unlikely to have detailed information as others have mentioned.  Also they are certainty unlikely to have proof that other beings would believe is credible ("oh the pot plant told me the cardinal's men kidnapped the kings daughter").

Also keep in mind they would also have their own personalities, traits and likely desires (as any other NPCs).  Some plants may not be at all concerned with people, or that potted flower in the windowsill may not want to give up any information unless you take it outside and plant it in the sunny garden with the other plants. 
Animals on the other hand have various loyalties and demeanor (some can be nice or mean inherently) and may even be cunning and want to be bribed (where gold and traditional wealth is of little value to them).  So a hound may want some meat from the kitchen it is never let into, or have been treated quite well by the passing murderer and thus feels some loyalty to them or that they aren't a bad individual.

All in all it can open up a lot of extra complexities and interesting twists to the game.

The answer is perspective.

During one session in The One Ring, one of the party's characters used a similar ability to talk to a murder weapon. What the DM did was probably the most brilliant thing I ever experienced while role playing.

He he described what happened but from the dagger's own perspective aka view point instead of describing straight up what happened in the room. So firstly the dagger never saw the murderer entering because it was in its scabbard. All it heard was a man's voice. Secondly while it witnessed the murder after being drawn, all it could really give was a detailed description of the victim's spleen. So while we the party got some clues, like we learnt that the murderer was a man, it didn't reveal the plot but it did make for a really dramatic session.

So my advice is, let the players keep using the spell/ability but don't give them a CSI witness statement.  Instead give them the information from the perspective of the plant or animal.

A mouse for example would probably only see the feet of the people in a room and noticing smells. So instead of saying a wood elf was in a room tell them the mouse say a pair of green booths whose owner smelled of the forest.

A tree might see centuries as days. So its description of events would be short flashes like stills instead of a narrative. First the tree saw one man in a room, then three men, then the room was empty except for the dead body of the first man.

This will allow your players to get clues without revealing the plot while making the spell seem much more thematically and dramatic.

If you want to make it realistic you would have to consider the different animals / plants that they are speaking to.  Chryckan mentioned a mouse, well a mouse only clear vision up to about 1 foot (30cm).  It's sense of smell and hearing are quite good however.  So if your players are human/humanoid it would be hard for a mouse to convey any sort of discription via smell or hearing.  A tree or plant doesn't have eyes and it's sense of hearing/smell is completely alien to us.  Maybe they sense a presence?  

Also take into account that peoples ability to relay information about a scene they weren't purposely observing is terrible at best.  Think about the last fast food place you went to.  Try to think how many people were there?  Can you describe your cashier or any of the other patrons?  Probably not, or your answers might be just you inventing memories when asked to think about it (something people do all the time).  

Unless the person was really remarkable the mouse would just say  he smelt like a "two leg" and leave it at that.  Why would he have any reason to take note of what happend unless it directly effected him

Well, what does a plant have to say? "Photosynthesis?"

Quote from Voidcritter >>

Well, what does a plant have to say? "Photosynthesis?"

These are some real world scientific facts about plans that we know to be true. (For trees at least)

They have a pain reaction.

They can different between light and dark.

They can communicate with other trees.

They can defend themselves against an attack (by parasites and disease similar to an autoimmune reaction in humans. So no tree ents brawlers).

Now lets say the party's ranger stumbles upon an area where a bunch a trees been ripped out of the ground and ask a nearby tree what happened.

Then just based on what we now to be true in our world the tree could tell him that "in the middle of the night (because it was dark) it heard the other trees cry out in pain."  It's not much but it is what a tree in our world would reasonably be able to notice and communicate.

Now in  a magical world trees can be completely sentient or have animistic spirits and souls the plant might notice a lot more. It's just up to the DM to decide.

First the DM has to decide how sentient/aware a plant is.

Next what it can and will perceive.

Lastly, which things falls in its sphere of interests. Meaning which things and events it will actually be bothered to take notice of. 

Then based on those three criteria there will be an small overlap which will consist of any information the plant can communicate to a character. It just probably won't be what a human or elf would notice if they witnessed the same thing happening.  Which brings us back to the OP's question. That being able to communicate with plants and animals probably won't be all that informative.