Speak with Plants is the same as Speak with Dead, really, just using a different medium. Firstly, you have to be looking for information of a specific sort. Then, you have to ask the right corpse/plant, ie one that would be in a position to have the information you want.
So if you're just walking through the forest and randomly cast Speak with Plants, then "Mmmm, warm," sounds like a perfectly reasonable answer. On the other hand, if you're chasing a band of raiders who kidnapped the farmer's daughter and you come to a fork in the road, you might travel a few hundred yards down one path and ask the roadside plants, "Did riders come past here?" If they have, then you know which way the bandits have gone! If not, then you know they took the other fork!
Perhaps if you come to an isolated farmhouse with a farmer and his one daughter, and are suspicious about the daughter, you could ask plants on the farm, "how many people live in the house?" When the plant answers back, "One" you know that something funny is going on.
Aside from its divination value, Speak with Plants works on creatures of the [Plant] type, like Treants. And if they are friendly, you can use the spell to ask them for a favor (as determined by the DM.)