The Druid is in an area where there's no natural vegetation. He throws a pot with plant(s) in it under the enemy feet and casts Entangle. How would you rule it?

Based on my reading of the spell (and I'm certainly no expert in Pathfinder) , it might work, but it does read very much like the 3.5e version of the spell, and would probably suck quite badly. It does not imply that foliage increases in size, and in fact specifically makes allowances for changes based on the nature of local plantlife, which means you are stuck with the small size and weak strength of the potted plant, if the GM is making a reasonable interpretation of the last line.

Also, note that it doesn't affect a 40 ft. radius spread, it affects all plants in a 40 ft. radius spread. Which means if you had an area partly vegetated, where half was overgrown, and half was blasted earth, the effect would only take place on the vegetated half. So you're stuck with only the reach of the plant, and the entangling effect of a single potted plant. It would also not be out of place for the GM to require an action to throw the plant, which involves both a to-hit roll, and delaying time when you can cast the spell. Which would doubly suck.

I would say that the effect is "plants in a 40 ft. radius".

You mean that it creates plants in a 40 ft. radius? Maybe, but I don't think that's how its meant. For example, Fireball has an area of effect of "20-ft.-radius-spread", not "fire in a 20 ft. radius". The things that are affected are the items in that spread which are burned, which could include anything, not just fire.

Entangle, to my mind, only affects plants, but causes them to react by entangling people. It is the way the spell invokes the necessity for plants to be present, by making them an intermediary medium.

Yes. That's what the plant(s) in the pot are for. I think that if the spell would care how many plants there are it would be written a little bit more clearly. I don't see anywhere in the spells description it saying that spots without plants don't entangle.

I think that it should affect the entire area and fill it with vines/plants temporarily. It's freakin' magic, why does it have to be based on existing vegetation? Granted that's an arbitrary DM fiat... but I'd still stand by it. Why penalize the player and force an otherwise useful spell into one that becomes virtually useless? After all, what dungeon crawl has vegetation in the corridor or the larger cavers? Where do you find vegetation in the ruins of any terrain except jungle? A lot of the druid spell choices are restricted by environment or being outside so I say go the opposite way, make them more flexible and less useless.

Edit: The situation where the DM says, "Actually Bob, only half the clearing has vegetation in it. The other half, where all the monsters are standing, doesn't have any vegetation in it, your entagle is useless... once again." is far too common a possibility otherwise. *shrug*

Well, that's obviously a question of balance, Ryfte. It's a good argument to change how the spell worked, but if ImperatorK was asking for how the rule should be interpreted, I think it's difficult to say that that's currently how it is written. I'd personally argue that inherent in Druid spells is that they are strong in natural areas, and weak in man-made areas, much like Cleric spells inherently have limitations based on the faith of the cleric, but that's obviously just a matter of opinion, which is entirely up to the GM in question.

Regarding the plant pot, Imperator, I don't think it's of much use. If your GM agrees that it creates plants, the plant in the pot is superfluous. If he doesn't, I don't see how the requirement to have plants is satisfied by a single plant which isn't affixed to the ground (beyond what that plant can do, obviously). The last line in the SRD text does seem fairly clear that the effect of the plants depends on the vegetation that is present, even if you don't like my interpretation of the "Area" line. There really isn't any language to signify that an base amount of plant material is required to "grow" to fill the area, at any rate. That is more just interpretation based on how you'd like the spell to work.

The last line concerns the earlier sentence in the same paragraph. If the plants have thorns, the spell deals damage. If the plants would happen to be poisonous, the spell could have some poison effect. That's a thing that depends on the plants and for the DM to decide upon. I do not think it has anything to do with the rest of the description.
The spell says there have to be plants. I throw a pot with plant(s). The requirement is met.

Short answer - No. But I'd be willing to brainstorm with the player on some ways for Entangle to be more versatile.

I agree - an interpretation of the rules, as is, would in my mind make that not work.

However - A GM could also go natural vs man-made house-rule. As long as there is some natural ground - perhaps Entangle works in some more creative way. Barren earth magically, temporarily has ruts, ridges, etc that lead to the same effects basically as Entangle at the same saves. Rather than Entangle, the rough - difficult terrain may cause enemies (who fail the save) to stumble some. Going to one knee and then immediately up - (same basic effect of failing an Entangle save).

Whereas this would not (perhaps work in a dungeon, castle.) Unless you really wanted to get generous and have it be more of a "the ground opposes you" in general spell. In that case, even in a castle, etc - it might cause limited tremors that shake up even paved/stone ground in a small area.

If I were GM, I'd rather any of the above than "just the rules" or a character carrying and throwing a potted plant around.

I'd let it work just for the fact that when I read what they did, I laughed.

The second time they did, I might rule otherwise.