I'm a first time DM for first-time players. I was very afraid of them all dying really early on because I read that the encounters can be very hard, there are only 4 players instead of the usual 5, and they all have very low HPs (7, 8, 10 and 15). Goblins have one-shotted some PCs on the very first turn of a combat.

The opposite turned out to be the case: the level 1 druid is the star of the game. (Her true strength at level 2, Wild Shape, hasn't even come into play yet!) Mainly the entangle spell seems very strong to me.

In Cragmaw Hideout, when they entered the Goblin Den with Yeemik (6 goblins total), she simply entangled a few of them and the party barely took any damage at all. Same thing happenned to Klarg. He was supposed to be a big boss with a lot of HP and damage, yet she simply entangled him, and they all had their turns attacking him.

I searched a bit because I thought I might have missed something that made this spell so overpowered. One thing I found on a few forums was people saying that entangle requires vegetation to work (and no one disputing this statement). Such exchanges often went like this:

It is a strong druid spell, but it is limited. First, you need vegetation. That makes it pretty useless in a city or dungeon. Many natural settings also will not allow entangle to work: deserts, tundra, etc.

This would indeed make the spell often useless during the Lost Mine of Phandelver adventure, but I can't find what they're talking about in any official description of the entangle spell:

Grasping weeds and vines sprout from the ground in a 20-foot square starting from a point within range. For the duration, these plants turn the ground in the area into difficult terrain.

A creature in the area when you cast the spell must succeed on a Strength saving throw or be restrained by the entangling plants until the spell ends. A creature restrained by the plants can use its action to make a Strength check against your spell save DC. On a success, it frees itself.

When the spell ends, the conjured plants wilt away.

The way I interpret this, it just says that plants sprout out of the ground, even if none were there.

Is vegetation required for entangle to work? And, if not, is there anything I'm missing about entangle that makes it not incredibly overpowered? I'm afraid the spell will spoil the fun of the campaign for my other players because they haven't really been challenged yet.