What is the point of the "Grasping Undergrowth" effect of the spell Druid Grove?

Question

The 6th level spell Druid Grove (XGE p.154) lets the caster pick various effects for the targeted area. One of the possible effects is

Grasping Undergrowth: You can fill any number of 5-foot squares on the ground that aren't filled with fog with grasping weeds and vines, as if they were affected by an entangle spell.

And the Entangle spell works the following way:

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

Since Druid Grove has a casting time of 10 minutes, it seems unlikely that you could successfully cast it during a combat encounter to activate the restraining effect from the Entangle spell (which has a casting time of only one action) and instead only get the difficult terrain. But difficult terrain is also overshadowed by another feature of the Druid Grove, "Solid Fog" that among other effects also causes "every foot of movement through the fog costs 2 extra feet", which overshadows the 1 extra foot caused by regular difficult terrain.

So, why would one choose to use the Grasping Undergrowth effect at all? (apart from the role-playing aspect of being able to use it as seats or a bed)

Answer 1

Some additional spell effects are incompatible with fog

The final section of the Druid Grove spell description describes how you could have gust of wind or wind wall in a couple locations. Both those spells disperse fog.

Placing Grasping Undergrowth in the location of where those wind effects are placed allows maximizing the effectiveness of the grove.

Fog is implied to be a gas by these two spells

Wind wall explicitly says

A wall of strong wind rises from the ground at a point you choose...

and

The strong wind keeps fog, smoke, and other gases at bay.

This means that wind wall explicitly works against the Solid Fog of the Druid Grove spell, as there is nothing in either description giving an exception to it.

In addition, the use of the phrase "and other gases" instead of just "gases" implies that fog and smoke are considered gases, even though both consist of objects suspended in the air (water droplets and soot particles, respectively.) Therefore, if fog is a gas, then gust of wind is effective against the fog as it

disperses gas or vapor.

A DM is of course free to rule how they like, but rules just do what they say they do, and without additional arguments from the rules, these effects affect the solid fog of Druid Grove, and so make it meaningful to use the "Grasping Undergrowth" in area affected by the wind spells.

Answer 2

Let invaders see what you want them to see

Sure, you could just fill the entire 90-foot square area of the druid grove with solid fog, and just force them to be blind and walk at 1/3 speed while anywhere in the area. But maybe you have something more elaborate in mind. You could create "corridors" of fog-free areas that appear to be a path of least resistance, to lead invaders in the direction you choose. And of course, you fill all the fog-free areas with grasping undergrowth, to make it a bit harder on whoever wants to come through. For example, you could lead them toward the area of your spike growth, with a suitable shiny object in plain view to lure them over it.

Another example would be a 10-foot wide corridor with a dead end and a gust of wind effect pushing toward the dead end, creating a trap that forces anyone stuck in it to venture into the fog rather than try to walk 60 feet upwind in difficult terrain. Now they can't just change their mind and turn around and leave after walking 5 feet into the fog. They have to push through 30 or so feet of solid fog if they want to escape.

Placing narrow corridors between fog walls also encourages a group of enemies to line up in a way that may be more convenient for someone aiming to catch all of them in a spell's area.

Make the spell harder to dispel

Druid Grove has a special interaction with Dispel Magic:

A dispel magic cast on the area, if successful, removes only one of the following effects, not the entire area. That spell’s caster chooses which effect to end. Only when all its effects are gone is this spell dispelled.

If you fill the entire area with fog, then a single Dispel Magic could remove all the fog, and with it the main impediment to movement within the grove. If you mix fog and undergrowth, you force an invader to cast Dispel Magic twice instead of once if they want to have unrestricted movement through the spell's area. (And that's assuming both castings are successful.)

Answer 3

It's probably a mistake.

You're correct -- difficult terrain is functionally the only effect you're going to get out of this, and is strictly inferior to Solid Fog. But that's actually pretty strange.

If the design team wanted the area to just be difficult terrain, there's no need to invoke the entangle spell to get there -- they could have easily just said "it's difficult terrain". Because of this, I think it's most likely that this was an oversight on their part. I think they intended that the Grasping Undergrowth areas to have the restraining effect on creatures and forgot that entangle doesn't work like web (or entangle got edited to only restrain on the initial casting after they had written druid grove and nobody noticed the dependency).

I'm not sure it actually matters that much.

That said, it's probably not all that relevant. While failing a strength save and getting restrained is bad, it's really not that much worse than trying to have a fight in Solid Fog, which is heavily obscured for the enemy and transparent to the caster's friends.

Restrained gives the target disadvantage on attacks, advantage on enemies attacking them, disadvantage on Dexterity saves, and they can't move from their space.

In Solid Fog, the target has disadvantage on attacks, advantage on enemies attacking them, and with triple movement costs, they may not be prevented from moving, but it's the next best thing.

The only real difference is the dexterity saves, but that's probably balanced by the fact that an enemy can tear free of restraining effects, while Solid Fog provides no real way to bypass it. It's largely a wash, in my view.

Answer 4

To Set a Trap

If you have a grove, camp site or other location that you want to protect. If you expect interlopers and can guess where they would approach (like along a deer trail, a well worn passage or near a doorway) or you can creatively encourage them to follow a certain path. If you can set sentries to watch that path and take advantage of the spell's effect when the "bad guys" arrive.

This could slow down attackers enough to give defenders time to organize a response. The spell also lasts 24 hours (it can be made permanent too), so it's perfect for defense.

Why vines instead of fog? Archers cannot typically see through the fog to shoot those caught in the vines.

Answer 5

The difference being that with the Fog it slows a creature down while the Grasping Undergrowth potentially stops them altogether while also giving them disadvantage on Dexterity Saves AND their Attack rolls.

Dont forget that mixing the effects prevents it from being easily dispelled as well. IF you combine it with the Spike growth Spell as well then you have yourself a really nasty thing.

Move too much take damage.

Move onto the wrong spot you might be "entangled" and get damage.

Stay in the fog and be blinded while others pepper you with attacks benefiting Advantage.

Adding Wind wall making it nearly impossible to blow away the fog or attacking at range.

Gust of wind pushing you back into spike growth dealing another instance of damage and potentially needing another save against "entangling" again.

Do not attack Druids in their Grove; should be a proverb in all fantasy settings out there.