How do Plant Growth and Slow interact with each other?

How do Plant Growth and Slow interact with each other?

If on spell-caster casts Plant Growth in an area and the other targets a creature with the Slow spell, what happens?

Supposing that the creature fails its saving throw against Slow would this mean that, while it is within the area affected by Plant Growth, its movement is two times, four times, or eight times slower?

I have got an inkling that the creature would only move 1 foot for every 8 feet of movement, but not sure is those two effects stack.

Thanks for your help with this.

While the outcome of Slow and Plant Growth appears to be the same - the creature takes longer to get from A to B - they achieve that outcome using different game mechanics.

Slow says: "An affected target's speed is halved".

Plant Growth says: "A creature moving through the area must spend 4 feet of movement for every 1 foot it moves."

So a creature affected by Slow moving through an area affected by Plant Growth would not only have its movement halved, it would also need to pay quadruple of that movement in order to move through the area. So the net effect would be a movement reduction to 1/8 of the regular movement speed.

Another argument which supports this rule interpretation is that while Slow affects the creatures in an area, Plant Growth affects the plants in an area. So the stacking rules don't apply because the two effects don't even target the same entities.

They work in parallel, but movement is 4 feet for every 1 in the area

In this case, slow reduces their movement by half.

Meanwhile, plant growth makes travel through the area cost:

... 4 feet of movement for every 1 foot it moves.

It's still the above movement cost, but their total movement opportunity has been halved from slow.

A use-case: 1 turn vs 9 turns moving 50'

Let's use a standard creature with a move speed of 30. Let's say that they are in the exact center of the plant growth area and are 50' away from leaving the effect.

Under normal conditions, they can traverse the 50' while utilizing Dash in one round.

But in this case, this creature under slow now has a move of 15.

And the plant growth makes every 1 foot of movement act like 4 feet.

So they can really just move 3' in a single move or 6' with a dash. It will therefore take 9 turns using Dash while under these two spells to clear the 50' to freedom.

• The end effect is that it takes 8 times as long for a creature to traverse this space if neither spell was in effect, so the asker was thinking along some kind of lines Feb 2, 2020 at 22:44
• @l0neGamer maybe, but that isn't what's happening inside mechanically. Feb 2, 2020 at 22:54
• It might be worth it to provide an example of the speed being halved due to Slow, and how that results in the 1/8 "effective movement".
– Mark
Feb 3, 2020 at 6:57
• @MarkwishesMonicawell Fair enough, i've added it - hows that? Feb 3, 2020 at 14:33
• @L0neGamer Yeah, and it depends on grid use or not, too. Feb 3, 2020 at 14:49