Would Spike Growth affect multiple floors or inclines?

Question

The description for Spike Growth says:

The ground in a 20-foot-radius Sphere centered on a point within range sprouts hard spikes and thorns. The area becomes Difficult Terrain for the duration. When a creature moves into or within the area, it takes 2d4 Piercing damage for every 5 feet it travels.

The transformation of the ground is camouflaged to look natural. Any creature that can’t see the area when the spell is cast must take a Search action and succeed on a Wisdom (Perception or Survival) check against your spell save DC to recognize the terrain as hazardous before entering it.

Say I cast spike growth at the top of stairs that descend 10 feet. Would the spike growth go down the stairs and onto the floor at the bottom, or would it only affect the top floor only?

Answer 1

Spike Growth affects the surface it's cast on. If there's an incline, it would likely follow the shape of the surface in that area, spreading along the slope within the 20-foot radius Sphere.

It's fine if an area starts on a first-level floor, extends across a staircase, and finishes on a second-level floor, as long as the valid surfaces are within the Area of Effect.

It won't affect walls or ceilings, and it won't jump between levels or floors that are separated by gaps (e.g. cliffs or large holes) greater than the diameter of the Sphere.

Also, the Area of Effect rules apply:

An area of effect has a point of origin, a location from which the effect’s energy erupts. The rules for each shape specify how to position its point of origin. If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect. To block a line, an obstruction must provide Total Cover. See also “Cover.”

DM's ruling

Your DM might decide otherwise based on their interpretation or any specific environmental factors.

Answer 2

Think of the point of origin as a lightbulb

When aiming an AoE spell, you first pick a point of origin, which is akin to aiming a laser pointer (except the caster can decide to stop at empty air). It either hits that point, or total cover blocks its path and then it hits the obstacle instead. Then the affected area extends from that point as defined by the rules, but spheres basically works like a lightbulb: everything that the light would hit (out to the radius) is inside the area of effect.

Spike Growth defines that the ground within the sphere becomes spiky. How you define "ground" is honestly up to the group (Does any floor count or does it have to be in contact with the earth? Do cliff sides count?), but it definitely affects multiple levels of elevation for as long as those are illuminated by the "lightbulb" point of origin.

In your example with the stairs, you would indeed cover the stairs down for as long as you can see the steps themselves from your point origin, and continue along the ground for as long as there isn't any ceiling blocking your view.

Answer 3

The spell says* (emphases mine),

The ground in a 20-foot-radius Sphere centered on a point within range sprouts hard spikes and thorns. The area becomes difficult terrain for the duration. When a creature moves into or within the area, it takes 2d4 piercing damage for every 5 feet it travels.

Because the spell indicates that it affects a sphere, we understand that when it says "area" it means "area of effect" in a general sense - a sense that is actually a volume - and not that that the spell itself acts only in the flat plane of a true area.

As such, it is easy to adjudicate. Any combat square within the sphere is affected. It will spread down stairs and to a floor below. If there are multiple levels on which one could stand within the sphere, each of them is affected.

For example, if the center of effect is on the central shaft of a spiral staircase, you might be able to get multiple circuits covered in the forty vertical feet between the lowest and highest point, depending on the slope of the stairs. However, if you are strict about the Total Cover rule, only the stairs below the point of origin would have their spikes on their upper surface. The stairs above that point would have the spikes on their undersides!

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*Props to Tarod for correcting the spell description to the appropriate edition text.

Answer 4

Spike Growth's AoE can include separate "grounds" on multiple levels

Technically, Spike Growth is limited to the "ground", rather than to a given "surface".

The ground in a 20-foot radius centered on a point...

So, on a technicality, the spell may actually be limited to "the surface of the planet". However, exactly what constitutes "ground" is up to DM fiat. This question suggests that the ground is any walkable surface, but that doesn’t really clarify anything, because that certainly includes walls and ceilings, depending on a relative plane of gravity or special ability, like Spider Climb.

It's feasible that, in an "underground" dungeon, each of the surfaces, especially the floor-like surfaces, might qualify as "ground".

Regardless, your intuition that the spell in not limited to a particular "ground" is correct. In your example, "... the top of stairs that descend 10 feet...", the spell would include ground in a 20ft radius from the point of origin, according to the Area of Effect rules--specifically Spheres.

A Sphere is an area of effect that extends in straight lines from a point of origin outward in all directions.

and

If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect. To block a line, an obstruction must provide Total Cover.

So long as you positioned the point of origin such that you could draw a straight line to each of the desired "grounds", the spell affects them. This would include each of the stairs, as well as any ground at the top of and the bottom of the flight of stairs, so long as it is within the volume of the sphere; the same goes for an inclined surface. No DM should prevent you from casing the spell e.g. on the slope of a hill, simply because it's an incline. You could not, however, affect ground on the opposite side of an obstacle. So, you couldn't affect the upper floor if the point of origin was blocked by the ceiling.

It's difficult to argue that ceilings and walls constitute "ground" in any natural linguistic sense. However, it would be reasonable for a DM to rule that certain walls or ceilings do qualify, especially insofar as they connect to ground--e.g., again in an "underground" dungeon, the ceiling and walls might constitute the "surface" of the planet.