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The transformation of the ground is camouflaged to look natural. Any creature that can't see the area at the time the spell is cast must make a Wisdom (Perception) check against your spell save DC to recognize the terrain as hazardous before entering it. (PHB, Ch. 11, Spike Growth).

The general rule is that making an ability check requires an action.

An ability check tests a character’s or monster’s innate talent and training in an effort to overcome a challenge. The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results. (PHB, CH. 7)

Based on that, a passive Wisdom (Perception) check does not seem to suffice for noticing the area covered by spike growth if one did not see the casting of the spell.

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

  • Is there any good reason to not require an Action to make the Wisdom (Perception) check to notice the spike growth if the creature has not seen the spell cast?

  • Put a different way, is there a good reason to apply a passive Wisdom (Perception) check for this case?


An example I'll use is: the party being chased, they go around a corner/pile of rocks, and the druid (trailing the others) casts the spell and then moves to catch up to the party. The pursuers didn't see the spell being cast, and unless they take an action to look at the area as they come around the pile of rocks/logs/other obstacle that obscured vision, they won't see that the spike growth was cast there, and begin taking damage as soon as they enter that area.

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No

If the NPCs are canny enough to reason that a Druid might cast Spike Growth and then slow down to investigate and/or request making a perception check, that would require an action.

If they don't, but receive passive checks (because, see below) those aren't actions. Just because the check can fail does not mean it is an active check or an action.

Also, in the very text you quote, is the statement:

A passive check [...] can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

Or, dare I say it, a hidden terrain feature that the NPCs have no reason to expect is there, but which could change their actions if they noticed it. I.e., exactly this situation.

I'm not sure you're obligated to do so in this case, but I certainly would, probably at disadvantage because they're chasing someone.


I originally wrote this answer thinking it was your PCs trying to detect the Spike Growth, rather than the pursuers, and that made it seem very obvious to me. When I analyze it from an NPC point of view it definitely seems both different and more difficult, but I don't think it actually is different.

I think the difficulty stems from the fact that I, as GM, would have to make the judgment call about whether they're smart enough to slow down and investigate, which is awkward because I have full knowledge not available to the pursuers.

But this is exactly what the passive wisdom check is for: I don't have to think hard, I don't have to contort myself, I just have to follow what my policy would be anyway-- a penalized passive check. Which is not an action.

(Does this mean PCs get a chance to think their way out of something the NPCs don't? Yeah. That's why they're the heroes.)

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    \$\begingroup\$ Your suggestion, GM/DM side, seems to be Passive Check with disadvantage. I like that. May adopt it. \$\endgroup\$ Mar 17 at 17:15
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    \$\begingroup\$ @KorvinStarmast yes, but to be clear I would apply it to PCs in the opposite circumstance. And the penalty is due to the circumstance of chasing/being chased. \$\endgroup\$
    – Novak
    Mar 17 at 17:33
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TLDR: The spell doesn't say an action is required to make the check, so that's not a requirement of the spell - but the DM can still rule that the situation requires it. The DM also decides whether the check is active or passive.

The text of spike growth says:

Any creature that can't see the area at the time the spell is cast must make a Wisdom (Perception) check against your spell save DC to recognize the terrain as hazardous before entering it.

Now compare that to, say, the text of a spell like disguise self (emphasis mine):

To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC.

Note that while disguise self explicitly states that one needs to use their action in order to make the check, spike growth does not. Ordinarily, it might be enough to simply point that out and leave it at that; spells do what they say they do. However, if you are under the impression that:

The general rule is that making an ability check requires an action

then we need to go a little deeper here, because...

That's not a rule

The rules on ability checks which you cite say that:

The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure.

From this statement, you conclude that "making an ability check requires an action," but that doesn't follow logically. When you do spend an action to attempt something that might fail (except an attack), then that does permit an ability check. But that doesn't mean that any time a DM calls for an ability check, you must have spent an action. "When it is raining, I always carry my umbrella" doesn't mean that any time I am carrying my umbrella, it must be raining.

Not all ability checks require that an action has been spent on them. For one thing, not all of your ability checks happen on your turn. In the list of examples in the PHB section on Using Ability Scores, we can see that Strength checks are called for when another creature tries to interfere with your climbing or swimming. But if they make such attempts on their turn, you don't have an action to spend, so if an action was required, you couldn't make the check.

Even on your turn, Dexterity checks are called for when you try to "keep from falling on tricky footing", and an Acrobatics check specifically for "when you're trying to run across a sheet of ice" - but how could you run (dash) if you have already used your action to keep from falling? Neither could you take the Cast a Spell action on any turn if you needed your action to "stay upright on a rocking ship's deck". You couldn't ever attack underwater if you needed your action to both make a Constitution check to "hold your breath" and to make the attack.

So, if not all ability checks require actions, how do we distinguish between those that do and those that don't? That's covered under Other Activity on Your Turn (emphases mine):

Your turn can include a variety of flourishes that require neither your action nor your move.

You can communicate however you are able, through brief utterances and gestures, as you take your turn.

You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.

If you want to interact with a second object, you need to use your action. Some magic items and other special objects always require an action to use, as stated in their descriptions.

The DM might require you to use an action for any of these activities when it needs special care or when it presents an unusual obstacle. For instance, the DM could reasonably expect you to use an action to open a stuck door or turn a crank to lower a drawbridge.

Thus, some activities (eg., free object interaction) never require an action, some (eg., some magic item use) always do, and some (eg., an unusual obstacle) might at the DM's discretion. For any of these activities that also have a chance of failure and thus require a check, it comes down to a DM ruling on how absorbing the activity is. If it is something you can do as part of a dedicated action, it might require a check but not your action. But if it is something that needs so much of your time or attention that it precludes you from doing something else on your turn, then that is a check and an action.

As an example of how I might run this, suppose the PC's in my party are walking through the woods when they are ambushed by a small group of brigands that the PC's suspect are screening a larger group. All I've said so far is that several rough-looking men, weapons drawn, have emerged from hiding places and look hostile - roll for Initiative. Once attacks start flying, one of my players wants to attack, but prefaces her attack with a question about what her PC can see and deliberately target.

No check required
"Are there any that look like they could raise a warning?"
"One of them is holding back from the fight and is carrying a large horn rather than a weapon; it is obvious. You can attack him or any of the others."

Check but not action required
"We've fought another group like this before - or maybe the same group - are there any that look wounded?"
[One of them has a bandaged hand, another a limp - but that is not obvious, so there is a chance of failure] "You can make a Perception Check with a quick scan. If you find one that is wounded and still want to target that one, you can."

Check and action required
"Who is in command?"
"No one has shouted any orders, but you might be able to pick out the leader from body language - I'll allow an Insight Check, but that will be your action - you would need to spend enough time so that you wouldn't be able to attack this turn."

DM's might reasonably disagree on where to draw the lines on this sliding scale, but the point is that they are empowered to make these decisions.

Passive Perception confuses the issue

Active Perception is not when a character 'actively looks' for terrain hazards, it is when the player actively makes a Perception roll (because the DM told them to). Passive Perception is not an intuitive sense the characters have even when they don't search, it is when a character actively looks but the DM determines the result by using their passive Perception score without the player making a roll.

Any ability check, active or passive, is a result of the character's activity and conscious intent; as described in the PHB's section on Ability Checks (emphasis mine):

An ability check tests a character's or monster's innate talent and training in an effort to overcome a challenge.

Passive ability checks are included as a subsection of the Ability Check rules and must follow the general rules for ability checks; they happen when the character is making an effort for them to happen. Further, while it can be assumed that most of the time your character is deliberately on the alert for danger, if they focus their attention on another activity they lose the ability to make passive Perception checks, as described in the PHB section on Noticing Threats.

The purpose of passive checks is explained in the PHB:

A passive check is a special kind of ability check that doesn't involve any die rolls. Such a check...can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

'Active' and 'passive' thus refer to what the player and DM are doing in the meta-game (rolling or not), but not what the characters are doing within the narrative. You should not be determining whether someone 'gets' an active roll rather than a passive check based on what their characters are doing, but rather on how you as a DM want the meta-game to proceed. What the character is doing is the basis for permitting a check in the first place, regardless of whether you decide that it will be active or passive.

To answer your question

If the pursuers are focusing on catching the fleeing party, and have no reason to suspect the terrain is not as it appears, they won't initially be "making an effort" to spot the danger specifically hidden on the ground. The lead pursuer will likely enter the spike growth and start taking damage. Once those farther back see this, that might give them pause in their pursuit. What happens next requires your ruling as a DM. The spell itself doesn't require an action to be spent, but is identifying the area of spike growth so demanding of their attention in this particular situation that it requires their action for the round in order to make the check? That's the DM's call.

Regardless of whether an action is required, when the pursuers make their checks do you want them to make active Perception rolls or allow them to use their Passive scores?

These are two separate decisions, and you will need to make both of them.

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  • \$\begingroup\$ "Not all ability checks require that an action has been spent on them." -> 10/10, this is the answer, especially since the spell does not require an action. And you included very good examples of ability checks that do not consume an action (like a DEX check to keep your balance while moving on tricky terrain). \$\endgroup\$ Mar 18 at 20:43
  • \$\begingroup\$ @Glasses2C_Sharp I think it's the answer, too! But OP also asked about Passive Perception. I suspect that adding my (IMO just as textually correct, but unpopular) view on Passive/Active checks means this answer won't get as positive a response as if I had just confined it to pointing out that ability checks don't require actions. \$\endgroup\$
    – Kirt
    Mar 20 at 2:46

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