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I was looking into getting the spell Plant Growth because we work with some agrarian societies in our campaign.

If you cast the 8-hour version that enriches plants in a 1 mile radius to double their next harvest over the same 1 mile every day, what stops it from increasing the harvest exponentially by further enriching the crops?

The theory being that even using a weekend of casting we could give a city/farmer x8 the harvest instead of just x2.

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  • \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already, and check out the help center for more guidance. \$\endgroup\$ – V2Blast Mar 31 '19 at 4:20
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Enrichment does not stack

The exact text of the 8-hour version of Plant Growth is:

If you cast this spell over 8 hours, you enrich the land. All plants in a half-mile radius centered on a point within range become enriched for 1 year. The plants yield twice the normal amount of food when harvested.

Importantly, the direct effect of the spell is to enrich the land and the plants growing on it, not to double the harvest. The first time you cast the spell, the plants within the radius become enriched. If you cast it again in the same place, there is no effect, because the plants are already enriched. No matter how many times you enrich the plants, they are still just enriched. The enriched plants, when harvested, yield double the normal amount.

(Note: You can arrive at this same conclusion using the DMG's rule for combining game effects, but my answer here doesn't rely on this rule or any particular interpretation thereof. Even if effects of the same name could stack, enrichment as described in the spell's text is a binary state that cannot stack and which can only cause plants to produce "twice the normal amount", which is independent of how many times the spell is cast.)

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    \$\begingroup\$ Important part of the RAW IS "The plants yield twice the normal amount" — so no matter what magic was used on the field before, effect is always 2 * normal, unaltered output. \$\endgroup\$ – Mołot Mar 31 '19 at 9:39
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    \$\begingroup\$ Surely the most pertinent thing to quote would be "Combining Magical Effects" from PHB pg 205 \$\endgroup\$ – Nacht Mar 31 '19 at 11:18
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    \$\begingroup\$ @Nacht That rule is tricky to apply here since the listed duration of Plant Growth is "Instantaneous", and there is a lot of disagreement about whether a spell can continue to have magical effects after its duration has expired. \$\endgroup\$ – Ryan C. Thompson Mar 31 '19 at 13:54
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    \$\begingroup\$ @Hobbamok One example would be Feeblemind, another instantaneous spell with an ongoing effect : rpg.stackexchange.com/q/146765/40516. Notably, RAI according to Jeremy Crawford is that the effect of Feeblemind is not dispellable. \$\endgroup\$ – Ryan C. Thompson Jan 9 at 2:33
  • \$\begingroup\$ @RyanC.Thompson I jumped the gun on this one :/ I already deleted my Answer below after reading some more about it and forgot this comment. Looking at the topic now: damn, what an unclear shitshow. I'd still rule around combining magical effects as well but damn. What a messy part of the rules \$\endgroup\$ – Hobbamok Jan 11 at 11:54

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