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.
Best Answer
Enrichment does not stack
The exact text of the 8-hour version of Plant Growth is:
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.)