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If you cast Plant Growth for its 8-hour version to

[...] 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.

and then come back the next day and cast it again the same way for the third time, would the total crops harvested be doubled, and then, quadrupled, and then octupled?
The campaign I am in is measured in years, so the impact of a druid (or in this case, nature cleric) who can spend 8 hours to double a 1/2 mile radius circle of food would be super helpful, but I need to know if I can prioritize one spot to make the growth exponential, vs going and casting in a different spot every day to just have a net double.

Eddymage
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EMTGO
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    What problem (at play) are you trying to solve with this question? Is this a world building question, a down time question, a 'how does magic impact druid relationship with NPCs' ... and have you consulted the "combining magical effects" section of chapter 10 in the PHB/Basic Rules? (I'll be interested to see the answers to this, as the druid in one of the campaigns I run has volunteered at least two castings of this spell for the local farmers ...) – KorvinStarmast Feb 08 '21 at 14:00
  • the campaign I am in is measured in years, so the impact of a druid(or in this case, nature cleric) who can spend 8 hours to double a 1/2 mile radius circle of food would be super helpful, but I need to know if i can prioritize 1 spot to make the growth exponential, vs going and casting in a different spot every day to just have a net double. the actual amount of food i create is important because of trade and population growth! – EMTGO Feb 08 '21 at 14:07
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    Sweet: if you could summarize that bit from your comment and add it to your question that would be great. – KorvinStarmast Feb 08 '21 at 14:11
  • would it be appropreate for me to ask a separate question about maximizing food production with magic over a long time period or could i expand this question? i assume separate question, but I'm not the best with this format – EMTGO Feb 08 '21 at 14:14
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    Ooh, I think that's a separate question since there may be additional spells from additional classes that would be of help. Drop in a link to this question as "related" would be my suggestion – KorvinStarmast Feb 08 '21 at 14:16
  • ok, i did it! i just put the url of this question in as the link, is that correct or am I missing a feature? – EMTGO Feb 08 '21 at 14:36
  • wow, odo, thats a lot! the world building implications of that could be interesting, you could functionally have a druid and farmer be able to feed the entire world a few times over, that would make the world focus on supply and travel a lot more then production itself – EMTGO Feb 08 '21 at 14:38

1 Answers1

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Multiple instances of the same spell do not stack

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

– Dungeon Master's Guide, p. 252 (note errata)

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

— Player's Handbook, p. 205

From these rules we can conclude that casting plant growth multiple times will only double the crop yield once. Casting the spell a second time will make the crop yield doubling last at least 8 hours longer than it otherwise would, but that is a very small benefit compared to coming back just under a year later and casting the spell again then.

Exempt-Medic
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    That quotes above are about duration, but plant growth is instantaneous, not even 'until dispelled' like true polymorph. Or are you suggesting that 'for 1 year' is the duration, even though the stated duration is instantaneous? For what it is worth I agree with the overall answer, but I am not sure the quotes entirely justify it. – SeriousBri Feb 08 '21 at 14:37
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    @SeriousBri The effect still has a duration, and the DMG's Combining Game Effects rule still prevents the stacking. – Thomas Markov Feb 08 '21 at 14:40
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    Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. – Thomas Markov Feb 08 '21 at 14:40
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    @SeriousBri It has a duration" of instantaneous, but that duration is unfortunately a bit of a misdirect. Since that specific part of the effect lasts for 1 year, the effect can't be stacked. – RevanantBacon Feb 08 '21 at 14:45
  • @Serious I wouldn't be opposed to a general question on "Can you stack multiple castings of a spell if its duration is Instantaneous?" Though there is already this question and this one and this one – Exempt-Medic Feb 08 '21 at 14:48
  • My question was mostly playing devils advocate since I didn't see the quotes were conclusive, but the latest question has provided perfect clarification, the key word being 'effects' in the DMG quote. +1. – SeriousBri Feb 08 '21 at 16:09