So the spell says it's instantaneous....but how long would the effects last on the land?
Forever? or at least a good while I assume. This seems like one huge drawback of the spell.
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Yup. It's Permanent.
You aren't temporarily Enlarging the Plants and you aren't magically enchanting them to be entangling. You're actually making them grow to become that dense and entangling.
Any spell that has a Duration will always say how long it lasts. This one is Instantaneous. According to PHB203...
Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can’t be dispelled, because its magic exists only for an instant.
Plant Growth is altering all of those plants to make them grow big and dense in an instant...and then the spell is over, and the plants are still huge.
Another example of a spell such as this is Stone Shape. It alters a hunk of stone into a different form. Once this is done, the spell has ended...but the stone is still in a different form.
This is both a boon and a detriment to the Plant Growth spell.
It's a boon because you are creating extremely difficult terrain that cannot be undone by breaking your Concentration, and cannot be torn down with a Dispel Magic spell.
On the other hand, it's a detriment because now you're going to have to either wade through all that hyper-dense foliage, or hack your way through it to clear a path.