Black Mulching

A 20-foot radius sphere of death appears at a point you choose within range. At the end of your turn, nonmagical plants that fit fully within the sphere's confines wither and die instantly, while those too large to fit inside begin to wither. Magical plants or plant-type creatures must succeed on a Constitution saving throw, suffering 4d12 points of necrotic damage on a failure, or half as much on a success.

As an action on your turn you can shift the position of the sphere to any point within 90 feet of you. If this spell is cast in an area affected by plant growth, it takes 2 rounds for nonmagical plants to wither. The way large plants, especially magical plants, are affected by this spell is something determined by the DM.