Planting oak saplings in swamps generate swamp trees. The only way swamp trees appear in minecraft is by natural generation in swamps. This mod allows the player to regrow these trees. If you take an oak sapling (the same kind that would drop from the swamp tree) and plant it within a swamp, it will spawn a swamp tree. If you grow the oak sapling outside a swamp biome, you still get the expected behavior of an oak tree. In this context, swamp biome means the swamp biome minecraft:swamp as well as the mangrove swamp biome minecraft:mangrove_swamp.
Requires Fabric Loader, but does not need Fabric API. As the mod is a simple mixin it should be easy to modify it to work with another loader, as long as it supports sponge mixins.
Works on both client and server.
Special thank you to nimids for cleaning my code and updating from 1.19.2 to 1.19.3.