How to prevent plant from growing under certain conditions

[NebelNidas]

Hello there, I'm currently porting Oceancraft to 1.16 and want to implement seaweed (for now an almost 1:1 copy of vanilla kelp). I don't want it to grow to the very top water source block of the sea/ocean, so in getPlacementState I check if there's a water block above, and it works perfectly.
But: This only prevents manual placing, the seaweed still grows to the last block by itself! How do I prevent this? I already tried setting canGrow to false and manipulating the grow or canPlaceAt function, but this didn't seem to have any affect/didn't work.

My code:

public class SeaweedPlantBlockBase extends AbstractPlantStemBlock implements FluidFillable {
    public SeaweedPlantBlockBase(VoxelShape bounds) {
        super(FabricBlockSettings.copy(Blocks.KELP_PLANT).nonOpaque(), Direction.UP, bounds, true, 0.14D);
    }

    protected boolean chooseStemState(BlockState state) {
        return state.isOf(Blocks.WATER);
    }

    protected Block getPlant() {
        return this.asBlock();
    }

    protected boolean canAttachTo(Block block) {
        return block != Blocks.MAGMA_BLOCK;
    }

    public boolean canFillWithFluid(BlockView world, BlockPos pos, BlockState state, Fluid fluid) {
        return false;
    }

    public boolean tryFillWithFluid(WorldAccess world, BlockPos pos, BlockState state, FluidState fluidState) {
        return false;
    }

    protected int method_26376(Random random) {
        return 1;
    }

    @Nullable
    public BlockState getPlacementState(ItemPlacementContext ctx) {
        FluidState fluidState = ctx.getWorld().getFluidState(ctx.getBlockPos());
        Block topBlock = ctx.getWorld().getBlockState(ctx.getBlockPos().up(1)).getBlock();
        return fluidState.isIn(FluidTags.WATER) && fluidState.getLevel() == 8 && topBlock == Blocks.WATER ? super.getPlacementState(ctx) : null;
    }

    public FluidState getFluidState(BlockState state) {
       return Fluids.WATER.getStill(false);
    }
}

In the original source code (Forge 1.8) it was solved with the canBlockStay function, but that doesn't exist anymore in modern Minecraft versions:

public boolean canBlockStay(World p_149718_1_, BlockPos pos) {
    return p_149718_1_.getBlockState(pos.up(1)).getBlock() == Blocks.water;
}

[NebelNidas]

I fixed it by overriding the randomTick function:

@Override
public void randomTick(BlockState state, ServerWorld world, BlockPos pos, Random random) {
	Block topBlock = world.getBlockState(pos.up(2)).getBlock();
	if (topBlock == Blocks.WATER) {
		super.randomTick(state, world, pos, random);
	}
}

Thanks to lolad#1000 on Discord!