How to create rare plants?

Hello everyone. I have problems with the spawning rates of my plants/flowers. I hope anyone had the same problem and found a solution.

I made a mod that has some flowers and other plants in it. The plants itself work fine, but i want some of them to not spawn in groups of 5-7 every 20 blocks. 

So i changed to code. In this code down here are the original settings:

    public void generateSurface(World world, java.util.Random randomGenerator, int chunkX, int chunkZ) {
        for (int i = 0; i < 1; i++) {
            int l6 = chunkX + randomGenerator.nextInt(16 + 8);
            int i11 = randomGenerator.nextInt(128);
            int l14 = chunkZ + randomGenerator.nextInt(16 + 8);
            (new WorldGenFlowers(((BlockFlower) mcreator_starBloom.block), BlockFlower.EnumFlowerType.DANDELION)).generate(world, randomGenerator,
                    new BlockPos(l6, i11, l14));
        }
    }

I changed the numbers 16; 8   to   32; 16  but nothing happened.

After some experimenting, i used the numbers 480; 240 and the plants are insignificantly rarer (spawn every 30 or 40 blocks). Instead it takes around !10 minutes! to spawn them in a new world, even if the world is already loaded and you can play on it. 

Is there another way to make them rarer? And what does the .nextInt(128) mean?