Did you know that you can spawn block randomly by replacing an existing block such as tall grass, grass blocks or other blocks in vanilla or custom biome?
PREVIEW OF THE GENERATION
This can be done using the ore generation settings under the block element generation tab. In the image above the yellow rocks were generated using the generation settings below.
As you can see I have the generation targeting the surface dimension and will only replace tall grass when generated. Also, note the amount per chunk and per group are both at 32, you can adjust these settings to have less dense coverage.
If you find it not enough coverage you can always make more of the same element and direct the creative pick item a 1.9.1 feature and the dropping item to one block of the same kind to give more coverage over more dense areas. This is what the settings for the second spawner blocks would look like...
THE SECOND SPAWNER SETTINGS
The photo above is the second block or the spawner block that targets the (Birch Rocks) Main block. Both blocks use the same settings. Below is one additional spawner added.
AFTER THE SECOND SPAWNER
The main thing you will want to try to avoid as much as you can is generating lots of blocks like tallgrass these rocks or even leaves with procedures. This can and will impact performance and cause the game to grass suddenly, its hard on the memory when this happens and is best to avoid using procedures like when block added or update ticks procedures that happen without interaction, you may be able to get away with when player right-clicks or destoryies item but try to stay away from automatic triggers due to how it can impact PC health and performance.