Savannah Tallgrass can only be found in Savannah Biomes on Savannah Grass. This type of grass is only available at world-generation and does not regrow currently. Such it will not start to grow on blocks of Savannah Grass that players have placed, not even on very large artificial Biomes.
Tallgrass can be placed by players directly onto the ground from the quickbar, but like common green Tallgrass it can also be placed into another patch of Savannah Tallgrass and by that will eredicate the latter completely (just like Red Mushrooms will vanish when blocks or objects like torches are being placed onto them).
When harvesting Savannah Tallgrass, occasionally Wheat Seeds can be obtained as an additional random harvest. Even Savannah Tallgrass that has been placed by players will sometimes "drop" Wheat Seeds when being harvested again. In this way, Wheat Seeds can be multiplied endlessly.
Just like green Tallgrass that grows on Grass, 4 patches of Savannah Tallgrass each can also be used as a Fuel for the Forge, however with a very low "heat level" providing only a slow melting/hardening process, comparable to ordinary Wood.