Plantera's Bulb

Plantera's Bulb is an object found in the Underground Jungle which can be destroyed with a Pickaxe. When destroyed, it summons Plantera.

Plantera's bulb

Spawning

In a Hardmode world, after killing all three mechanical bosses (The Twins, The Destroyer, and Skeletron Prime) at least once, the bulbs will spawn somewhere in the Underground Jungle. They can be destroyed with a pickaxe. Upon doing so, Plantera will be spawned. The bulbs give off a glowing particle effect, which helps to locate it. On the minimap, the bulbs are light pink in color. This can sometimes be confused with Pots or Heart Crystals. Multiple bulbs can be found in the same Underground Jungle. If there is a plant or piece of vegetation in the way of a bulb's spawn, the bulb will replace it.

Farming

Bulb Farms

Because some of the most valuable Plantera drops are incredibly rare, bulb "farms" are very common. They are made by digging long tunnels of mud on top of one another in an Underground Jungle in order to create space for the bulbs to spawn. The player then uses the map to spot the bulbs and is able to easily break them and fight Plantera. Due to the structure of the tunnels it is very easy to fight Plantera because the player simply has to run along the tunnel damaging the boss in order to get the kill. Generally, the more tunnels the player digs the faster Plantera's bulbs spawn. It is also important to note that the spawn of vegetation in the tunnels does not affect the spawn rate of the bulbs, so it is unnecessary to clear bushes and other jungle life out of the "farms".

Notes

Destroying the bulb with the Clentaminator can summon Plantera, but it's not recommended since Plantera may enrage.
Plantera's Bulb can be located on the map, by pressing M (default) or back (xbox 360), and scanning the jungle area that you have explored. It appears as a 2x2 pink square. You don't have to have your player pass it to get it on the map, just explore the jungle area and once that area is lit up, you can find the bulb.
Destroying a second bulb while fighting Plantera will not summon a second Plantera.
Once all the 3 hard-mode bosses are killed, the player will receive the message "The jungle grows restless..." which signals that the bulbs will spawn.
Aside from jungle grass being present, the conditions for a bulb to spawn are the same as Zombies.

Bugs

Sometimes when trying to destroy a bulb, it only destroys the block you used your pickaxe on (fixed by destroying any other block of it).
Sometimes when the bulb is only half spawned in, it could be broken with a sword.
If a bush grows where a bulb was broken, another bulb will spawn.
If a bulb is broken where it spawns from a grown bush (Bug above), the bulb will be broken in tiles.
Defeating the three Mechanical Bosses before defeating the Wall of Flesh (using summoning items Mechanical Worm, Mechanical Skull, and Mechanical Eye) causes the message "The jungle grows restless..." to appear, but may cause Plantera's Bulbs to not spawn.