Plant Kingdom is a blue-skied, untamed jungle with enormous flora like leaves, flowers, plant stems and fungi everywhere. Covering the horizon are ginormous undergrowths like moss stalks and flat fungi stretching into the sky, leaving the jungle bottom covered in darkness with only a few bioluminescent fungi to illuminate it. Along with fertile brown soil and small round hills, thick grass covers the ground and ledges like a neat blanket. The stage initially starts on a beach though.
Plant Kingdom features several exclusive gimmicks, such as plant-themed Gymnastic Bars, Vines and mushrooms. There are also stem tubes (based on Winding Tunnels/High-speed elevators) that suck up the player at one end, transport them through the tube, and launch them out of the other end with a burst of pink petals. As a bonus, the stem tubes let the player use Trick Actions after being launched out. There is also a humanoid tree that scoops the player up and tosses them sky-bound. Trick Actions can be performed once released.
Unique obstacles in Plant Kingdom include tree stumps: short and simple obstacles identical to those in Leaf Storm. They can be destroyed with a Spin Jump/Axel Jump or Super Boost/Fire Boost. The stage also has a few underwater sections, though they do no contain Air Bubbles, leaving the player at greater risk of drowning.
Towards the end of Act 1, there is a 3D section where the player has to grind on three vines; the player can freely switch between the three vines in order to avoid iron balls and collect Rings.
The boss of Plant Kingdom is Ghost Rex, a large mechanical Tyrannosaurus. It can be fought after clearing Plant Kingdom Act 2. On the first time however, the player can only fight it with Sonic the Hedgehog.
In battle, the Ghost Rex attacks with bites and ramming. Halfway into the fight, Ghost Rex begins stamping on the ground causing it to cave in, sending itself and the character flying down into a cave where the fight continues. It then begins attacking with extended head-slams. To defeat the Ghost Rex, the player has to attack its head until its health is depleted and land the final blow.