From the Super Mario Wiki
Revision as of 09:59, 12 April 2012 by Akfamilyhome
Spiky Plants are strange, round plants with red spikes seen in Super Mario Galaxy and Super Mario Galaxy 2. They hurt Mario or Luigi when touched, so the only way to destroy them is by attacking them with a Stretch Plant, a coconut, or by being pulled and eaten by Yoshi (only in Super Mario Galaxy 2).
There are three kinds of variants, the standard variant, the bigger variant and the floating variant. The bigger variants, after being destroyed by a coconut, will reveal a Sling Star or Launch Star leading to the next planet. The floating variants serve as obstacles while Mario rides a Floaty Fluff and are more common in certain galaxies, including the Gusty Garden Galaxy (in Super Mario Galaxy), the Yoshi Star Galaxy, and the Tall Trunk Galaxy (in Super Mario Galaxy 2).
When touched by the Co-Star Luma, Spiky Plants will retract their spikes, allowing Mario to pass over them safely without needing to destroy it.