From the Super Mario Wiki
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Venus Flower Traps are enemies that only appear in Super Mario Galaxy 2. They only appear in the Honeybloom Galaxy and the Honeyhop Galaxy. Venus Flower Traps resemble flowers and can be used as platforms, but if Mario or Luigi steps on one of these, it will soon close up, devouring any object on it. If Mario or Luigi is swallowed, he will lose a wedge of health. Venus Flower Traps are usually airborne or floating over shallow ponds. After snapping shut for a few seconds, it reopens and can be used again. They resemble Piranha Plants because they look similar when they close their mouth. One can avoid lasting damage from these, though. If a coin is floating above it, one can get trapped by the flower and spat into the coin, having full health but still flashing red from damage.