Translation guide
The corolla is the collective term for all the petals of a flower. This is a botanical term, and the most common Japanese equivalent is 花冠 (かかん). In everyday conversation, people simply say 花びら (petals) or 花 (flower).
The learner wants to refer to the corolla in a scientific or botanical context.
The corolla of this flower is a vivid red.
Technically means 'petal', but sometimes used in contexts where the collective petals are discussed. Not a direct synonym for corolla, but often appears in similar contexts.
The petals are fused to form the corolla.
The learner wants to talk about the colorful part of a flower in casual conversation.
The common word for 'petal(s)'. While not a direct translation of 'corolla', it is what people normally say when referring to the petals collectively.
The cherry blossom petals are falling.
Simply 'flower'. In many contexts, this is sufficient and more natural than a technical term.
Use 花冠 (かかん) only in scientific or botanical discussions. In everyday Japanese, 花びら (petals) or simply 花 (flower) is much more common and natural. Using 花冠 in casual conversation may sound overly technical or stiff.
This flower is very pretty, isn't it?