You're looking outside of your window while you eat breakfast. It's spring, so there are some flowers now that weren't there before. You write a Facebook message about it.
The flowers are starting to bloom!
Flowers start as little buds on a plant. Then they grow larger and open up to reveal the flower. This process is called "blooming".
When a plant only has a few open flowers, you can say that it's "starting to bloom".
After most of the flowers are fully opened, you can describe it as "in bloom":
I see that your tulips are in bloom now.
And when the flowers start to turn brown, dry up, and go away, you say that they're "wilting".
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