If something is sepalled, it has sepals, which are the outer parts of a flower that protect the bud before it opens.
The word sepalled is used mostly in science or when talking about flowers.
Examples:
The sepalled flower was ready to bloom.
The sepalled bud was still closed tightly.
I saw a sepalled flower in the garden.
The sepalled plant looked like it was wearing a green coat.
The sepalled rose had the most beautiful petals.
The sepalled bud was like a present, waiting to be opened.