Connotation for bouquet and ribbon’d in o captain my captain

The connotation for "bouquets and ribbon’d wreaths" is celebration.

Explanation:

Take a look at the following lines from Walt Whitman's poem "O Captain! My Captain!":

O Captain! my Captain! rise up and hear the bells;

Rise up—for you the flag is flung—for you the bugle trills,

For you bouquets and ribbon’d wreaths—for you the shores a-crowding,

For you they call, the swaying mass, their eager faces turning;

When the speaker mentions the bouquets and the ribbon'd wreaths, what he means is that America is celebrating. As we know, the poem is an extended metaphor for both Lincoln's death and the end of the civil war. The lines above represent the celebration of victory by the Union. The speaker is describing a festive scene - the ship (America) arriving to the port (the end of the war) and being welcomed with bouquets and ribbon'd wreaths to celebrate its safe return (victory).