Her use of the word “moor” is brilliant.
One definition could be straightforward in that a “moor” is marshland / poor soil where the “Captives” are “denied the plucking” of something beautiful (the flowers in the bouquet). This alludes to the labor humanity is burdened with after the fall out of Eden which she write about in the previous poem, “Angels, in the early morning” where the morning plenty turns to sand. Here she is talking about how this world, though it can be beautiful, it’s mostly a swamp and paradise is in the next life.
“Moor” can also be as in a ship being secured which references her images of a boat sailing to paradise. Multiple times she has used this image, such as with Noah in “Once more, my now bewildered Dove” where the ship has survived the voyage and the faithful search for dry land (hence the swamp imagery in today’s poem used as a metaphor for the poor soil of mortal land). She also uses the boat metaphor in “Went up a year this evening” where she watched her friend sailing off to a new land, which recalls her earlier poem “Could live – did live” of the spectators watching Jesus. Yet here the spectators are not joyous, but rather compared to “Captives”, prisoners to this mortal world until they are set free and must, until then, stand on the shore and watch their friends make the journey one by one and alone. Thus her “Nosegays” could also be a funeral bouquet.
“Moor” can mean outsider (think Othello who was Shakespeare’s ultimate outsider), and this alludes to the sense of humanity being out of place within this mortal realm, as if our true home is somewhere beyond, and not here (in Venice, nor in Cyprus where they sailed to).
Finally, “moor” can also mean ‘More’ as in wanting more which alludes to the point of a “prayer” in that the “Captives” are wanting for something better, they have no other “errand” than to be like the dove in “Could live, did live” who fly out with hope and faith that there will be dry land where the flowers grow all day.