In just a few lines, the speaker says that everything their mother left behind comes down to a single rose. The word "legacy" in the title leads us to expect money or property, but the poem offers something small, living and delicate instead. The rose is "bearing all," which can mean it carries the whole inheritance, and it might also suggest it bears, or endures, loss.
The last two lines are the most puzzling. "Deducting out the green / for air only" sounds like an accountant working through an estate, subtracting what doesn't count. The green, the stems and leaves, is taken away, and what remains is the bloom, with the green given over to air. I read this as a way of paring the mother's life down to its essence: color, scent, beauty, and not much more.
The mood is quiet, restrained and tender. The speaker doesn't sound angry or desperate, but the plain, almost businesslike tone seems to hold back grief, and it leaves the feeling to the image of the rose.