Cold

We hear the "cold in the earth" refrain twice in the poem, so we know for sure that the speaker's lover is definitely "cold in the earth"—as in, dead as a doornail. We see it in the first three stanzas when the speaker is really emphasizing the distance she feels with her former special someone. Her lover is cold (and dead) while she's still kicking in the living world—you don't get much more distant than that. The difference between the two is what's most important in terms of death, loss, and learning to cope with the pain of realizing someone is "cold in the earth" without that beating heart one comes to love so much.

• Lines 1-2: We hear it first in connection with the speaker's lover being "far, far removed" in the grave. So we know that coldness is symbolic of the difference and distance between the dead and the living.
• Lines 7-8: Those fern leaves cover his grave but the speaker worries that her thoughts no longer hover there. And since he's there "forever, ever more," we get the feeling that she feels even more distance between herself and his final resting place. 
• Lines 9-10: While her lover is cold in the earth, "fifteen wild Decembers" have passed. He's still cold in the earth and she still remembers. So things have changed, but the most important thing—her lover, cold in the earth—remains the same.