The poem contains a few vivid metaphors that help to illustrate the speaker's relationship to her mother's memory.
The speaker's mother died suddenly, and the speaker didn't get to witness her burial. As such, the speaker says, "a romance began." Lacking the closure of goodbye or burial, the speaker's relationship with her mother was left unsettled. Describing her obsession with her mother's memory as a romance reflects the speaker's intense desire to be reunited with her mother.
The speaker also coped with her loss by romanticizing her mother in her memory—she viewed her through rose-colored glasses. Using another metaphor, she says that the "ivy-mother turned into a tree." Ivy is a vine that clings to other plants, often smothering them. This metaphor suggests that in real life, the speaker's mother was a domineering, suffocating presence—someone the speaker perhaps longed to get away from. But after she died, the speaker's grief warped her memory of her mother, changing her into a tree: something sturdy and solid, that provides shelter. The tree metaphor suggests that the speaker wants to remember her as being more nurturing than she actually was.
But this version of her mother remains always out of reach. The speaker chases after her, but she is like a "rainbow," impossible to catch. Thus, the speaker becomes the one with "tendrils [...] that clutch."While in life her mother was clingy and overbearing, in death it is the speaker who clings to her memory of her mother.
In the last stanza, the speaker imagines her mother's voice saying to her:
[...] You are the ghost
With the bat-voice my dear. [...]
This metaphor implies that the speaker has become a shadow of herself. Her mother's death has had a profound and lasting impact on her life, to the point that she's the one who is really "lost." Like a bat, she is blindly trying to find in her mother's absence. Note, too, that bats navigate using echolocation; as such, the metaphorical phrase "ghost with the bat-voice" hints at the way that the speaker is calling out in the dark to her mother, her voice is constantly echoing about in her mind.