The speaker in "The Woodspurge" experiences a connection with nature that is similar to the way Romantic poets emphasized the bridge between the individual imagination and the vitality of the natural world. Unlike the Romantic bridge which emphasized an imaginative/creative connection with nature, Rossetti is illustrating a more sensual, dreamlike connection. The speaker is grief-stricken, aimless, and looking for sensual significance: something in the external world to balance, makes sense of, or cancel out those internal feelings.
In the first stanza, death is invoked and we know from the second and final stanzas that the speaker is coming from a state of profound grief:
My lips, drawn in, said not Alas!
My hair was over in the grass,
The speaker is now sitting in this second stanza, too grief-stricken and aimless to verbalize (Alas!) his grief. The speaker's tone is melancholy, depressed, but also moving towards an unconscious trance. His focus shifts from grief to a dreamlike but heightened experience of perception. The woodspurge just happens to be the intoxicated (captivated) speaker's object of observation. In the last stanza, he suggests that no wisdom can come from this grieving (he never notes what the cause of the grief is). Instead, the thing that he will remember is the image of the woodspurge.
From perfect grief there need not be
Wisdom or even memory;
One thing then learned remains to me-
The woodspurge has a cup of three.
The speaker has effectively submitted to a kind of trance wherein he is focused on his sense perceptions. It is like a meditation. The speaker concentrates on particular, albeit mundane, things to the point that he is more in tune with those things than he is with his grief. In the first stanza, when he walks, the wind moves; when he sits, the wind is still. He is now being guided by nature; not his inner thoughts/grief. The shortness of the words and the simplicity of the poem illustrate the speaker's focus on the simplistic things around him.
There is something very simple but profound happening here. One interpretation of this poem is about how we remember seemingly trivial things in times of intense psychological feelings. For example, one might remember a train whistle during a funeral of a loved one. We focus on such things during times of good and bad psychological intensity as a meditation and as a dual function of distraction and integration. In other words, one might dwell on the train whistle (or woodspurge) to push the grief aside for the time being. But one might also dwell on the woodspurge to attribute greater significance to the woodspurge because, at a time of such grief, a person is inwardly and outwardly looking for answers: looking for significance in everything.
That said, this is about looking and about perception. The speaker does not make some profound statement about grief. He is describing his heightened sense of external perception during a time of inner turmoil.