This quotation from Nathaniel Hawthorne’s The Scarlet Letter occurs in chapter ten, when Hawthorne begins to give the reader a more in-depth look at the relationship between Roger Chillingworth and Reverend Dimmesdale. Dimmesdale, not recognizing Chillingworth as an enemy, has accompanied him on a pharmacological expedition in the country. When Dimmesdale questions Chillingworth about the origin of some unusual plants, he tells Dimmesdale that he found them
“growing on a grave which bore no tombstone, nor other memorial of the dead man, save these ugly weeds that have taken upon themselves to keep him in remembrance.”
In the next paragraph he refers to them as the “black weeds.”
In their discussion, the weeds take on a metaphorical (hence, symbolic) significance, allowing Chillingworth and Dimmesdale to talk about what is really on their mind. When Chillingworth says the weeds “grew out of his heart [referring to the dead person in the grave] and typify, it may be, some hideous secret . . . “ he is really taking a jab at Dimmesdale and his secret: the fact that he is Hester Prynne’s secret lover and the father of Pearl. It is not yet clear to the reader how much Chillingworth has figured out about this situation.
When Dimmesdale responds with, “Perchance he earnestly desired it [the revelation of his sinful past], but could not” he is secretly confessing that he would like to admit his sin, but is for some reason unable to do so.
So the black weeds represent unconfessed sin. The weeds are black to emphasize the idea that unconfessed sin harms the sinner. By cloaking the conversation in imaginative terms, (referring to the grave and the dead man), Dimmesdale and Chillingworth avoid discussing themselves and giving away any personal information. This keeps the suspense and subterfuge in play for the rest of the story.