What is the symbolism of the scaffold, poisonous plants, weeds, and forest?

In The Scarlet Letter, the weeds that grow from the grave – sprouting from the heart of the sinner – are visible manifestations of his sin. For Dimmesdale, who notices these weeds, they represent his worst fear and, strangely, his greatest longing – exposure. He comes to believe that his sin will also sprout from his heart somehow, that his sin will reveal itself to the town without his knowledge, as the weeds grow from the sinner’s grave without his permission. He makes his fears manifest when he carves the A upon his own breast, and flagellates himself in the night, marking his body with the evidence of his sin. Likewise, the poisonous plants that grow uncultivated in the wilderness can be viewed as the product of sin, which will gradually poison the sinner and even cause his death if it is not rooted out. Dimmesdale’s failing health is evidence of the gradual death his unrevealed sin is bringing about.

The forest has two symbolic values in the novel. For the townspeople, it is the dark, wild place where the devil dances and where witches’ covens congregate under the light of the moon. The wild natives are at home in the forest, but no civilized Christian would ever feel safe there, or make the forest his home.

For Dimmsedale and Hester, the forest is a place to hide from the prying eyes of the townspeople and from Chillingworth. They can relax in the dimness of the woods, knowing that their tormentors won’t follow them. It is also a place of release for them and for their daughter, Pearl. They can be together as a family, speak openly with each other and the sun even shines through the canopy of the trees and onto them like a blessing. Hester can take off her A and let her hair down, and Pearl – the imp of evil, the symbol of their sin -- can frolic and play, almost like a normal child.

Dimmesdale is almost envious of the fact that Hester’s sin was publicly revealed on the scaffold. Though it is a humiliating punishment, it also unburdens the sinner by exposing the sin. He pleads with Hester, in his sermon, to reveal the name of her partner in sin, but she won’t. He is too weak to reveal his guilt; she is too strong to do so. So Dimmesdale visits the scaffold in the night, hidden by the darkness, although the reader sees his longing for the release that exposure would bring. He finally gets his opportunity at the end of the book when he, Hester and Pearl gather on the scaffold as he dies.

So though he is drawn to the forest where sin can be hidden, he longs for the scaffold where sin can be revealed. He fears the unbidden growth of weeds signaling his sin, yet the toxins that build up from that lack of release finally kill him.