Aconite

Aconitum, commonly known as aconite, monkshood, wolf's bane, leopard's bane, mousebane, women's bane, devil's helmet, queen of poisons, or blue rocket, is extremely poisonous.

Hannah McKay grew it on her property.

Summary
Regarding Aconite, Dexter thought, "Beautiful and dangerous -- just like Hannah."

Through research, both Dexter Morgan and crime writer, Sal Price, presumed that Hannah McKay used Aconite (via ingestion) to cause heart attacks in at least two individuals -- her first husband and Beverly Grey, her mentor. Dexter discovered that Hannah was the sold beneficiary of Beverly's will, inheriting her gardening business. Sal had obtained a sample of Beverly's blood by bribing the funeral director, and it tested positive for Aconite. Debra Morgan tried to prove that Hannah murdered her husband by having his body exhumed. However, since he was not embalmed, it was impossible to prove Aconite poisoning.

After Hannah's handyman informed Dexter that all wildflowers grew on the southwest corner on the property, Dexter located a patch of the plants near Hannah's greenhouse. Some had recently been harvested, but he was unsure for what purpose. The thought crossed his mind that she was after him, even though he had previously witnessed her poisoning a rabbit. ("Do the Wrong Thing")

When Sal Price became a threat to her, Hannah contaminated his pen (which he always chewed on) with Acontie, and Sal died from a heart attack in Dexter's apartment. ("Chemistry")

Trivia

 * The web article that Dexter learns about Aconite is a knockoff of wikipedia.org and nearly identical to the Wikipedia article Aconitum -- minus a few details, added and changed.
 * Aconite allows someone to kill silently, while being detached from the act.
 * Women, in general, use poison more often then men to commit a murder.
 * The only post mortem signs are those of asphyxia.