a Little History

Pre-Crisis
Dr. Pamela Lillian Isley, a promising botanist from Seattle, is seduced by Marc LeGrande into assisting him with the theft of an Egyptian artifact containing ancient herbs. Fearing she would implicate him in the theft, he attempts to poison her with the herbs, which are deadly and untraceable. She survives this murder attempt and discovers she had acquired an immunity to all natural toxins and diseases

Post-Crisis: Life in Seattle and Gotham
Post-Crisis, her origins were revised. Pamela Isley grows up wealthy with emotionally distant parents. She later studies advanced botanical biochemistry at university with Alec Holland under Dr. Jason Woodrue. Isley, a timid, shrinking violet, is easily seduced by her professor. Woodrue injects Isley with poisons and toxins as an experiment, causing her transformation. She nearly dies twice as a result from these poisonings, driving her insane. Later Woodrue flees from the authorities, leaving Pamela in the hospital for six months. Enraged at the betrayal, Pamela suffers from violent mood swings, being sweet one moment and like poison the next. After her boyfriend has a car accident after mysteriously suffering from a massive fungal overgrowth, Isley drops out of school and leaves Seattle, eventually settling in Gotham City.

She begins her criminal career by threatening to release her suffocating spores into the air unless the city met her demands. The Batman, who appears in Gotham that very same year, thwarts her scheme, and she is incarcerated in Arkham Asylum for the Criminally Insane. From this point on, she would have a kind of obsession with Batman, being the only person she could not control. Over the years, she develops plant-like superpowers, the most noticeable being a lethal toxin in her lips; she is able to literally kill with a kiss.

In subsequent issues, she states that she only started a life of crime to attain sufficient funds to find a location to be alone with her plants, undisturbed by humanity. A few years later, she attempts to leave Gotham forever, escaping Arkham to settle on a desert island in the Caribbean. She transforms the barren wasteland into a second Eden, and is, for the first time in her life, happy. It is soon firebombed, however, when an American-owned corporation tests their weapons systems out on what they think is an abandoned island. Ivy returns to Gotham with a vengeance, punishing those responsible. After being willingly apprehended by Batman, she resolves that she can never leave Gotham, at least not until the world was safe for plants. From then on, she dedicates herself to the impossible mission of "purifying" Gotham.

At one point, Batman travels to Seattle to ascertain information on Pamela Isley's life before she became Poison Ivy. Here, it is stated that both of Pamela's parents are dead. When and why they died has been left undetermined.
While in Arkham, Poison Ivy receives a message through flowers that someone is to help her escape. That night, two women, Holly and Eva, successfully break Ivy out and bring her back to their employer. She is less than happy to discover that it is the Floronic Man, formerly known as Dr. Jason Woodrue, her former college professor that conducted the experiments on her. The only human portion of him remaining is his head, while the rest of his body is plant-based.

After striking a deal with him in the underground tunnels of Gotham, Ivy receives a trunk full of money in return for samples of her DNA. Woodrue intends to combine their DNA to create a "child", all while flooding the streets of Gotham with high-powered marijuana. The purpose of this is to create a world economy run on hemp and to have their offspring control it. Batman intervenes, but is overcome by Woodrue's henchwomen, Holly and Eva. However, Ivy turns on Floronic Man and lets Batman go to fight the intoxicated maniac. In the end, Batman decapitates the Floronic Man, and Ivy escapes with her money.

At times, Ivy demonstrates positive, even maternal traits. When Gotham City is destroyed in an earthquake, rather than fight over territory like most of Batman's enemies, she holds dominion over Robinson Park and turns it into a tropical paradise. Sixteen children who are orphaned during the quake come to live with her, as she sympathizes with them, having herself suffered a traumatic childhood. She cares for them like sons and daughters, despite her usual misanthropy.

That winter, Clayface (Basil Karlo) pays Ivy a visit, hoping to form a bargain with her. This would entail her growing fruits and vegetables, having the orphans harvest them, and him selling the produce to the highest bidder. She wants nothing to do with the plan, and she attempts to kill him with a kiss. Clayface overpowers her, however, and imprisons Ivy and the orphans for six months in a chamber under the park's lake. He feeds her salt and keeps her from the sun to weaken her. Eventually, Batman comes and discovers the imprisoned orphans and Ivy. The two agree to work together to take Karlo down. Batman battles Clayface and instructs Robin to blow up the lake bed above, allowing the rushing water to break apart the mud, effectively freeing Ivy. She fights Karlo, ensnaring him in the branches of a tree and fatally kissing him. She then proceeds to sink him down into the ground, where he becomes fertilizer for Ivy's plants. Batman, originally intending to take the orphans away from Ivy, recognizes that staying with her is what is best for them, and they remain in her care until the city is restored. Also, as part of a bargain to keep her freedom, Batman arranges it so that Ivy provides fresh produce to the starving hordes of earthquake survivors. Soon after, Ivy finds Harley Quinn, who had almost been murdered by the Joker, among the debris of the earthquake and nurses her back to health. The two have been best friends and partners-in-crime ever since.

After Gotham City is reopened to the public, the city council wants to evict her from the park and send her back to Arkham Asylum, as they are uncomfortable with the thought of a "psychotic eco-terrorist controlling the equivalent of 30-odd square blocks". They also mistakenly believe that the orphans in Ivy's care are hostages. The Gotham City Police Department threaten to spray the park with R.C. Sixty, a powerful herbicide that most certainly would have killed every living plant in the park, including Ivy, and more than likely do harm to the children. Ivy refuses to leave the park to the city and let them destroy the paradise she had created, so she chooses martyrdom. It is only after Rose, one of the orphans, is accidentally poisoned by Ivy that the hardened eco-terrorist surrenders herself to the authorities in order to save the girl's life. Batman says that, as much as she would hate to admit it, Ivy is still more human than plant.

Later on, she and other Gotham characters are manipulated by the Riddler and Hush. Her task is to hypnotize both Superman and Catwoman however, she abandons Catwoman to be killed by Killer Croc, and Batman is able to keep Superman busy in a fight long enough for the Man of Steel to break out of the spell. Soon afterwards, the Riddler, who is being chased and attacked by Hush, approaches Ivy and seeks her protection. Ivy, who is angered by the manipulation, battles the Riddler physically and psychologically. She comes to physically dominate her opponent, humiliating Riddler and temporarily breaking his spirit.

Poison Ivy comes to believe that her powers are killing the children she had looked after, so she seeks Bruce Wayne's help to reverse her powers and make her a normal human being once more. Soon after, she is convinced by Hush to take another serum to restore her powers and apparently dies in the process. However, when her grave is visited shortly thereafter, it is covered with ivy, creating the impression her death would be short-lived.
Shortly after, Poison Ivy appears briefly in Robinson Park, killing two corrupt cops who killed one of her orphans. Though whether this takes place before or after the aforementioned storyline is unknown.

"One Year Later", Ivy is alive and active. Her control over flora has increased, referred to as being on a par with Swamp Thing or Floronic Man. She also appears to have resumed her crusade against the corporate enemies of the environment with a new fanaticism, regarding Batman no longer as a main opponent, but as a 'hindrance'.
After arriving back from a year-long absence, Batman discovers that Ivy has been feeding people including "tiresome lovers", "incompetent henchmen", and those who "returned her smile" to a giant plant which would digest the victims slowly and painfully. She refers to these murders as a "guilty pleasure". In an unprecedented event, her victims' souls merge with the plant, creating a botanical monster called Harvest, who seeks revenge upon Ivy. With the intervention of Batman, however, she is saved. Ivy is left in critical condition, and the whereabouts of Harvest are unknown.