Upon feasting on the blood of its first victim, the petal was able to multiply, thus creating more petals. At first, they only fed on birds and squirrels. The new petals would go on to find more victims, enabling them to multiply thousands of times in the space of just one day. (TV: Co-Owner of a Lonely Heart)
As the petals were multiplying at an alarming rate, the Governors stepped in to ensure the threat was dealt with. Dorothea Ames, who worked for the Governors, enlisted Miss Quill on a mission to force Charlie to use the Cabinet of Souls to destroy every single petal and stop the invasion.
Both Miss Quill and Charlie had reservations about using the Cabinet, as Quill believed that they should execute the Shadow Kin rather than the petals and Charlie believed that they shouldn't use the Cabinet at all. As the petals advanced and began murdering humans, Dorothea pulled out a gun and threatened to kill Charlie's boyfriend Matteusz if they didn't do as she asked.
While this was happening, April had just been proclaimed Shadow King and so used her newfound army of Shadow Kin to rid the world of the petals. Ames later revealed that she and the Governors had actually expected this outcome, and that they had calculated that all of the killer petals probably only shared a single collective soul, meaning only one Rhodian soul would have been needed to kill them all. (TV: Brave-ish Heart)