Sprout is wild and tends to be aggressive, often trying to bite people when he was still confined in a flower pot. After he rooted into the ground and grew to enormous proportions, he became very dangerous, spreading his vines like weeds to attack victims. However, he apparently calmed down after being uprooted and contained. He also seems to like Lilo.
Experiment 509 was the 509th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to rapidly spawn into an uncontrollable killer forest of vicious duplicates. 509 and the other first 624 experiments were deactivated and smuggled to Earth by Jumba during his mission to capture Experiment 626.
All of the experiment pods were released and scattered across the island of Kauai.
After describing 509's powers to Lilo, Jumba, unwilling to risk activating 509 due to the destruction the latter would cause, locked the pod away.
However, Lilo and Stitch stealthily stole 509's pod from Jumba's desk drawer the following night. Lilo then activated the experiment, named him Sprout, transplanted him, and fed him super-fertilizer.
The next day, Lilo took a rapidly growing Sprout to an outdoor orchid competition. When Mertle approached Sprout and noticed he was not an orchid, Sprout lashed out at her, causing Mertle to accidentally break his pot open.
Sprout then rooted into the ground and grew into a massive forest of killer plants, which took over the Kokaua Town fair, causing widespread panic.
Eventually, Stitch managed to lasso the original Sprout, uproot him, and drag him into a pineapple-shaped water tower slated for demolition, trapping the latter and causing the forest to die off without the original Sprout.
Lilo was unsure if this was truly Sprout's one true place, but it was the best she and Stitch could do on such short notice.
The first 624 experiments, including Sprout, were rounded up by Leroy and taken to a stadium to be destroyed. However, Lilo, Stitch, Jumba, Pleakley, Reuben and Gantu arrived before the experiments could be destroyed.
Sprout participated in the following battle between the experiments and the Leroy clones.
The Leroys soon gained the upper hand in the battle, but were defeated when Lilo, Stitch, Reuben and several other experiments performed the song "Aloha ʻOe", which caused the Leroy army to shut down due to the original Leroy's fail-safe. Sprout joins them for the song by playing the drums.
Sprout has made one appearance in the Stitch! anime and was mentioned in another. Though he did not officially appear in the second episode, his son did. It is revealed that Sprout can produce seed pods and launch them into space, where they cling to asteroids. One of these pods landed on the island and grew into a "Sproutling" (509-A) that carried out Sprout's original programming before being neutralized.
- Sprout was first revealed in a DVD bonus feature of Stitch! The Movie in the experiment gallery.
- Sprout is an obvious parody of Audrey II from Little Shop of Horrors.
- Sprout bears a resemblance to the Piranha Plants from the Super Mario Bros. games and Bloomsday from the Wii game Wario Land: Shake It!.
- Sprout's holographic silhouette appeared in both "Amnesio" and "Fibber".
- Sprout is described by the experiment computer screen as, "Experiment 509. Primary function: Sprout into an uncontrollable forest of destruction."
- In the battle between the Leroy army and the experiments at the end of Leroy & Stitch, Sprout did not spawn into an uncontrollable forest of destruction like he did in his episode.
- In the English dub of Stitch!, Sprout has a son named Sproutling (a.k.a. Experiment 509-A).