Springfield Lemon Tree
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The tree was planted over a hundred years ago, when Springfield was founded by Jebediah Springfield and Shelbyville by Shelbyville Manhattan. Because it was a "sweet moment" in the town's history, the Springfield townspeople chose to plant a lemon tree, because lemons were the sweetest fruit available at the time. The tree was very near the point where Jebediah and Shelbyville parted ways, so it ended up being very close to the boundary between the towns of Springfield and Shelbyville.
The tree was a source of income to the kids of Springfield, because they used its lemons to make lemonade which they sold at lemonade stands. The lemon tree was once stolen by a group of kids from Shelbyville, who uprooted the tree and planted it in an impound lot in the city. It was returned to Springfield when Bart came up with the idea to have Ned Flanders park his RV in a "No Parking" space with Rod, Todd, Martin, Database, Nelson, Milhouse, and their fathers hiding inside. When the RV was impounded at the lot, they uprooted the tree, stationed it on top of Ned's RV and escaped back to Springfield. Along the way, they went under a low archway which scraped the tree off the top of the RV and knocked half of its leaves off.
The rescuers returned the lemon tree back to the hilltop where it belonged, albeit missing all of the leaves on one side. Meanwhile, in Shelbyville, the incident was portrayed as a great victory where the people of Shelbyville "banished the awful lemon tree forever, because it was haunted." To mark the occasion, they planted a turnip tree, and the kids of Shelbyville got to drink turnip juice when the story was re-told.
The tree has become a popular attraction and tourists are encouraged to visit it. However, tourists are strongly discouraged from picking the lemons as they (as well as all the fruit in Springfield) are radioactive, evidenced by their giving off a barely audible hum. Also, international law forbids any fruit grown in Springfield from leaving town: In addition to the radioactivity, it hosts dangerous parasites and is a strong attractant for mutated fruit flies.
The lemon tree was later seen in winter, still missing a lot of its branches on one side.
 The Simpsons: Tapped Out