Pineapple is a splicable consumable which was added in Super Pineapple Party 2015. This item can only be created during the Super Pineapple Party, and can only be effectively used during the week long event.
To grow one, seeds that don't normally splice into anything need to be spliced during the event. However, the sum of the seeds rarity must add up to a rarity of 4 to 50, in order to work. Beyond that, rarity does not affect the trees at all. Higher rarity does not create more Pineapples. Pineapple Trees take four hours to grow.
When 200 Pineapples are consumed, the country flag by your name turns into a Pineapple and your Pineapple luck is increased (Full of Pineapples mod). When 400 Pineapples are consumed, your skin and clothes changes to yellow, with green hair and hats, further increasing your Pineapple luck (Turning into a Pineapple mod). When 600 Pineapples are consumed, you turn into a pineapple body (Pineapple Form mod) and you obtain the Too Many Pineapples achievement as well as maxing out your Pineapple luck.
At this point, you cannot feed yourself any more pineapples. Others must feed you more, until you reach 620 Pineapples at which point you will explode and lose your Pineapple Form mod and your Pineapple luck, forcing you to start over.
You obtain items by feeding a Pineapple to a Pineapple-bodied player (someone who has eaten 600 Pineapples, but not exploded yet). This will give you one of four mods based on your current Pineapple luck level: Pineapple Magic if you have no other Pineapple mod, Double Pineapple Magic if you have the Full of Pineapples mod (200+), Triple Pineapple Magic if you have the Turning into a Pineapple mod (400+), and Ultimate Pineapple Magic if you have the Pineapple Form mod (600+). Each of these mods cause a pineapple to revolve around you for 20 minutes, then creates a random Super Pineapple Party prize for you. Having higher levels of Pineapple Magic increases your chances of getting better Pineapple items.