You are able to do this after.
You are able to go back to the past.
Speak to the NPC with the Jefferson next to the Portal. He will give you a Mystery Candy. You need to plant this and return to the future via the Portal and speak to the NPC in the top right hand corner.
Speak to the NPC as Jefferson
After that point you will need to equip the following Costumes in the same party.
UPGRADED Hotdog
UPGRADED Wolfman
UPGRADED Superhero
Then talk to him as every Costume without changing any of the Costumes. He will then give you Fertilizer.
Return to the past and use the Fertilizer on the Plant and return to the future.
No make your way to the back entrance of the research labs and go upstairs on to the top of the building and make your way into the Tree via the Door.
Walk up the inside of the Tree and out and you'll find the Chest with the Costume inside.