Bonsai
noun
Tray planting is bonsai. It makes sense because that is exactly what you're doing: planting a tree in a tray!
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
アメリカでも盆栽が買えるんですか？
Can you buy bonsai in America too?