Bonsai
Bonsai Tree
noun
Tray planting is bonsai. It makes sense because that's exactly what you're doing: planting a tree in a tray!
盆栽 refers both to the art of cultivating these miniature trees in small trays (or pots), as well as to those bonsai trees themselves.
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?