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.