。
She's growing tomatoes in the garden.
Literal
She [topic-は] garden [at-で] tomatoes [object-を] is-cultivating.
栽培する ('to cultivate/grow') is a more formal and deliberate word than 育てる ('to raise/grow') — it implies purposeful agricultural cultivation rather than casual gardening. トマト is a loanword from English. ~ている marks the ongoing state of cultivation.