。
The trees have started to put out leaves.
Literal
Trees [topic-は] leaves [object-を] put-on began.
葉をつける ('to attach leaves') is the standard collocation for plants growing leaves; literally 'to attach the leaves (to itself).' ~始める is a productive verb-stem auxiliary meaning 'begin to X' — attaches to almost any verb. 木々 (the kanji 木 doubled) is the plural form 'trees,' a reduplication strategy Japanese uses for a small set of nouns (人々 'people,' 国々 'countries,' 山々 'mountains').