sin ‧ gly /ˈsɪŋɡli/ BrE AmE adverb
[ Word Family: noun : ↑ single , singles, ↑ singleness , the singular, ↑ singularity ; adverb : ↑ singly , ↑ singularly ; adjective : ↑ single , ↑ singular ]
alone, or one at a time:
Plant the trees singly or in small groups.