Entry 1 sense Webster, 1913 Undergrowth /ŭn'-dẽrg-rōthˌ/ · Un·der·growth · IPA /ˈʌndɚˌɡɹoʊθ/ 01 n. That which grows under trees; specifically, shrubs or small trees growing among large trees. 1. That which grows under trees; specifically, shrubs or small trees growing among large trees.