at the roots, or on the roots of the tree, or of something similar (see Molina)
Alonso de Molina:
inelhuayocan. ala rayz, o enla rayz del arbol, o de cosa semejante. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 38v. col. 2. Thanks to Joe Campbell for providing the transcription.