underplant
verb
un·der·plant ˌən-dər-ˈplant 
underplanted; underplanting; underplants
transitive verb
: to fill around, under, or among with lower-growing plants
underplant a tree with pansies
Example Sentences
Recent Examples on the Web Our gardens reflect our patriotic colors and spirit when our beautiful red, white and striped roses are underplanted with blue companion plants. 
Rita Perwich, sandiegouniontribune.com, 30 June 2017
Rita Perwich, sandiegouniontribune.com, 30 June 2017 Word History
First Known Use
1891, in the meaning defined above