window box
US /ˈwɪn.doʊ ˌbɑːks/
UK /ˈwɪn.doʊ ˌbɑːks/

1.
bloembak, vensterbankbak
a long, narrow container for growing plants, typically placed on a windowsill or attached to the outside of a window
:
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She filled the window box with colorful petunias.
Ze vulde de bloembak met kleurrijke petunia's.
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The old building had charming window boxes overflowing with geraniums.
Het oude gebouw had charmante bloembakken die overliepen van geraniums.