Sentence:  Such small, exquisite enclosures as the Sundial Garden beside the house, planted by Mr. Howard with near-black and white perennials chosen to bloom sequentially for 10 months of the year, or the Azalea Walk, with its <WSD>underplanting</WSD> of bulbs and season-extending clematis, cannot take care of themselves. 
Meaning of the word marked with <WSD> tags:  To plant underneath.