Complex sentence: He planted an arboretum of exotic trees and amassed a collection of brooms, then little employed in horticulture, and assembled every new vegetable he could find, raising the quality of the gardens to the highest level set by the English at Kew.
Split into simpler sentences: He planted an arboretum of exotic trees and amassed a collection of brooms, which were at the time little employed in horticulture. He then assembled every new vegetable he could find, raising the quality of the gardens to the highest level set by the English at Kew.