Answer: Often the background planting is on the north side of a long border, where it won't cast shade on plants that need full sun, or ocasionally to the east side so that the border receives full afternoon sun. Borders which receive only full morning sun are best for a selection of plants requiring part-shade.
Much depends on microclimate: For example, some plants needing full sun in England actually require partial shade during the hot summers in the United States. Another problem those larger trees and shrubs may eventually cause is root competition for water and nutrients. Part of the skill accomplished perennial gardeners exhibit, especially those with mixed borders, is the ability to blend the plants attractively while still meeting their cultural requirements. Often it is a matter of trial and error over time combined with attentive and thoughtful observation. Also, talk to gardener's whose borders you admire, and keep reading. It all helps!
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