As you know, in classical literature the dead are referred to as shades, but here in Milton's simile "shades" merely means "shade trees."

在古典文学里，逝者常被用树荫来指代，而在这里弥尔顿的比喻“树荫“仅表示“叶绿成荫的树“

耶鲁公开课 - 弥尔顿课程节选