House Enthusiast's beautiful description of the Salem Witch Trials Memorial in Salem, Mass., illustrates how important symbolism is to a site — and how including it subtly can be especially powerful.
In the case of this memorial, no signage announces the site, just subtle engravings of the statements made by the accused as well as their names, method of execution, and dates of death. Those engravings are on twenty granite benches which levitate on the face of a wall, creating tombstones for the deceased, whose ignominious executions precluded any tombstones of their own. According to the site's designer, James Cutler, the black locust trees planted in the courtyard were chosen as "the tree from which these innocents were hung." And I would say it's not irony but careful planning that sites the memorial adjacent to Old Burying Point, the graveyard in which lies the grave of Judge John Hathorn, whose witchcraft verdict led to the deaths of the accused.
These and numerous other details would have no significance outside of this site. And if more obvious means of communication had been chosen — say, a large plaque announcing the memorial, or upright "tombstone" pillars instead of the horizontal granite slabs — the visitor would be less intrigued, less inspired to contemplate rather than spectate.
In the garden, we generally seek to symbolize less grisly memories, but the lessons are the same: use a soft brush to illustrate with contrast, color and texture; find clues that are specific to the site, and its residents; and don't state the obvious — often a whisper is louder than a shout.