Symbolism, Imagery, Allegory
Can we just say that roses represent love and call it a day? No? Fine.
Susan loves all flowers, but roses hold a special place in her heart. For example, she spends a lot of time planting and caring for a bunch of yellow climber roses when the family lives in the valley. Oliver even replants these same roses when the family moves into their McMansion in the mesa several years later. In our minds, this makes roses a symbol of Susan and Oliver's seemingly undying love.
But—as we know—some serious trouble befalls the relationship when Susan cheats on Oliver, indirectly leading to their youngest daughter's death. In the face of this tragedy, Oliver does something seriously symbolic: he "heavily, impassively, expressionlessly [...] tears the roses from the ground and leaves them lying" (8.7.80). In one swift move, Oliver has effectively told Susan that he doesn't love her anymore.
The story doesn't end there, though. We eventually learn that when Oliver is an old man, he designs a hybrid rose that he names in honor of Agnes. This is an exhaustive process: it takes Oliver "two or three years" to make "a dozen roses [...] trying for just the right one" (9.1.86). Interestingly, he never tells Susan about this. Although his pride may prevent him from shouting it from the rooftops, this represents Oliver coming to terms with Susan's betrayal and quietly admitting that he still loves her.