Why does the writer compare the rising roses of her green house of the north with the asphodal's of the god's garden?

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Never have I found in the greenhouses of the North such heart-satisfying roses as the climbing roses of my southern home. They used to hang in long festoons from our porch, filling the whole air with their fragrance, untainted by any earthy smell; and in the early morning, washed in the dew, they felt so soft, so pure, I could not help wondering if they did not resemble the asphodels of God's garden.

The writer compares the rising roses in her northern greenhouse to those from her childhood home and says they are lacking. This is a reference to the fact that in the South the roses bloom wild and free out of doors, while in the North they have to be carefully propigated and protected from the weather. Asphodels of the god's garden is a reference to mythology and an immortal flower said to grow in the Elysian fields.