Final answer:
The phrase is a metaphor that signifies the union of nature and human thought.signifies the union of nature and human thought.
Step-by-step explanation:
The phrase "A rose sanctuary will I dress, wreath'd trellis of a working brain" is a metaphor that signifies the union of nature and human thought. It suggests that the mind is like a beautifully adorned rose sanctuary, cultivated by intellectual effort. The use of imagery from nature, such as roses and trellises, emphasizes the creative and imaginative nature of the mind.