Derek's situation suggests he is trying to understand the factors affecting the growth of his rose bushes after moving to a colder, drier climate. Since he has already considered water availability and increased watering doesn’t seem to improve the situation, the next logical step would be to investigate other environmental factors that could be influencing their growth.
Given the options provided, the most relevant scientific question he should ask next is:
D. Does temperature affect the growth of the rose bushes?
This question addresses a key factor in plant growth, as temperature can significantly impact plant health, development, and blooming. Exploring this aspect could provide valuable insights for Derek's situation.