In the opening chapter we are told Catherine was not fond of the heroic enjoyment of watering a rose bush.
Catherine was indfferent to flowers.

This stands in contrast to Catherine telling Henry his sister [Eleanor] has taught her to 'love a hyacinth'.
This detail may signal JA wanted us to see Eleanor as like a gothic heroine.

It appears Northanger Abbey had hyacinths. I believe around 1800, hyacinth dells were a feature of picturesque landscape gardens.

Catherine 'learns to love a hyacinth' under the influence of the Tilney siblings. She is learning a lesson in beauty, taste and importantly, as Heather says, in feelings.

Henry would like Catherine to learn to love other flowers and 'may come to love a rose.'- for her to continue in the habit of learning to love.

Well, that's my interpretation !