This flower and its history in England reminds me of what you have told us about the pineapple.

Thank you very much, Julie.

Following are my musings, based on Julie's information, so read at your own risk ;-)
1.
An exotic flower, and therefore expensive. The General grows this at the Abbey too, I think.
He clearly loves a garden.

In the following, 2 musings, perhaps JA is being sarcastic.
2.
Hyacinth is a Florists flower, grown under strictly defined set of rules or standards.
The atmosphere at Northanger Abbey seems to be under the same strictly defined set of rules or standards by the General-maybe that's why he loves to grow the pineapple and the hyacinth.

Catherine has learnt to love a hyacinth, but Henry would very much like her to love a rose - freeer (?)

3.
Another explanation.
Like the setting and goings on in 'Udolpho', hyacinth is foreign too.
Henry thinks it a good idea if she learns to love a rose - symbol of England?

He very much seems to say that to Catherine later on when he says, '..remember the country and the age we live in.'
He would like to say that Catherine is so taken up with something that is foreign.