I have done this OP and it was disheartening. Very much a hobby rather than any meaningful income. I have a (fairly) 'succesful' 10k+ Instagram photography page and regularly am contacted by brands, magazines and newspapers etc asking if they can use 'x' image in an article/issue/brochure for free. I said yes a number of times in the past thinking it would be good 'exposure' and have seen my photos in various magazines, online articles, on national trust brochures, a P&O ferry poster, and so on. For a bit of fun it's fine but just not profitable. I would make more money doing sponsored content to be honest but I don't do any. It's just for fun/a hobby.
So many people are willing to do this however that there is very little need for most companies to pay for an image or a stock image, why would they! With camera phone quality what it is and camera equipment being so affordable now, high quality amateur photography is a saturated market IMO. These companies are very active on social media and have staff on the look out for images they can use for free via this method, I get several weekly. If you agree they will send you a consent form re: right to use an image, then usually ask you to send them it in high res.
Stock image websites pay virtually nothing either and like you, I looked into the greeting card/print concept at the time but production costs are high so again it isn't ever going to be a big money maker. I would say I made a few hundred pounds in 6 months but it wasn't worth the hours put in. Photography greetings cards are far less popular than illustrative ones and you'd need to spend a lot of time canvassing local businesses to stock them too. I'd firmly agree with the advice given that when it does sell it is more likely to be iconic local imagery or landmark based, rather than flora/fauna, unfortunately.
I think you'll also find a lot of gardens/tourist locations use a combo of in house and the social media to procure their imagery. I have twice been approached about an in house role, one was voluntary, the other very part time (8 hours a week) and included sole use and right of photos taken on the premises.
It's really tough! Definitely check out Facebook groups for the up to date info though. Would love to see your pictures!