I have a small garden with only white tiger lilies and a dandelion and the tiger lilies did give me some tcs at first, now the last couple of harvests, all I've gotten is gold and I'm wondering if this is a change or a bug? I really like the tower shields they give and hope it is just a bug.
When you get 999, then you won't be able to get any more. So, these are your options:
1) Go into your spell book and place the treasure cards you want into your side decks for future use.
2) Sell them. For the best price, head to the Bazaar or,
3) Quick Sell them. Open up your spellbook and click the upper left corner where the yellow treasure card icon is. You can click "Quick Sell" to sell off extra treasure cards you don't want right from your house. You will get a lower price than the Bazaar, but there is a convenience in selling off a lot at once without having to leave your garden.
Another warning for you, make sure your reagents don't go over 999, because the same problem will crop up with them.
I never even thought of that but it is definitely possible! I'll check (as soon as the game is stable again, I can't log in for more than a few seconds thanks to ongoing attacks). Thank you too for the detailed explanations for how to resolve the problem and the tip about the reagents, it was all super helpful!