How it works
Underneath the map and the quizzes, one idea does the work: show a plant again just before you would forget it.

The question
A photograph and four names
Sometimes the other way round: a name and four photographs. The other options are picked to be believable, never at random.
The boxes
Know it well, see it less often
Each plant sits in a numbered box. Right, and the wait doubles: a day, two, four, out to sixty-four. Miss it, and it comes back tomorrow.
Typing
When you are ready, no options
After a few boxes they fall away and you write the name yourself. Small typos are fine.
Check-ups
Old favourites come back
Plants you have learned return after a couple of months, just to keep them fresh.
Golden leaves
What you earn, you keep
Finish a level and its leaf is yours for good.
Your pace
Set it up however suits you
Questions per quiz, how far the boxes go, how much old material is mixed in, and a daily reminder at a time you choose.