okay, i've looked it up myself, and it basically does this:

If you can level the sapling in this event to high enough EXP, the new sapling (an item drop you'll get at the end of the event) will be able to level up a Flower Knight from Level 1 all the way to Level 80 in one feed.

Required EXP for level 1 to 80:

5★ Flower Knight: 196200 (153000 with 9x100 year Manyus)
6★ Flower Knight: 204015 (160815 with 9x100 year Manyus)

So what you're saying is that if you feed the sapling too much, you're basically wasting resources, since she'll give more xp than you need. Makes sense.