Answer:
You need to have 100 FP with Daryll before he will let you in the basement. Giving him a Colored Grass will give you 9 FP with him once per day. But if you give him another you won't gain as much FP. So if you give him one per day you'll only need ten (you get one FP for talking to him once per day). But if you do it all in one day you'll need to give him much more.