Your other options are:
Tropius (Great Marsh)
Bellsprout (Route 224 and 230)
Oddish (Route 224 and 230 @ Night)
Snover (Route 216, 217, Acuity Lakefront, Mt. Coronet Peak)
Cherubi (Honey trees)
Cacturne (Route 228)
I would not recommend a Grass mono in Platinum, considering 2 out of E4 will trash you (Aaron and particularly Flint), and you will meet Candice and Byron will difficulty. You can't get Lileep until after you get the Nat dex, so you essentially have no counter for Flint. I know you're looking for a challenge, but doing a Grass mono in Platinum might be too much.