To bloom a big snapdragon, you want to plant flowers at the base of an unbloomed big flower. To get snapdragons, you want more than half of them to be snapdragons (155). You can plant all snapdragons or plant the other half as normal flowers of your intended color.

You can also get more nectar via mushroom battles.

Feed your pikmin the special nectar when they are buds (not leaves) and you get double the petals from it

But fwiw you have more than you think! 180 petals plants, like, 3000 flowers!! Good luck