Featured with permission by MagicHadi

Plumage Peach: Ysheva harbor, next to where you fight Zeke. Melodious Melon: to the right of the entrance in the argentum flight deck. The point with the taller crops. Flutterheart grass: has a quest marker. Some gymnastics from the airborne throne. Sumpkin: Torigoth. Tall grass patch east and slightly north of Kloom Farm. Dawn Hydrangea: starting from the greatspine boundary on the upper-left area of torigoth, head toward the shipyard until you see a path to the left leading to a cave. Go from there, go across the branch of the bigass tree, and you’ll see a platform to the right. Hop on to it and pray you get a good amount cuz its a pain in the ass to walk there 20 times. This spot also gives sumpkins if you wanna kill 2 birds with one stone. EDIT: You can also start from Umons Shipyard to get the dawn hydrangea. Idk which is faster. EDIT 2: You can get Sumpkins far faster at the Torigoth flower patch, north of the relay base. Though its next to torigoth you can only fast travel to it on the upper level-right map. EDIT 3: Torigoth flower patch also has dawn hydrangea’s if your botany level is high enough, though their drop rate is far lower. EDIT 4: As a commentor pointed out, you can get both dawn hydrangeas and sumpkins way faster if you unlocked clear sky beak fast travel point in gormott. It’s recommended that you use that area above any of the others since it provides far more materials. EDIT 4: Plumage Peach can also be found south of corrins house in fonsett village.