Seeds and Flowers are used during the Spring event -- the same one where you can find Easero and Kurrabi eggs, usually held around Easter or Earth Day.
I don't recall what year this planter event was released nor do I know what year each set of flowers were released, but I'll be doing my best to catalogue these in release batches!
If you see any seeds or flowers I'm missing, please let me know! I missed out on participating for a couple of years so everything after set 3 is murky for me.
2016
Paordula Sprout - Paordula ||| Chryshaefu Pod - Chryshaefu
Perilus Seed - Perilus ||| Orchsearo Seedling - Orchsearo
Goibit Sprout - Goibit ||| Lumicyid Beans - Lumicyid
Snapgondra Seed Pod - Snapgondra ||| Dandysirleon Seed - Dandysirleon
Angel Trumpet Seeds - Angel Trumpet ||| Otachitail Seed Cluster - Otachitails
Zaphodil Bulb - Zaphodil
2017
Pitchcu Plant Seed - Pitchcu Plant ||| Drachidily Seed - Drachidily
Bleeding Wyrae Fungus Spore - Bleeding Wyrae Fungus ||| Song Mirabine Seed - Song Mirabine
Noctis Head Seed - Noctis Head ||| Liyure Jaw Seed - Liyure Jaw
Jaaku Cherry Seed - Jaaku Cherry ||| Dog-Toothed Violet Seed - Dog-Toothed Violet
Foxglove Seed - Foxglove
2018
Bleeding Heart Seeds - Bleeding Heart Flowers ||| Bears Breech Pods - Bears Breech
Black Bat Seeds - Black Bat Flower ||| Goatsbeard Root - Goatsbeard
Lizard Tail Seed - Lizard Tail Flower ||| Malalslip Seed - Malalslip
Elepod - Elephant Head
2020
Hammer Orchid Seeds - Hammer Orchid ||| Shrimp Plant Seed - Shrimp Plant
Intes Husk Pod - False Intes Fruit ||| Galta Fin Seed - Galta Fin Flower
Drindian Glory Seed - Drindian Glory Flower ||| Ezahni Primrose Seed - Ezahni Primrose Flower
Chimby Pod - Chimby Fruit ||| Draqua Lotus Seed - Draqua Lotus Flower
Kayoki Ear Pod - Kayoki Ear Carnation ||| Kioka Lily Seed - Kioka Lily Flower
Leverene Lace Seed - Leverene Lace ||| Ashdrop Bulb - Meiko Ashdrop
Meragon Fin Seed - Meragon Fin Orchid ||| Narwiscus Seed - Narwiscus Flower
Tesuri Ribbon Seed - Tesuri Ribbon Flower ||| Roditore Berry Pod - Roditore Berries
Omnily Seed Pod - Omnily Flower ||| Skeleton Seed - Skeleton Flower
Butterfly Bush Seed - Butterfly Bush ||| RSTU001 Bean - RSTU001 Sprout
RSTU002 Daisy Seed - RSTU002 Daisy Flower ||| Jahra Orchid Seed - Jahra Orchid Flower
Ardur Lava Lily Seed - Ardur Lava Lily Flower
2021
Habromeliad Seed - Habromeliad ||| Aerix Paradise Seed - Aerix Paradise Flower
Ahealily Seed - Ahealily ||| Divuin Glowbell Seed - Divuin Glowbell
Aukirillus Seed - Aukirillus ||| Quelis Bellflower Seed - Quelis Bellflower
Murrayle Ferox Seed - Murrayle Ferox ||| Kurranemone Seed - Kurranemone