God of War Ragnarök doesn't just ask you to slaughter the population of the nine realms, like in the Crucible or when hunting Odin's Ravens. There are more peaceful side quests, like Treasure Maps, and of course, picking flowers.
This service starts after freeing Freya from her curse, when you pick up your first flower , in one of the kingdoms.
The real purpose of this quest is probably to push you to visit certain places again, in order to trigger additional dialogues, as the story progresses.
Despite the effort required to complete it, it only brings 750 XP to Kratos, as well as 750 XP to Freya. It is especially the trophy "The Florist" which justifies searching everything to find the flowers, in order to obtain the platinum trophy.
The flower can be found in the second optional area, the Forbidden Sands of the Alfheim Desert. You have to stop the sandstorm in Le Chant des Sables before you can pick it up. You can find her at the back of an alcove in the western section of the area.
You have to wait until you have obtained the Draupnir Spear to access the flower. It is located in the dwarf city of Nidavellir. Near the Svartalfheim Tavern, a pit can only be crossed with a hitch point created by the spear. On the other side is the flower.
Hard to miss, this flower can be found right next to Brok's forge in Le Creuset. But to access the area, you will first need the two seed halves, which can be found in Svartalfheim. So you need the Draupnir Spear too.
After defeating Garm for the second time, the flower can be found near the Arsenal of the Dead. Instead of scaling the wall and following Atreus, use your spear to create a hook point off to the side. Then climb up, then turn right. This will allow you to reach an isolated tower, with a corpse and the flower.
No flower is easy to find. That of Vanaheim is hidden at the very end of the optional zone, in the North of the kingdom. You have to advance far enough in the story to save Freyr, then follow the dog's tracks, which will lead to the Crater. Then you have to restore the water supply to the river in the Jungle. Eventually, you'll be able to access the flower, not too far from the sinkholes fast travel portal.
End of the game and remaining flowers
After completing the main story, go to Ratatosk, opposite Sindri's house, to get an extra bag of seeds. Use these seeds to unlock new realms, as well as new passages to previously unlocked realms.
With the new seeds, you will be able to visit the Angrboda Treehouse with Kratos and Freya. This opens up quite a few possibilities for dialogue with Angrboda, by the way. The flower is in the passage a little to the south of the area, at the end of the dead end.
The destruction of Asgard has caused a new passage to appear between the training area and the Crow Tree. It leads to the ruins of a strange Ase prison, full of interesting objects and unexpected encounters. You will also be able to find the Flower of Asgard inside a cell.
The kingdom of Nilfheim is the only one to contain two flowers. The second can be found in the Mist Fields, to the east of the area. This requires using the new access to the area, unlocked with one of the seeds given to you by Ratatosk.
It's not a mistake on your part, you didn't miss the flower of Midgard. It's just not accessible until the end of the game. After unlocking access to the Woodland Sanctuary and Freya's house in southern Midgard, you can say hello to the turtle house.
The last flower is on the left side of the turtle, at the bottom of another dead end.
An earth dragon sits behind the locked door to the Forbidden Sands in Alfheim. Here's a written strategy guide with tips and tricks for defeating him on maximum difficulty, as well as a video of the fight in this mode.