The Sea of Flowers is a region that allows you to grow daisies, pink roses, red tulips, and marigolds.
To access it, you'll first need to unlock the Cartographer, a building that allows to access a number of regions that specialize in certain aspects of farm life and server as expansions of sorts to your main farm area. You can find it to the west of your farm:
The Cartographer is available to players level 40 and up for 15,000,000 Thalers or at level 15 and up for 150 Gold Bars.
If you participated in the 2017 Valentine's Day Event and unlocked the Sea of Flowers during the event, you won't need to unlock it again. If you didn't unlock it during the event, you'll be able to unlock it on the cartographer's map for 7,500,000 Thalers or 100 Gold Bars.
Unlike other regions, the Sea of Flowers does not have any prerequisite tasks.
The Sea of Flowers starts out with 25 plots which are ready for use immediately; additional plots can be cleared by removing obstacles (regular obstacles and special ones which can only be cleared for Gold Bars) and expanding the field, similar to how it works on the main farm.
In addition to daisies, pink roses, red tulips, and marigolds, you can also place the Florist in the Sea of Flowers - it will allow you to make beautiful bouquets out of the flowers you harvest!