Use the Glasshouse to keep and manage your Friendship Flowers! The Glasshouse is located in your Garden near the ocean.
The first Glasshouse slot will be unlocked at Garden level 5. If you haven't unlocked the Glasshouse slot but has a Friendship Flower, the flower will go to the Gift Box automatically.

Friendship Flowers can only grow when you place them on your neighbor’s Glasshouse plot. The Flower placed on the Glasshouse plot will be cared for by that neighbor and you will receive a reward if your Flower is crossbred by your neighbor.

Here are the steps to place your Friendship Flower on your neighbor's Glasshouse plot:

To place a Friendship Flower in your neighbor's Garden, tap on your Glasshouse, and you will be able to see which Friendship Flowers you have that can be put into foster care.
Tap on the "Foster Care" button and a neighbor panel will appear allowing you to choose which neighbor you want to ask to foster your Flower. After you arrive in neighbor's Garden, tap on their Glasshouse plot (the four red tiles plot) and you will be able to see the panel as below.

Place the Flower you want onto the plot and your neighbors can take care of your Flower for you. If they decide to crossbreed your Flower with one of their Flowers, your neighbor will get a Flower and you will receive a special Flower Package.

How to take a Friendship Flower back

If you want to take back a Friendship Flower from the neighbor, do the following steps:
1) Tap on the Glasshouse in your Garden. You'll see a list of your Friendship Flowers.
2) Find the Friendship Flower that you want to take back, and then tap on the small yellow circle with the arrow icon inside of it and confirm that you want to take this Flower back.

Note: the Friendship Flower that was taken back will return to the initial stage of growth, and its new owner will have to grow this flower again from the very beginning.