After you acquire a new plant, visit the Outskirts Greenhouse to plant it.

Once there, tap the chair icon on the bottom-left of the screen to enter "Arrange" mode. This mode makes all empty pots visible and allows you to plant new plants or move existing ones.

Tap any empty pot to see which of your plants can be placed there. You can also tap on a plant already in a pot to move its location.

Note: Plants require specific pot sizes. The outline color of a pot tells you what size plant it can hold. For example, a pink-outlined pot is for medium-sized plants.

When you're finished arranging your plants, tap the chair icon again to exit "Arrange" mode. You can now care for your plants as usual.