Instructions
Raise your gardener level to 29 to make bullrush seed appear in the store. At level 29, there is also a chance that Seedos, the roaming seed lover, will give you one for free.
Open the General Store interface. Buy as many bullrush seeds as you need for 62 coins each. When you are asked where you want to put the seeds, place them on the ground anywhere (for now).
Go back to the garden. Replace a patch of grass with mud, if necessary. Dig a pond with your shovel. Bullrush must grow around mud and water. You won't be able to plant them anywhere else.
Pick up the bullrush seed and hover the cursor over the spot where you want to plant it. Select "Plant." Give the plant three doses of brown fertilizer -- if you have it. This increases the plant's value.
Wait for the bullrush to mature. At this time, you are able to break off the brown plant head and give it to Pinatas for food. Certain Pinatas need to eat the plant if you want them to mate or become residents.