Can be found in Activities > Outdoors (sign) and Decor > Paintings (decals)
This object is pretty simple to use. You can click on it and choose to label it so when you hover over it you know exactly what types of plants are in that part of the garden.
This sign is blank, you’ll likely want to add some lettering to it so that it looks better. I made a bunch of simlish decals that are included in this download so you can use for some variation.