Hmm, that's an interesting one because it's not a discrete object, it gets drawn out like walls or a fence. So perhaps look in the same place as those, rather than looking near the normal gardening objects?
What I lack in decorum, I make up for with an absence of tact.
If you are looking to make plants, you probably want Garden - Plant - [name of plant]. The foliage and dirt mound will be pulled when you clone it, and it appears to be the same texture for all plants. If you want to make actual plants, clone the Garden - Crop - [name of crop]. All crops appear to use one texture, each in it's own location on one texture.
If you want a replacement for the dirt, fway did an invisible one. I've done a one that looks like a dirt pile I can upload if that it what you are looking for.
Otherwise I'm thinking they are like roofs, and possibly not clonable.