Yes, you can cursor over the dirt, and get the option to do nothing or clear the spot, dirt, whatever, I forget the exact wording. At any rate, you can clear and plant something different. I plant the alchemy ingredients I want, and when I have enough of any particular one, I clear the spot and plant something else. It's great for alchemists. The greenhouse wing is cool, and I have it on one house for another character, but my current character finds the garden plot is quite suitable for her needs.
I had to give up on my alchemist, as he got too
much gold from using the greenhouse, and the game kinda got boring

It's insane for leveling up alchemy though.