You've got a pretty solid start, but there are still some areas that you should look into. For instance, proportions on the wheels compared to the greenhouse. Even if you had 20' wheels, the side visibility would still be about 4 inches out the sides. Also, when drawing your wheels, try to do them in one stroke, not several. You can practice elipses on a seperate paper, eventually it'll get to the point where you can draw the shapes of the tires and wheels in one smooth stroke. Other than some proportion problems, you're on to a good start.
Apparently I have a tendency of being too harsh with my help, so I hope that didn't come out too offensive.