You did a lot right: You used a tripod, you used a small aperture for lots of DoF, and a sufficiently long exposure time. On the "areas for improvement" front, to be brutally honest the subject just isn't that interesting (ref your offer of use). Also, you were too close; notice the significantly over-exposed areas in some. That said, they're a good start.
As an exercise it was a good idea. You captured the light well with some overexposure in spots. I'd like to have seen you move around some. Getting further back would have been good, also going down on the ground and shooting upward while filling the frame with the color may have made the subjects more appealing. When you do a series try taking a few and then looking at your results and critiquing what you have so far and then look for ways to improve.