(I'm an amateur) Your pictures are nice. I think if you ever photograph the area again, take in to consideration your lines, and background. You could change your angle maybe making the cacti appear to grow towards the sky, or maybe get right in on the plant (with caution to the needles) and try to get a depth of feel image.
What lens are you using? If it is the kit 18-55mm, that is all I use and you can make some very nice images. If you have a Photoshop program you can get your horizon lines straight.