Not every exposure has to be "perfect", or "centered" on the meter. However, I do agree that the cactus could use some more light. If you are using a long enough shutter speed, I would bring a spotlight with you so you can light paint the cactus. You can try it from different angles for some really neat effects. I like the exposure on the sky, the cactus just needs more light.
thanks to you both, I think you are right. The first pic does have artificial light. The light used has three settings and I use the middle one. Next time I'll use the brightest setting. The second pic has no artificial lighting at all. Thanks for your opinions