I just went to the Scenic Express website and almost fell out of my chair. Those desert plants are great. While some are definitely done better then others, they all are pretty respectible.
I think I might be bugging them with emails to get these into the HO market somehow. Not sure who actually manufactures these desert plants, but I think they would do well if made in HO scale.
At a train show in Anaheim, California several years ago, a vendor had beautiful Joshua trees and palm trees in HO scale. I think his Joshua trees look better then Scenic Express.
He offered flowering and non flowering Joshua trees. I bought a couple of both his non flowering Joshua trees and palm trees. Oh how I wish I knew where to get more. I seem to recall the man lived in Riverside, but I have been unable to find out more and he has not been to a local show since that one time.
I thought I had a better photo of the desert section of the layout showing the Joshua trees, but these are the best I could find:
And this is the best I could find showing some of the palm trees:
Oops almost forgot. Plastruct makes some prickly pear cacti in HO scale. They come in an awful bright green plastic color. I painted mine with paints on hand. The cactus part was painted PollyScale Depot Olive, while the little pears were painted with PollyScale Milwaukee Road Maroon. No photos I have on hand, but if you'd like I can take some and post here.