Answer: Wow! Shady and moist soil in the low desert isn't something we see very often! My first question is how shady and how moist? Full shade all day? Or partial shade, such as the filtered light beneath a tree canopy. And how moist? Does it stay moist year around? Why? Constantly wet soil isn't good for roots because they don't receive sufficient oxygen. Also, you didn't mention how much space or what type of plant. Groundcover, a shrub? So, determine those issues. Then here's a few possibilities for you. I hope one of these will work for you.
Full shade or partial shade. Common houseplants, such as spider plant or philodendron aren't used much outdoors, but in the right conditions they do quite well.
Yellow dot/Wedelia trilobata. Groundcover that takes full sun to full shade and moderate water.
Mexican honeysuckle/Justicia spicerga. Grows 2-3 feet high and spreads. Orange trumpet flowers attract hummers. Takes a fair amount of shade and variable water.
White plumbago/Plumbago scandens. Takes full shade and moderate to low water. Susceptible to powdery mildew if air circulation isn't good.
Grape Ivy/Cissus trifoliata. Native vine that takes full sun to full shade, but is fairly low water use.
Sago palm/Cycas revoluta. Not a true palm. Takes partial to full shade and moderate water.
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