The most drought resistant plants are cacti and other succulents.

Then you only need water your bananas plants which aren’t even technically trees but berries.[1]

Footnotes

Hi Dorie, thanks for the A2A. At first, I thought you were crazy. I grew up in Houston where there is literally water about 6 inches down in some yards. Lots of people grew bananas there, although we had to wrap them in newspaper in the winter in order for them to produce bananas, as they only seemed to do that on last year’s growth, and a freeze would kill them back to the roots.

But now I’m in Colorado, and you’re in California, and I wouldn’t have thought to try growing them in either place. Still, I often check in on the blogs at Drought Smart Plants, Succulents and Water Wise Perennials and I remember seeing an article about growing bananas in a dry climate. Before I point you to that article, I’d say that there aren’t any good choices for drought tolerant plants to grow immediately next to your banana trees, as the bananas want a lot of moisture. However, as the article below mentions, your best strategy may be to segment the garden a bit to provide a nice moist micro-climate for the bananas, and have other native species nearby that don’t necessarily share the moisture.

Anyway, here’s the article. Hope it helps!

Growing Banana Trees in a Drought-Tolerant Garden