i would use a dramatic large succulent like an Agave. Great structure against the white wall, and great shadows as well.

Sphaeralcea ambigua - Apricot mallow - beautiful small orange flowers, can get 4x4 and will flower all year round if it is happy. Takes reflective heat and is beautiful. I would plant it in the ground if possible.

@Kerry, Sphaeralcea Ambigua might be a perfect plant for this location. The flower color perfectly enhances a boring dark concrete planter, while contrasting to the white wall. That section of the garden is boring and needs color.

Would this grow in gritty mix? I noted that this plant wants very alkaline soil, and the bark in the gritty mix tends to make it a bit more acid. I am not sure that garden lime would blend all that well into the gritty mix given the size of the particles. I could also put it into a well-draining soil that has <30% peat and mostly pumice and coarse sand for the inorganic components. That soil would be much wetter but does drain well. But that soil could have alkalinity adjusted easily.
How deep do the roots go on this plant?

I only see this available online as seeds. Most nurseries will carry it this time of year?

It is a beautiful plant! It is native to the desert and can be planted in a mixture of soil and decomposed granite. It will want good drainage, but will appreciate some summer water since it is native to an area that gets an occasional thunderstorm during our hot months.

Not sure if it will thrive in your soil based on your description but I would be happy to give you a free plant from my garden and you can test it out and see. Where are you in the bay area?

@Kerry thanks for Calscape information. If you want to enable messaging on this site for a short time, let me know and I will pass location information.

It may be too late, but if you do not want any varmints living under your shed I would suggest stapling some hardwire cloth all around the bottom and having it extend out six inches further once it hits the ground.

@christopher Have had those sheds about five years now and mainly they attract bunnies and cats. :) That fox periodically eats all our bunnies. I have definitely followed your advice on access points to the crawl space of the house. We had a major rat problem and sealing the entrances fixed it quickly.

Lantana comes in orange yellow pink colors, can shape into tree or ground cover, blooms profusely and drought tolerant. Lorapetalum with green or purple leaves and bright pink flowers. California lilac ceonanthus grows in different shades of blue. also grow as ground cover or small tree. They all tolerate full sun, will appreciate light shade in the afternoon, blooms ,and drought tolerant
Hibiscus is a

Nandina 'Firepower' if you just want something tough, dependable, low-maintenance, and well-behaved. 'Firepower' does not flower so no seedlings.