Do you have a photo?

Unfortunately no, I don't -- I am having issues with my phone (it's ancient). I tried to take one.

You mention yellow and crispy...makes me think not enough water.

Nursery pots are good at cooking roots when the temps are hot. Not sure how much you watered but in those pots...they need to be soaked thoroughly.

Maybe not enough! I did water it daily, but it's a big, black ceramic pot -- I am guessing 25 gallons but could be more (about 3 feet around and about as deep?). I bet anything I did not soak it well enough down into the roots, which would make a lot of sense.

I'll try delphiniums in the garden, proper. It's heavy clay soil and holds a ton of water.

Delphiniums REALLY don't do well in hot, humid climates. They like Scandinavian style winters and summers. Or English.

It's not humid or that hot here. It's hot if you go inland, but I live relatively near to the coast, so it's cooler here, and it rarely goes over 85-90 before August, very low humidity (dry, very dry. I live in Wine Country near Sonoma, but more towards the coast in a cooler microclimate). Delphiniums are a huge favorite here and normally thrive in peoples' gardens in the Bay Area. Highly sold at nurseries and bloom especially in July.

I do think I underwatered them for the size of the pot though.