I'm a devotee of Chinese vegetables and have been trying to grow pok choi here north of Denver. I have heavily amended clay thanks to my livestock and access to an excellent watering system. The choi comes up fine but immediately separates into single stems, I've never gotten good "bulbs" or stout stems. We only two seasons here typically: winter and summer. Summers are windy, hot and very dry, hence the good watering system.
Add as much compost as possible to break up the clay, leaves, grass clippings, vegatable waste from the kitchen, or bought organic compost. We have clay here in Dallas and must amend the soil in order to grow anything other than Bermuda grass.