If you want to purchase some tasty bok choy, visit an Asian market. What you probably won't find there is a standard-size variety. It may be the part of Asia that most of the immigrants are coming from but the dwarf varieties are what I find in the markets. They are what seem to be called "Shanghai bok choy."
Inspired by that, I bought Mei Qing Choi from the very first. I have probably tried over 10 different varieties, the last 20 years. And, I will continue to try new bok choys but, likely, continue to grow Mei Qing every year also.
Apparently, the seed companies have learned that other gardeners have discovered the dwarfs and the choices have grown
. I've enjoyed growing "Toy Choy" but those are tiny, little guys! You can't wait long for them to make much size - they will bolt.
Both very cold and hot weather will cause bok choy to bolt. Remember, you can eat them at that stage but, of course, vegetative growth has pretty much come to a stop. You really need to LOOK at your bok choy every day! They will keep quite well in the fridge so grab them
- especially those ultra-dwarf types!
edited to add: I tried Lulan bok choy from New Dimension Seed Company last year. Not a lot different from Mei Qing but those plants grew real well and were especially