The Shanghai Bok Choy 上海菜 is the most commonly brought and used vegetable in Chinese cuisine. It has a green bulbous stem and darker green oval leaves. The Shanghai is not to be confused with the White Bok Choy 奶油白菜. (there's no differentiation in non-Asian markets).
In the supermarkets you will find the full sized Shanghai or the "baby" Shanghai. Now let's explain "baby" or "苗". The "苗" means sprout or baby. It is the younger form of the vegetable. There is a future post with examples for this topic. Now back to the Great Shanghai! Below are all examples of Shanghai Bok Choy. Now most Shanghai is grown in Mexico by small to mid size growers. There are also your local farms that grow them in California, Florida, NY and NJ. (that I am aware of.)
So, how do we know where our Shanghai is grown? We don't. Currently many of the Asian supermarkets (brick and mortar or online) do not label where their produce is from, since it is the consumer's responsibility to be aware. Therefore, please wash all your produce thoroughly!
When choosing your vegetable, look at the bottom where it's cut. Depending on the vegetable it should be a white-green. It will turn yellow-ish the longer it sits. Every product reacts differently depending on how its handled and stored. From left to right: OLD, not so old, and the sample on the right is the freshest.
Shanghai Bok Choy can be used in a simple saute with garlic and salt, or braised with meat, the middle picture is braised porkbelly with whole Shanghai. Or bring the water to a rolling boil, add Shanghai cut or whole, boil for 2-3 mins depending on size of vegetable, drain, plate and drizzle with Oyster sauce or ponzu.