Pac Choi and Choi Sum

Pac Choi (sometimes spelled "Bok Choy") and Choi Sum are two Chinese vegetables of the species Brassica rapa, so they're related to the mustards, and more distantly, the cabbages. These two vegetables are amazingly easy - they germinate and grow so quickly that it's all about timing. They can be harvested as baby greens or as full sized plants, but always before they bolt and form flowers.
Pac Choi is an urn-shaped plant, with thick, succulent petioles (stems) that may be white or light green, topped by deep green, tasty leaves. If you eat the leaves raw, the mild mustard taste is a bit stronger, but they are perfect for stir-frying. The petioles remain crisp, while the leaves cook down, a bit like spinach. In Chinese, choi means "vegetable," and pac choi means "white vegetable" - a reference to the stems.
Choi Sum (or Yu Choi Sum) grows round, flat leaves around a central stem, and then forms a small head of flower buds. Just before the first flowers open is the ideal time to harvest, by cutting right at the ground level and using the whole plant.
Both types grow best (more slowly, without bolting) in cool weather in spring and fall. Pac Choi, in particular, is suited to very early spring plantings, and harvesting into late fall. With cloche protection, Pac Choi may be harvested in winter.
Sow these plants in short rows every 2-3 weeks for a longer harvest. You can still plant them throughout the summer, but you'll need to stay on top of them. The bolting process can unfold over only 48 hours! (Family: Cabbage, Brassicaceae)