Mustard, or mustard greens, is a species of the mustard plant Brassica juncea. It is more pungent than the closely related Brassica oleracea greens (such as kale, cabbage, collard greens among others), and is frequently mixed with these milder greens. Mustard spinach is a very close relative (Brassica perviridis).
Mustard greens thrive best in cooler weather and can provide two crops per year – one in the spring and again in the fall. When nipped with frost, the taste of mustard greens becomes even more distinct. In particular, the variety Southern Giant Curled Mustard exhibits a pronounced horseradish-mustard flavor.
The leaves, the seeds, and the stems of mustard greens are edible. Traditional Southern recipes include mustard greens and/or turnip greens (as well as other types of cool-season greens) being cooked for a long period with ham hocks or other smoked pork products. Mustard greens are a very good source of vitamins A and K, and they are rich with iron.