Potherbs are edible green leaves that you cook up to eat. You may also see them called Greens or Winter Greens. The reference is not to something “grown” in a pot, but rather “cooked” in a pot.
When you cook up spinach, as opposed to serving it fresh in a salad, you can refer to it as a “potherb.” Other examples of Potherbs are beet greens, collards, kale, mustard leaf, turnip greens, spinach, and Swiss chard.
Potherbs, in British English, is considered by most an antique word. But, it is not just an older medieval term and concept as some sources might suggest. It is used a great deal still in Southern US kitchens. And the word “Potherbs” still has its uses. It is a bit clearer than the term “greens”, which is used on some restaurant menus to indicate a salad of fresh, uncooked leaves — e.g. “with mixed greens.”
The definition of Potherbs, though, gets a bit fuzzy after you define it as something leafy that you cook. Do you classify rapini, which is a form of broccoli more valued for its leaves than florettes, as a Potherb? Then what about cabbage, if you cook it?
Food writers often qualify Potherbs by saying that, in common with “regular herbs” used for flavouring, Potherbs are edible plants that don’t develop a woody stem (though certainly a good number of herb stalks do tend to go what we would think of as woody, even if plant biologists disagree with us.) Some sources, covering that base, say the difference between an herb and a bush or a tree is that even if herb stalks do get woody, they don’t survive from year to year. The stalks die completely during the winter whereas those of a bush or tree do not.
Many things that were previously eaten as Potherbs in western culture would now be considered just “survivalist” food, items such as Dandelion, Dock, Horseradish leaves, Mallow, Purslane and Stinging Nettle.
With some plants, only the young leaves are used as “Potherbs”, the older leaves being too bitter, tough or stringy to bother with.
Many Potherbs are served with some kind of fat — bacon fat, butter, fatty pork, or oil — to mellow their taste.
Most Potherbs don’t last long after picking, and require using up within a few days.
They usually require a good washing, and some will require the cooking water to be changed a few times to wash away toxins or bitterness.
Occasionally you will see the word Potherbs also being used to refer to (flavouring) herbs that you use in cooking, such as thyme, sage, basil, etc. But as you can use *any* herb in cooking, this sense would be so vague as to render the word of little utility. Plain “herbs” are used in small quantities for their flavour; “Potherbs” are used in large quantities as a vegetable.