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Leaf vegetables, also called potherbs, greens, vegetable greens, leafy greens or salad greens, are plant leaves eaten as a vegetable, sometimes accompanied by tender petioles and shoots. Although they come from a very wide variety of plants, most share a great deal with other leaf vegetables in nutrition and cooking methods.
Nearly one thousand species of plants with edible leaves are known. Leaf vegetables most often come from short-lived herbaceous plants such as lettuce and spinach. Woody plants whose leaves can be eaten as leaf vegetables include Adansonia, Aralia, Moringa, Morus, and Toona species.
The leaves of many fodder crops are also edible by humans, but usually only eaten under famine conditions. Examples include alfalfa, clover, and most grasses, including wheat and barley. These plants are often much more prolific than more traditional leaf vegetables, but exploitation of their rich nutrition is difficult, primarily because of their high fiber content. This obstacle can be overcome by further processing such as drying and grinding into powder or pulping and pressing for juice.
Leaf vegetables contain many typical plant nutrients, but since they are photosynthetic tissues, their vitamin K levels in relation to those of other fruits and vegetables, as well as other types of foods, is particularly notable. The reason is that phylloquinone, the most common form of the vitamin, is directly involved in photosynthesis. This causes leaf vegetables to be the primary food class that interacts significantly with the anticoagulant pharmaceutical warfarin.
During the first half of the 20th century, it was common for greengrocers to carry small bunches of herbs tied with a string to small green and red peppers, these bundles were called "potherbs."
Leaf vegetables are typically low in calories, low in fat, high in protein per calorie, high in dietary fiber, high in iron and calcium, and very high in phytochemicals such as vitamin C, carotenoids, lutein, folate, magnesium as well as vitamin K.
A primary source of dietary inorganic nitrate for nitric oxide production in the body is from leafy vegetables, in particular spinach and arugula. Nitric oxide is a natural cardio-protective that contributes to cardiovascular health and reported to be responsible for the anti-hypertensive effects of plant-based diets such as the DASH diet and the Mediterranean diet.
The vitamin K content of leaf vegetables is particularly high, since these are photosynthetic tissues and phylloquinone is involved in photosynthesis. Thus, users of vitamin K antagonist medications, such as warfarin, must take special care to avoid leaf vegetables entirely (or else eat a very carefully monitored and constant amount of one or more of them, which is very difficult). Green beans, peas, and green fruits usually have too little vitamin K to cause problems for users of these medications, and while other plant tissues (fruits and non-green vegetables) and meats contain some vitamin K, it is usually too little to cause large changes in coagulation status with warfarin. (Note: the cyanobacterium Spirulina, due to its photosynthetic nature, contains significant vitamin K).
If leaves are cooked for food, they may be referred to as boiled greens. Leaf vegetables may be stir-fried, stewed, steamed or consumed raw. Leaf vegetables stewed with pork are a traditional dish in soul food, and southern U.S. cuisine. They are also commonly eaten in a variety of South Asian dishes such as saag. Leafy greens can be used to wrap up other ingredients into a edible package similar to how a tortilla is sometimes used. Many green leafy vegetables, such as lettuce or spinach, can also be eaten raw, for example in sandwiches or salads. A green smoothie enables large quantities of raw leafy greens to be consumed by blending the leaves with fruit and water.
At least 80 different kinds of greens are used, depending on the area and season, including: black mustard, dandelion, wild sorrel, chicory, fennel, chard, kale, mallow, black nightshade, lamb’s quarters, wild leeks, hoary mustard, charlock, smooth sow thistle and even the fresh leaves of the caper plant.
Preboggion, a mixture of different wild boiled greens is used in Ligurian cuisine to stuff ravioli and pansoti. One of the main ingredients of preboggion are borage (Borago officinalis) leaves. Preboggion is also sometimes added to minestrone soup and frittata.
In the cuisine of the southern United States and traditional African-American cuisine, turnip, collard, kale, garden cress, dandelion, mustard, and pokeweed greens are commonly cooked, and often served with pieces of ham or bacon. The boiling water, called potlikker, is used as broth. Note that water in which pokeweed (poke sallet, poke salad) is boiled contains toxins removed by the boiling, this water should be discarded.