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LEAF VEGETABLES

LEAF VEGETABLES. Leaf vegetables are a diverse and eclectic group of plants comprising several different taxonomic plant families: Aizoaceae, Amaranthaceae, Asteraceae (Compositae), Basellaceae, Boraginaceae, Brassicaceae (Cruciferae), Chenopodiaceae, Convolvulaceae, Cucurbitaceae, Euphorbiaceae, Malvaceae, Phytolaccaceae, Polygonaceae, Portulacaceae, and Tetragoniaceae. In the literature, leaf vegetables are commonly known as "greens" and "potherbs." They are grown for their tender, succulent, and normally green leaves, and are usually cooked before eating, thus the name "potherb." Alternatively, the salad greens, for example, lettuce, radicchio, and endive, are usually eaten uncooked. Nevertheless, the leaf vegetables can be added fresh to tossed salads, giving the salad color and novel flavors. Not included in the group are those plants with leaves that serve as an important herb or flavoring ingredient, but do not constitute the main ingredient in the dish, such as cilantro, parsley, rosemary, etc.

Additionally, there are a large number of plants the leaves of which are eaten in certain parts of the world although the leaves are actually a secondary crop. For example, in Southeast Asia, chili pepper (capsicum) leaves are eaten, but it is the fruit that Americans usually consume. Other examples of plants whose fruits or roots are the primary crop but whose leaves are also consumed are peas and beans, plantain, cassava, cucumber, radish, and sweet potato.

Leaf vegetables may be cool-season or warm-season crops and can be grown as annuals or as perennials. In addition, some leaf vegetables are adapted to the tropics, while others are adapted to the temperate climates. Depending on location, leaf vegetables are either a main crop or treated as a minor crop. The more important leaf vegetables, based on dollar value, are spinach, kale, collards, mustard greens, and Swiss chard. Other leaf vegetables such as New Zealand spinach and dandelion are popular with home gardeners and are grown on a limited scale by market gardeners.

Leaf vegetables are among the most nutritious vegetables on a fresh weight basis and are also among the world's most productive plants in terms of nutritional value per unit area, in part because they grow rapidly, allowing several crops or harvests in a season. Although some of the constituents are lost during cooking, they still contribute significant amounts of provitamins A and C and several minerals. Leafy vegetables are also good for the eyes. Age-related macular degeneration is a leading cause of blindness among individuals over the age of 50. A research study in Massachusetts found that people who ate spinach, collards, and other dark green, leafy vegetables five or six times a week had about a 43 percent lower risk of the disease than those who ate it less than once a month. The typical shelf life for most leaf vegetables is ten to fourteen days.

Major Leaf Vegetables

Spinach. Spinach (Spinacia oleracea var. inermis ) is a member of the Chenopodiaceae family, which also includes table beet, Swiss chard, sugar beet, and amaranth. Spinach is native to an area near present-day Iran and was first cultivated by the Persians more than 2,000 years ago. Records of its use are meager, but it is believed that cultivation of the crop developed during the period of the Greek and Roman civilizations. It was introduced to China in 647 C.E. and apparently was transported across North Africa to Spain by the Moors by 1100. Two seed types exist, one having a smooth, round shape and the other an irregular, prickly shape. The crop was known in Germany in the thirteenth century only in the pricklyseeded form. Smooth-seeded spinach, which is used exclusively in today's commercial production, was not described until 1552. The colonists introduced spinach to the Western Hemisphere, and it was listed in American seed catalogs by 1806.

Spinach is the most important leaf vegetable in the United States. The edible portion of the plant is the compact rosette of fleshy leaves attached to a short stem. Leaves vary from ovate or nearly triangular to long and narrow arrowhead shapes; the latter are a characteristic of more primitive types. Leaf margins may be smooth or wavy, and surfaces are smooth, semisavoyed to heavily savoyed (crinkled). The crinkled appearance of the savoy tissue results from differential growth of parenchyma tissues between leaf veins.

When plants have attained marketable size, which, depending on the season, can be 30 to 80 days, and when overwintered as much as 150 days, they are pulled or undercut below the stem. Each plant will have five to eight fully developed leaves. Intact plants are trimmed, and several are tied together in bunches and packaged. Not all hand-cut spinach is bunched. Some are bulked into harvest baskets and sold in that manner. Savoy types are preferred for the fresh market because the leaves are dark green and resist compression during packing, thus allowing for better aeration, cooling, and postharvest life. Most of the commercial frozen spinach is machine-harvested. The machines have cutting blades adjusted to cut four to six inches above ground level to reduce the amount of petioles harvested with the leaves. The smooth or semisavoyed leaf types are generally used for machine harvesting because they yield more and are easier to clean. A limited amount of greenhouse spinach is produced in northern Europe during the winter.

Because spinach has a high leaf-surface area and a high respiration rate, it must be cooled rapidly to prevent weight loss and decay. Overheating will destroy quality. Thus, rapid cooling is essential to reduce wilting and weight loss. Vacuum cooling can give satisfactory cooling within ten minutes, usually applied after bulk packing and washing. Hydrocooling (cold water application) takes longer than vacuum cooling but is more feasible for small market operations. Following hydrocooling, excess water must be removed by centrifuging; otherwise postharvest diseases begin to develop. The product can then be stored under shaved ice to preserve freshness.

A serving (1½ cups) of cooked spinach has forty calories and provides 70 percent of the recommended daily allowance (RDA) for adults of vitamin A, 25 percent of the RDA of vitamin C, and 20 percent of the RDA of iron (see Table 1). Spinach also contains high levels of calcium, phosphorus, and potassium. It has moderate levels of protein. However, not all constituents of spinach are nutritionally beneficial. Oxalic acid in spinach reacts with calcium to form calcium oxalates. Excessive oxalic acid may interfere with calcium absorption in humans, a condition particularly serious for infants. Levels of oxalic acid are substantial in all spinach cultivars, although apparently less in savoy types than in smooth leaf types. Oxalic acid is also found in many of the other leaf vegetables, including chard and, especially, rhubarb. The leaves of rhubarb are toxic and should never be consumed. The stalks should be fresh when eaten.

An additional problem relates to the accumulation of nitrogen in the nitrate form, especially in spinach fertilized heavily with ammonium nitrate and grown under high temperatures and low light intensity. Nitrates convert to nitrites in digestion, and nitrites will oxidize hemoglobin to form methemoglobin. This substance can lead to methemoglobinemia, a disorder of humans and ruminants. Nitrates can also form carcinogenic nitrosamines. These toxic constituents in spinach do not present a risk when the crop is grown with proper fertilization and is consumed as part of a balanced diet.

Swiss Chard. Swiss chard (Beta vulgaris var. cicla ) is a type of beet developed for its large crisp leaves and fleshy leafstalks rather than for its roots. Early civilizations utilized the roots as a medicine. The first records of cultivation indicate that the Eastern Mediterranean region, not Switzerland, was the place of origin. Aristotle wrote of seeing a red chard in 350 B.C.E. From this leaf plant was selected the swollen root form, the table beet. Although large acreages are not common, it is grown widely to supply local markets.

Swiss chard leaves are of best quality just when fully expanded or slightly earlier but remain succulent throughout the season as long as the leaves are harvested at the proper size. The succulent, glossy, dark green leaves are usually slightly crinkled or savoyed. Sometimes the fleshy white leaf midribs are separated from the leaf blade and prepared much like celery or asparagus. The midrib color can be white, red, yellow, pink, or green. Swiss chard is prepared for the market by washing thoroughly, grading, and bunching. Storage is not recommended, but it can be kept for short periods.

A serving of Swiss chard (3½ oz.) provides 130 percent of the adult RDA of vitamin A and 25 percent of the adult RDA of vitamin C (see Table 1). Like spinach, which is related, Swiss chard has high levels of oxalates.

Kale and Collards. Members of the Cruciferae family, kale and collards (both Brassica oleracea var. acephala ) are known as "greens" and "soul food" in the southern United States, where they are most popular. Unlike cabbage (Brassica oleracea var. capitata ), neither kale nor collards forms a headthus the name "acephala" which means 'forming no head'. Kale and collards are the oldest forms of cabbage and are native to the eastern Mediterranean region of Europe or to Asia Minor. The use of kale as a food dates to 2000 B.C.E. or earlier. The Greek philosopher Theophrastus described a savoyed form of kale in 350 B.C.E. Traders and nomads introduced these leaf vegetables to other parts of the world, and they were introduced to the United States from Europe in the seventeenth century.

Both kale and collards have dark green leaves that form a rosette-like whorl toward the apex of erect unbranched stems. Even though collards and kale belong to the same taxonomic group, they are quite distinct. Collards differ from kale mainly in leaf shape and flavor. Collards have large, broad, flat, smooth leaves with smooth leaf margins; kale has a greater variability of leaf types. Most kales have largely upright heavily curled leaves. The decorative leaves of kale have given rise to its use as an ornamental plant. Flowering kale is very attractive for landscape plantings and is edible though not very palatable. The term "flowering" derives from the shape and coloration of the plant, which resembles a flower, and does not refer to actual flowers.

Kale is also called borecole ("winter cabbage"). The name "collard" is a corruption of colewort or colewyrt, Anglo-Saxon terms meaning young cabbage plants. Curly leaf forms of kale occur because of disproportionate growth along leaf margins, whereas the savoy (crinkled) appearance is due to nonuniform growth of portions of the leaf laminae. Kales and collards are the hardiest of the cole corps; when properly acclimated, they can tolerate temperatures to 0°F or lower and they are often overwintered. In addition, kale and collards have good tolerance to high temperatures (80° to 85°F), although growth stops at about 85°F. However, the best-quality kale collards are grown in the cooler part of the year. In general, collards are more heat tolerant, while kale is better adapted to cooler weather.

Both kale and collards are biennial, meaning that they will flower after an extended exposure to cold weather. A vigorous collard plant may reach a height of three to four feet. Kale is somewhat smaller. Two general types of kale are grown for the market: curly leaf (the most widely grown) and smooth leaf. Of the curly leaf forms, Scotch kale is rather light green, with very ruffled, finely divided leaves; it may be dwarf or tall, with the dwarf form preferred. Because of the curly leaves, one of the commercial production problems is removing sand from the leaves.

Collards can be harvested by cutting young plants. Large plants can be cut off, or the lower leaves can be removed during the season. Leaves of both crops should be young and tender. The flavor of kale is sweeter after a frost, and many prefer to harvest at that time. After harvest, the leaves or small plants should be washed, graded, and bunched or packed. Shipments are made with an ice covering to preserve freshness. When necessary, kale and collards can be stored for ten to fourteen days at 32°F at 90 to 95 percent relative humidity.

Both kale and collards excel in food value, with kale superior to most vegetables in protein, vitamin, and mineral content. On a fresh weight basis, kale is among our most nutritious vegetables. One serving (3½ oz.) provides 200 percent of the adult RDA of vitamins A and C and 13 percent of the calcium RDA for adults.

Siberian kale (Brassica napus ) has other common names such as Hanover kale, Hanover salad, spring kale, and Hanover turnip. It is a cool-season crop that belongs to the Brassicaceae family. Siberian kale cultivars vary considerably in appearance. The plant might best be described as resembling the ordinary collard, but it is not as curly as kale. The leaves form a rosette, and are usually smooth like collards rather than hairy like turnip leaves. The petioles vary from purple to white. Although it is sometimes compared to the turnip in growth habits, it does not form a fleshy root like turnips. Young tender leaves are used in cooking.

Amaranth. Within the genus Amaranthus, more than fifty species, including both cultivated and weedy species, are eaten as greens. The cultivated species are collectively called "amaranth." Another crop of the amaranth family grown in tropical Asia for its edible leaves is Celosia argentea. In Southeast Asia, the many cultivars of this species are usually classified by leaf color and shape. Common names for Amaranthus include bush greens, Chinese spinach, hon-toi-moi, pigweed, and tampala. Because of the large number of species used, there is considerable variability in growth habit, leaf shape, color, inflorescence characteristics, and utilization. Leaf shape and color also vary considerably among the different seed accessions. Some are red, others are green, while others

Nutritional constituents of leaf vegetables
Vitamins
Crop Water (%) Energy (cal) Protein (g) Fat (g) Carbohydrate (g) A (IU) C (mg) Thiamin (mg) Riboflavin (mg) Folate (mcg) Niacin (mg) Ca (mg) P (mg) Fe (mg) Na (mg) K (mg)
Amaranth 92 23 2.5 0.3 4.0 2,900 43 0.03 0.16 85 0.7 215 50 2.3 20 611
Broccoli Raab 92 18 1.8 0.2 2.0 2,700 70 0.05 0.07 71 0.5 125 45 1.5 40 250
Chaya 80 64 6.2 0.6 10.7 194 0.2 0.20 1.6 234 76 2.8 58 270
Collard 91 30 2.5 0.4 5.7 3,800 35 0.05 0.13 166 0.7 145 10 0.2 20 169
Dandelion 86 45 2.7 0.7 9.2 14,000 35 0.19 0.26 27 0.8 187 66 3.1 76 397
Garland Chrys. 93 21 1.6 0.2 4.4 14,675 37 0.03 0.22 77 0.9 56 32 3.1 52 571
Ice Plant 94 05 0.7 0.2 0.3 2,000 23 0.04 0.06 0.3 90 26 0.6
Indian Mustard 91 27 2.7 0.4 4.9 6,000 93 0.11 0.19 187 0.8 181 46 2.0 33 374
Kale 84 50 3.3 0.7 10.0 8,900 120 0.11 0.13 29 1.0 135 56 1.7 43 447
Malabar Spinach 93 19 1.8 0.3 3.4 8,000 102 0.05 0.16 140 0.5 109 52 1.2 24 510
Mustard Greens 91 26 2.7 0.2 4.9 5,300 70 0.08 0.11 159 0.8 103 43 1.5 25 354
N. Zealand Sp. 94 14 1.5 0.2 2.5 4,400 30 0.04 0.13 15 0.5 58 28 0.8 130 130
Pokeweed 92 23 2.6 0.4 3.7 8,700 136 0.08 0.33 1.2 53 44 1.7 23 242
Purslane 94 16 1.3 0.1 3.4 1,320 21 0.05 0.11 12 0.5 65 44 2.0 45 494
Sorrel 93 25 0.74 0.1 3.8 010 10 0.03 0.04 0.3 130 21 0.9 6 360
Siberian kale 87 42 2.8 0.6 8.3 3,100 130 0.07 0.06 28 1.3 205 62 3.0 70 450
Spinach 92 22 2.7 0.4 3.5 6,700 28 0.08 0.19 194 0.7 99 49 2.7 79 558
Swiss Chard 93 19 1.8 0.2 3.7 3,300 30 0.04 0.09 14 0.4 59 46 1.8 213 379
Turnip Greens 91 27 1.5 0.3 5.7 7,600 60 0.07 0.10 194 0.6 190 42 1.1 40 296
Water Spinach 91 26 3.1 0.4 4.6 4,600 50 0.07 0.17 1.1 84 49 2.7 43 385
Data per 100g raw sample. 1 IU = 0.3 μg vitamin A alcohol. Vitamin C = Ascorbic acid
source: U.S. Department of Agriculture, Agricultural Research Service, 1999. USDA Nutrient Database for Standard Reference, Release 13. Nutrient Data Laboratory. Available at, http://www.nal.usda.gov/fnic/foodcomp; V. Rubatzky, and M. Yamaguchi, World Vegetables, 2nd ed., International Thomas Publishing, 1997; H. D. Tindall, Vegetables in the Tropics, AVI Publishing, 1978.

may be variegated, usually with purplish patterns on a green background. Major leaf vegetable amaranth species are Amaranthus tricolor, A. lividus, A. dubius, A. gangeticus, A. blitum, and A. hybridus. A. spinosus, known as uray, is a vegetable of some importance in the Philippines. Amaranth is grown not only as a leaf vegetable, but as a grain ( A. caudatus ) in subtropical and tropical climates of Africa.

The green-leafed variety of vegetable amaranth (A. tricolor ) has been offered in the United States as the cultivar tampala. It is as acceptable as spinach when cooked, but not raw. A. lividus, known as bondue, is grown for vegetable uses in tropical Africa. Young plants of A. leucocarpus are a leaf vegetable in Algeria; in addition, the seed is made into candy. The green form of A. gangeticus, Chinese spinach, is most commonly cultivated for use as boiled greens in Asia.

For many of the leaf vegetable Amaranthus species, centers of diversity are Central and South America, India, and Southeast Asia, with secondary domestication in western and eastern Africa. The greatest diversity of leaf amaranths is found in India. The leaf amaranths are popular, low-cost, and a good protein source for the populations of many tropical, subtropical, and temperate regions.

Most leaf-type amaranth plants are erect, about one foot to three feet high, and produce numerous small flowers on terminal and axillary spikes. When harvested, plants are pulled with the roots left on to facilitate bunching. In another method, partial leaf removal is made with regrowth permitted for successive harvesting. Frequent harvesting, every seven to ten days, tends to delay flowering and encourage new shoot and leaf growth. Postharvest life is relatively short because of rapid wilting of the tender foliage.

Some general disadvantages of amaranths are the early, short-day flowering response and low-temperature sensitivity of some species and the high calcium oxalate content in leaf tissues. Nevertheless, these plants supply large amounts of provitamin A, vitamin C, protein, and fiber. Amaranth is not as high in vitamin A as spinach, but other constituents are comparable.

Mustard Greens. Mustard is often used in a generic sense to identify somewhat morphologically similar brassicas even though they are different species. All belong to the Crucifer family and are native to Central Asia and the Himalayas. For instance, black mustard (Brassica nigra ), white mustard (Sinapis alba), and Ethiopian mustard (B. carinata and B. juncea ) are all called mustard greens. However, in the United States "mustard greens" normally refers to Brassica juncea var. crispifolia. When Brassica juncea seeds are ground, they produce the famous Dijon mustard. Mustard greens are strong flavored and pungent although the inner leaves are relatively mild and quite suitable for raw salad use. It is an annual cool-season plant with its early growth in a basal rosette. Leaf form can vary among cultivars. Some cultivars have large leaves while others have leaves that are broad toward the apex. Within both forms are cultivars with curled or smooth leaf margins. The young tender leaves are harvested approximately seven weeks after sowing when they reach six to eight inches in height and before they become tough and woody. Plants are cut by hand, washed, and packed. They are packed for transit in the same way as spinach.

Minor Leaf Vegetables

Several leaf vegetables are grown to a very limited extent on commercial acreage to meet a small but steady demand. One may find these leaf vegetables growing more frequently in home gardens. Some of these crops are normally thought of as weeds; however, they are grown because they feature some prominent characteristic that is favored among specific ethnic groups. These crops include broccoli raab, chaya, dandelion, garland chrysanthemum, garden sorrel, ice plant, kangkong, Malabar spinach, New Zealand spinach, orach, pokeweed, and purslane.

Broccoli raab (Brassica campestris ) is also known by such names as raab, rapa, rapini, broccoli turnip, spring broccoli, cima di rapa, taitcat, Italian turnip, and Italian mustard. It is a highly regarded leaf green in Italy and other Mediterranean countries. The plant resembles turnip tops and sprouting broccoli but develops a much smaller, less compact inflorescence. The leaves are cut with the seed stalks before the flower buds open. There are two forms of broccoli raab: rapine, or spring raab, and rappone, or fall raab. Other than the season of maturity, there is no difference in appearance or flavor. Both go to seed very rapidly. In areas with a moderate climate, raab may be planted in the fall and overwintered to produce an early spring crop. Rappone seems superior to rapine for these fall plantings. In most areas, both are spring planted for early or late summer harvest. The harvest system is similar to that used for collards and turnips; the leaves and the flower stalks are tied together and sold as bunches. Raab is very perishable and must be marketed immediately.

Chaya (Cnidoscolus chayamansa ) is a little-known leaf vegetable of dry regions of the tropics. The leaves are used as a green vegetable in many Latin American countries. The name "chaya" comes from the Mayan word for the plant, ixchay or chay. Other common names are tree spinach, chaya col, kikilchay, and chaykeken. Chaya is a large leafy shrub reaching a height of about six to eight feet. It somewhat resembles a vigorous hibiscus or cassava plant. The dark green leaves resemble okra leaves. The domesticated cultivars have little to none of the offending featuresstem spines and leaf hairsfound in wild chaya. It is reported that "pig chaya" is one of the very best eating varieties. Plants are continuously harvested. Large leaves are cut into manageable pieces before cooking. Chaya is a good source of protein, vitamins, calcium, and iron. However, raw chaya leaves are highly toxic because they contain hydrocyanic glucosides. One minute of boiling destroys most of the glucosides.

Dandelion (Taraxacum officinale ) has been encountered by almost everyone as a weed in lawns and gardens. However, there are cultivated varieties of dandelion that make excellent cooking greens. The dandelion is a European native with low-spreading deeply notched leaves forming a rosette pattern as they emerge from a central tap root. The varieties used as a leaf vegetable have been selected for their leafiness and freedom from bitterness. The three major cultivars are Thick Leaf, Improved Thick Leaf, and Arlington Thick Leaf. The leaves are an excellent source of provitamin A, vitamin C, calcium, and several other minerals. Given that the plant is a perennial, the leaves are harvested by cutting below the whorl to keep the plant intact. The leaves are then washed, graded, and cooled.

Garland Chrysanthemum (Chrysanthemum coronarium ) is also called edible chrysanthemum, chop suey greens, shungiku in Japan, and tong hao cai in China. It looks very much like a leaf version of the flowering ornamental chrysanthemum. A native of the Mediterranean region, it was introduced to Asia via contact with European traders. It is now a popular cooked green in Korea, Japan, and China. Leaf shape varies from lobed to highly indented. Daisy-like flower heads are yellow or yellowish white and are also eaten. All plant parts have aromatic flavor qualities, becoming most pronounced in older foliage.

Garden sorrel (Rumex acetosa ) is a perennial plant that is closely related to rhubarb and buckwheat and is sometimes referred to as dock. However, the term "dock" has been used in Great Britain to include all members of the family Polygonaceae. Owing to its tart flavor, it is sometimes called sour dock and sour grass. In fact, sorrel derives from the Old French surele, meaning 'sour'. Garden sorrel is of Eurasian origin with long, thin light green or reddish green, slightly crinkled, arrow-shaped leaves. Other Rumex species similar to garden sorrel are French sorrel (R. scutatus ), spinach rhubarb (R. abyssinicus ), patience dock (R. patientia ), and Indian sorrel (R. vescarius ). French sorrel differs in being a short plant with branched stems that exhibit a semireclining growth habit. Leaves are arrow-or fiddle-shaped, more succulent, and smaller than garden sorrel. French sorrel is used like garden sorrel but has a milder taste. Spinach rhubarb is eaten like spinach, and the petioles are like rhubarb. Patience dock looks similar to garden sorrel although the plant is stouter and taller and has larger leaves and a noticeably stronger taproot than garden sorrel.

Ice plant (Mesembryanthemum crystallinum ) is a little-known vegetable of the southern hemisphere. Ice plant is so named because of the shimmering silvery dots that cover the leaves. It has also been called fig marigold, frost plant, diamond plant, midday flower, and dew plant. This is not to be confused with New Zealand spinach, which is sometimes referred to in gardening books as New Zealand ice plants. Ice plant is a perennial that does best in hot, dry climates. It is grown as an annual when used as a green vegetable. The leaves are picked as wanted once the plant has several leaves and is well-established. The slightly acidic, fleshy leaves are boiled and served like spinach.

Malabar spinach (Basella alba & B. rubra ) is also known as Ceylon spinach, climbing spinach, gui, acelga trepadora, bretana, libato, vine spinach, Indian spinach, and Malabar nightshade. The red leaf form belongs to the rubra species, while the green form is classified in the alba species. Basella alba has an African or Southeast Asian origin, while Basella rubra is thought to have originated in India or Indonesia. Malabar is not a true spinach, but its leaves, which form on a vine, resemble spinach and are used in the same way. Malabar spinach can be grown from seeds or cuttings. The vine is normally trellised. Two vines are sufficient to supply a small family all summer and fall. The thick, fleshy leaves are cut off together with some length of the stem to keep the plant pruned to a desired shape. When cooked, Malabar spinach is not as slick in texture as many greens such as spinach. The Bengalis cook it with chopped onions, spicy chilis, and a little mustard oil. The mucilaginous texture is especially useful as a thickener in soups and stews.

Mizuna (Brassica juncea var. japonica ) is an Oriental cooking green also known as potherb mustard, kyona, Japanese greens, and sometimes California peppergrass. It is widely grown in Japan but is found only occasionally in gardens in the United States. Mizuna is twelve to eighteen inches tall with yellow-green leaves that are smooth and a bit fuzzy, similar to curly mustard, but with a different leaf shape. Leaves of mizuna are deeply notched, narrow, feathery, and quite attractive. A single plant may have as many as 180 leaves clustered together in a compact, twelve-inch diameter bunch. It withstands frost and light freezes and is not quick to seed even in periods of warm weather that occur during the winter months. Leaves are ready for use any time after three weeks of growth. Leaves are removed as needed, keeping enough young foliage to continue the regrowth.

New Zealand spinach (Tetragonia expansa ) is indigenous to New Zealand and became widely cultivated after it was introduced to Europe. It was introduced to England by Captain Cook in 1771 and was used on his voyages as a source of vitamin C. Presently, little is grown commercially in the United States, but it is popular with many home gardeners. Not a true spinach, it does somewhat resemble spinach in appearance and is used similarly. The plant is large, growing to a height of two or more feet in a spreading and branching habit of growth, and has thick succulent leaves. The young tops are harvested for boiling, and each harvest encourages new branching. Unlike many of the leaf vegetables, New Zealand spinach is a warm-season crop with very wide adaptation. It is an excellent source of fresh greens throughout the summer and is also frost-sensitive. Its flavor is comparable to that of spinach, but milder and without the astringency. In its early growth, New Zealand spinach is entirely vegetative. As it begins to develop, however, it soon produces flowers from the leaf axils. The flowers are considered undesirable for the market. Like spinach, tissues contain oxalates that render calcium nutritionally unavailable.

Orach (Atriplex hortensis ) is a hardy branching monoecious annual of the Chenopodiaceae family that is a substitute for spinach. It is also commonly known as mountain spinach, French spinach, and sea purslane. Some variations of the name are orache, arache, and orage. The name derives from the Old French arrache, a corruption of the Vulgar Latin atripica, from Latin atriplex ; these in turn were from the Greek word for orach, atraphaxus. It is sometimes called salt bush because of its tolerance of alkaline soils. The plants have a tolerance to drought and salinity and are adapted to a broad temperature range. Orach is considered to have originated in northern India and has been used as a medicinal and food plant for more than 2,000 years, making it one of the oldest cultivated plants. It was widely grown until the eighteenth century but is of little commercial importance today although it is returning in popularity as an ingredient in mesclun salads. It is grown as a substitute for spinach in Europe and in the northern plains of the United States. It is seldom seen in the tropics. Its leaves are slightly crimped, soft, and pliable and are shaped like arrows that are four to five inches long and two to three inches wide. Plants can attain a height of five to six feet. A rosette of leaves first develops, followed by a seed stalk that can grow to a height of six to nine feet. There are four common varieties of orach. White orach is most often grown because it is the most tender and best flavored. The leaves are very pale green, almost yellow. Red orach has dark-red stems and leaves. Green orach, also called Lee's Giant orach, is very vigorous, with a stout, angular, branching stem. The leaves are rounder, less toothed, and darker green than those of the other varieties. The fourth is a copper-colored variety that is now much sought after by specialty growers.

Orach is a cool-season vegetable and is grown much like garden spinach. It is quick to bolt in summer. Although stems quickly elongate, flowering is slow, and plants tolerate growing temperatures too high for spinach. Young leaves may be harvested and the plants will continue growing for multiple harvests. Orach has a mild flavor much like that of spinach, but it contains less oxalic acid. Even when the plant goes to seed, young leaves are usable. However, old leaves are not palatable and are not harvested.

Pokeweed (Phytolacca americana ) is a native plant throughout eastern North America. Other common names are inkberry, pigeon berry, coakun, pocan bush, scoke, garget, and poke salad. The branches bear clusters of flowers and dark red fruits that resemble the berries of a nightshade; pokeweed is therefore sometimes called American nightshade. It is a large-rooted perennial with a strong-growing tip, reaching up to ten or more feet in height. The top dies down in cold weather. There is little cultivation of pokeweed in the United States or elsewhere because it is gathered from the wild. All plant parts are poisonous. The young tender shoots and the older leaves may be eaten if boiled. The bitterness, and by association the poisonous compound, is removed by boiling and pouring off the cooking water until all the bitterness is removed.

Purslane (Portulaca oleracea ) is known by various names such as kitchen purslane, garden purslane, and in Spanish, verdolaga. One of the more descriptive names for this plant is in Malawi, where it translates to "the buttocks of the wife of a chief," because of the shape of the leaf. The exact origin of purslane is not known, but it is reported to have been used more than 2,000 years ago in Iran or India. Purslane is a popular vegetable in France, several other European countries, and Africa, especially in Egypt and Sudan. It was introduced to the United States from Europe. The name "purslane" derives through Old French porcelaine from Pliny's Latin porcilaca. The cultivated forms are upright and more vigorous than the weedy form. It is a summer annual with small, oval, juicy leaves clustered at the ends of smooth, purplish-red, prostrate stems that arise from a single taproot. The leaves are usually stripped from the stems and are prepared like spinach. The taste is a cross between watercress and spinach. An undesirable quality of purslane is that its foliage, like that of spinach, contains oxalic acid and tends to accumulate nitrates.

Water spinach (Ipomoea aquatica ), also known as kangkong, water glorybind, water spinach, water convolvulus, and swamp cabbage, is an important green leaf vegetable in Southeast Asia, Taiwan, Ceylon, and Malaysia. It is speculated to have originated in India but is now widely grown throughout the tropics. Water spinach can become an undesirable weed. The Florida Department of Natural Resources must issue a special permit to anyone wanting to grow it in Florida. There are two major forms (cultivars) that are cultivated in two ways, either upland (dry) or swamp (wet). Ching Quat, an upland variety, has narrow leaves, while Pak Quat, a swamp variety, has arrowhead-shaped leaves. The plants produce a trailing hollow vine that is adapted to floating in aquatic environments. The leaves are light green and look somewhat like sweet potato leaves. The upland types are started from seed or cuttings and are grown on trellises. Plants are often grown in nursery beds for transplanting later to the garden. Taking cuttings from plants in the nursery beds is the usual method. Harvest may start six weeks after planting. The swamp types are usually planted with twelve-inch-long cuttings planted in mud and kept moist. As the vines grow, the area is flooded to a depth of six inches, and a continuous flow of water is maintained through the field, similar to the way watercress is grown. Harvest begins four weeks after planting. When the succulent tips of the vines are removed, lateral and upright branches are encouraged. These branches are harvested every seven to ten days. All parts of the young plants are eaten. The crop is fragile and requires rapid and careful handling to minimize damage and wilting. It is eaten like cooked spinach. A canned product is often available in ethnic markets.

Conclusions

Leafy vegetables are consumed in most cultures and regions of the world. They consist of a wide range of different plants, yet no matter which leaf vegetable is used, it is usually prepared like spinach. The leafy vegetables contribute significantly to a nutritious diet. As a food source, the leafy vegetables are some of the best sources of provitamin A and vitamin C and supply good amounts of iron, folate, and other essential minerals. They are also an excellent source of phytochemicals, which aid in fighting heart disease and cancer.

See also Cabbage and Crucifer Plants; Herbs and Spices; Lettuce; Vegetables.

BIBLIOGRAPHY

Bose, T. K., and M. G. Som, eds. Vegetable Crops of India. Calcutta: Naya Prokash, 1986.

Chan, Harvey T., Jr., ed. Handbook of Tropical Foods. New York: M. Dekker, 1983.

Daloz, C. R., and H. M. Munger. "Amaranth: An Unexploited Vegetable Crop." HortScience 15 (1980): 383.

Duke, James A. Handbook of Phytochemical Constituents of GRAS Herbs and Other Economic Plants. Boca Raton, Fla.: CRC Press, 1992.

Herklots, Geoffrey Alton Craig. Vegetables in South-East Asia. London: Allen & Unwin, 1972.

Larkcom, Joy. Oriental Vegetables: The Complete Guide for Garden and Kitchen. Tokyo and New York: Kodansha International, 1991.

Maynard, Donald N., and George J. Hochmuth. Knott's Handbook for Vegetable Growers. 4th ed. New York: Wiley, 1997.

National Academy of Sciences. Underexploited Tropical Plants with Promising Economic Value. Washington, D.C.: National Academy of Sciences, 1975.

Rubatzky, Vincent E., and Mas Yamaguchi. World Vegetables: Principles, Production, and Nutritive Values. 2nd ed. New York: Chapman & Hall, 1997.

Ryder, Edward J. Leafy Salad Vegetables. Westport, Conn.: AVI, 1979.

Stephens, James M. Manual of Minor Vegetables. Gainesville, Fla.: University of Florida Press, 1988.

Tindall, H. D. Vegetables in the Tropics. Westport, Conn.: AVI, 1983.

Weaver, William Woys. Heirloom Vegetable Gardening: A Master Gardener's Guide to Planting, Seed Saving, and Cultural History. New York: Henry Holt, 1997.

Yeager, Selene. New Foods for Healing. Emmaus, Pa.: Rodale Press, 1998.

Paul W. Bosland


Spinach

Apparently the time of the introduction of spinach into China is well-recorded in Tang Dynasty history (618907 C.E.) because it marked the flow of many new food offerings of grain and seed from Tibet. Also, the trade routes to the West went through Tibet, and presumably spinach may have been traded from its origins in Persia. One story is that it was included as a part of the bridal offerings that were carried into China with the marriage of Princess Wencheng to the Tang Emperor Taizong sometime before 641 C.E. This marriage was well recorded in both Han and Tibetan writings and folklore. It is also known that during this same period many other food products flowed (such as pepper and cardamom) into China. This trade resulted from the extensive connections Emperor Taizong had established with the western Asian region. While it is possible that spinach came into China through the marriage, it was also likely that it could have come anytime during the later reign of this emperor after establishing all of these connections. It is also relevant to note that this period (618641 C.E.) also marks the time when both tea and porcelain were first extensively traded with the west.

Solomon H. Katz



Amaranth

Amaranth is one of the most ancient crops of the Aztecs of Mexico, who domesticated it about 5000 B.C.E.; it appears to have been independently domesticated also among the Inca of Peru. Among the Aztecs amaranth was cultivated first in the chinampa system; fertile algae-rich mud from their garden canals was made into seed beds, and the amaranth was allowed to grow to about eight to ten inches in height and was then transplanted to fields in the higher ground, where it was usually intercropped with maize. After the Spanish conquest, the continued use of amaranth was prohibited because the red color of the seeds reminded the Spanish of blood and its connection to traditional Aztec religious practices.

Solomon H. Katz



Purslane

Purslane has very high levels of the linoleic omega 3 fatty acid, which is essential and relatively low in the U.S. diet as compared with the higher levels of the omega 6 fatty acids. This imbalance of the ratio between the two classes of fatty acids may be the basis of an important nutritional imbalance.

Solomon H. Katz


Leaf Vegetables

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