The Morning Glory family (Convolvulaceae) is found worldwide in all
climates but the culinary ones all belong to the genus Ipomoea which
inhabits only tropical and subtropical regions. A few of the other genera
produce seeds which are sometimes ingested but not for the purpose of
Sweet Potatoes & U.S. "Yams"
Sweet potato vines are members of the Morning Glory family native to
Central America and the Caribbean. They are unrelated to true potatoes which
are Nightshades. They produce an edible "storage root" which is non-toxic
and under proper "curing" conditions converts part of its starches to sugars
making it sweet.
Sweet Potato, White -
[kumara (New Zealand - Maori), C. Ipomoea batatas]
These are the standard grocery store white sweet potato. Sweet potatoes
(red or white) are not particularly sweet at harvest but are "cured" at
85°F for 4 to 7 days during which some of the starch is converted to
sugar. Pictured are 1/2 pound potatoes.
Select firm sweet potatoes with no soft spots. Do not refrigerate
Sweet Potatoes. Temperatures below 55°F will make them hard and ruin the
flavor. They should be stored loose (not in plastic) and kept away from
sunlight. Stored at 55°F to 60°F and 90% humidity they'll keep for
several months but in a normal household environment they should be used
within a couple of weeks.
U.S. "Yam" -
[Red Sweet Potato, C. Ipomoea batatas]
This is not actually a yam but a sweet potato variety with red skin and
orange flesh. The term "Yam" was adopted from the African "nyami", the name
used for sweet potatoes by Southern slaves. This was to differentiate it from
the white sweet potato in commerce. Unlike True Yams they
have smooth skin as other sweet potatoes do.
Pictured are a whole 2 pound and a cut piece from a 1/2 pound sweet
potato. This "Yam" has orange flesh that is moister and sweeter than that
of the White Sweet Potato but all other characteristics
are pretty much the same.
Water Spinach - [Swamp Cabbage;
Ong Choy - variously spelled (Cantonese); Pak Hung, Pak Bung (Thai);
Kang Kong (Malay, Filipino); Kang Kung, Rau Muong (Vietnam); Toongsin Tsai
(Mandarin); Chinese Watercress, Water Convolvulus, Water Morning-glory;
Ipomoea aquatica also Ipomoea raptans (not common)]
This semi-aquatic plant is a controlled substance within the USA.
Importing plants or seeds and/or growing plants without a permit are all
illegal - see USDA
Plant Profile. Growing it or even transporting it is totally illegal
in some states.
Here in California it's a quarantined crop but may be transported
without a permit within the state. Enough growers have permits it's in good
supply, and that's a good thing because if it wasn't our
Asian population might try sneaking it into our waterways like they
have with snakehead fish.
There are several varieties of this vegetable, including a long leafed
variety that can be grown in damp soil (Ching Quat) and a wider leafed
variety that requires free water (Pak Quat). The wider leaf variety is
preferred in Asia but is rarely available here, probably to keep the
growing areas farther from our waterways. The photo specimens are about
19 inches long.
Details and Cooking.
These tropical vines, native to Africa and Asia, are quite unrelated to
the Sweet Potato vines and are in the same class as lilies. They produce root
tubers which are generally toxic, some less so, some more so, but it's best to
avoid eating any of them raw. Some require tedious pounding and leaching to
make them edible but these are not sold as vegetables in the USA. The
varieties sold here just require a little cooking for detox.
Yam (True) - [boniato (Spanish),
nyami (African), D. Dioscorea batatas and similar]
Yams are grown in Africa, the Caribbean and Latin America, but not
in the U.S. except a few in Florida. Unlike sweet potatoes they have a
rough scaly skin, and can grow to as much as 150 pounds, though they
are generally marketed at only a few pounds.
New Zealand Yam - [oca, oka (Andean),
Neither a yam nor a sweet potato but a completely unrelated tuber from
the Andean area of South America.
Nagaimo - [Nagaimo, Yamaimo
(Japan); Glutinous Yam, Cinnamon vine, Japanese Mountain Yam, Chinese
Yam; Ma (Korea); Shan yao, Huai shan, Huai shan yao (China);
Native to Southern China this yam has been cultivated in Japan from very
early times. Known in the U.S. mainly as an invasive weed, this vine with
arrowhead shaped leaves grows large tubers below ground and small tubers
above ground. It spreads mainly by dropping these small "air potatoes"
and rarely flowers.
Nagaimo is roughly cylindrical and may be two or three feet
long, but the photo specimen was 24 inches long, 2-7/8 inches diameter and
weighed 3-1/4 pounds. The skin is tan and the flesh is white and, unlike
other yams, sticky and more than a bit slimy. The two Japanese names,
nagaimo and yamaimo (shorter, thicker), are applied depending on the
root shape. Details and Cooking.
Violet Yam - [Purple Yam; Ube,
Halaya (Fillipino); Ratalu (India); Uhi (Hawaii); Dioscorea alata]
This violet fleshed yam, now easily available in Southern California in
Asian markets, is native to East Asia. It is very sweet and used mainly for
deserts, particularly in the Philippines. In India it is also used for
deserts but also appears in mixed vegetable dishes. Cooked, its purple color
darkens and intensifies.
Yampi - [Cushcush, Indian yam,
napi; Yampi, Yampie (Jamaica); Maona (Peru); Mapuey (Puerto Rico); Aja (Cuba);
Cara doce (Brazil). Dioscorea trifida]
Native to the Caribbean and tropical Central America this yam is grown
for its starchy roots and used similarly to
Cassava. It contains the bitter
toxic alkaloid discorene which is eliminated by cooking. The cut
ends of the photo specimens were dipped in wax to prevent drying out and
shriveling. The largest of the photo specimens was 11 inches long, 2-1/2
inches diameter and weighed 1 pound.
Raw yampi are very mucilaginous but this goes away when cooked. Cooked
the texture and flavor are pleasant and much like a waxy potato but more
tender and crumbly and noticeably sweeter. While yampi is a very fine
edible, it is overshadowed in its growing region by manioc (cassava) which
produces a higher yield and is more durable in storage.