Tuesday | October 17, 2017
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When is a yam a sweet potato?

Editor’s note: This is a reprint.

What is the difference between yams and sweet potatoes? Are they the same by a different name?

Many years ago orange colored sweet potatoes were introduced to the Southern U.S.. In an effort to distinguish these from the traditional white-fleshed potato, producers called them yams, which is the Anglicize African word, nyami.

Today, most of the starchy tubers consumed in the US and labeled as yams are in reality sweet potatoes. Yet yams and sweet potatoes are not the same; in fact, they are quite different from each other.

As far as botanical order is concerned, they are at opposite ends. The sweet potato is a dicot, set in the morning glory family. Yams, on the other hand, are a monocot, closely related to grasses. The sweet potato, whose sweet and moist flesh varies in color from white to yellow and orange, is native to South America; the skin is typically smooth. Garnet, Jewel, and Beauregard are orange fleshed sweet potatoes that often masquerade as yams in the local supermarkets.

In contrast, yams are dry and starchy and rather bland. While yellow or purple in color, the skin is rough and a bit shaggy. Yams are native to North Africa and Asia.

They range in size from that of a small potato up to 150 lbs. Yams are a primary agricultural crop in West Africa, where 95 percent of the world’s yam crop is grown. Incidentally, both yams and sweet potatoes can be purple.


Dear Dr. Sakovich: Thank you for sharing your knowledge with us to help make Big Island a better place to live. My question is about Ironwood trees that appear to be in distress. Growing quite extensively around the base of several Iron trees is carpet of greenish, yellowish brown moss. The trees are de-foliating and have substantial die back. The Norfolk pines growing nearby, also with this moss carpet seem to be doing fine. Also there are some coco palms which appear not to be affected by this same moss carpet around their bases.

Across the yard, I have a dead ohia tree which has the same moss carpet at its base.

Is there any relationship with the trees and this moss carpet? Could the moss be presenting a PH problem? If so, would removing the moss help the ironwoods? Would a fertilizer improve the health of the Ironwoods? Any ideas would be appreciated.

Again, thanks for sharing your wisdom with Big Island gardeners and small farmers. Kind Regards, M.W.

Yes, there is a relationship between the vigorous growing moss, and lichens too, and the decline of the ironwoods, but not the way most people think. The trees are growing poorly not because of the moss or lichens, rather just the opposite.

Trees decline first, shedding leaves and thus allowing more light to penetrate to the bark of the trunk and limbs. This, in turn, will cause a new growth spurt in the moss or lichens.

The moss growth is not the problem. Yet, because ironwood trees are a rather hearty lot, the decline is quite puzzling. They grow in a wide range of soils, including poor ones. They are tolerant to drought, wind and salt spray as well as some waterlogging: all in all, a hardy tree even though intolerant to shade.

One clue to the decline comes from Guam. On that island, research has shown a higher incidence of decline in ironwoods in disturbed locations such as golf courses, schools and housing subdivisions, than in natural, undisturbed areas.

Ironwoods do well in poor soils because they have the ability to fix nitrogen. If the soil is extremely low in phosphate or potassium, adding those nutrients may help the trees. The bottom line is don’t waste your time and money removing the moss.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at askthegardenguy@earthlink.net.


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