CARHUAMAYO, Peru—Natural-products companies based in the U.S., Europe and elsewhere have steadily built a market for maca, a small turnip-like root that grows in high mountain areas, and that is believed to give a burst of energy, especially as a sort of natural Viagra.

This year a flood of buyers from China swooped into the Junin region of central Peru to buy up as much of the root as possible. That led to a tenfold increase in the price of maca, and in some cases even more, growers say.

Peruvian exporters say the frenzy to find maca has led to broken long-term supply contracts. Global natural-products companies say they are in danger of being pushed out of the market. Police say the aggressive demand has led to sometimes violent thefts of sacks of maca in Peru.

Sales of semiprocessed maca, dried and ground up on small farms and processing plants in the area, have boomed. The government of Peru has sounded the alarm bell that raw maca is also being smuggled out. Officials say the Peruvian maca is used to improve lesser-quality maca grown in China.

Peruvian regulations prohibit the export of unprocessed maca. Peru’s tax agency confirms it seized tons of unprocessed maca before it was smuggled out this year. Some of that was being sent to Asia alongside other prohibited products like sea horses and shark fins, officials said.

More than a decade ago, the price for maca was so low that farmers might not even bother to harvest it. Now growers say they can sell a pound of maca root for about $13 or more, 10 times more than at the beginning of the year. Most maca is cream-colored. Growers say the coveted black maca can fetch about $45 dollars a pound.

For some companies in the U.S., there will be little or no maca available until next year’s harvest begins around midyear. Even then, local growers say that some of next year’s crop has already been bought by buyers from China.

Some maca is being grown in China, but buyers covet Peruvian maca, which has been grown for thousands of years in the virgin soil about 13,000 feet above sea level in the Junin region. Smaller amounts are grown elsewhere in Peru and in neighboring Bolivia.

In China, maca is being marketed as an alternative to ginseng, another highly prized root that has been used in traditional Chinese medicine as a health tonic and cure for male sexual dysfunction but has become increasingly scarce in the wild.

Consumers are already feeling the pinch in the U.S., where many people add maca powder to drinks or food. It is also commonly available in capsules or as a liquid extract used as an additive to tea or chocolate.

“The price of maca may have crossed a line. The extract market could collapse,” said Chris Kilham, a consultant with Naturex, a French health and nutritional ingredients provider.

Sevananda Natural Foods Market in Atlanta was out of bulk maca powder for most of September and October, said Betsy Abrams, a buyer for the food co-op. Ms. Abrams said she normally buys 25 pounds a month from distributors in the U.S. to keep up with customer demand, but she couldn’t find any recently.

“They’re real frustrated when they come in and see my jar upside down,” she said. “I say I just can’t get it because China is buying up the supply.”

Mark Arment, owner of The Maca Team, a maca dealer in Maryville, Tenn., said consumers in the U.S., Europe and Australia can expect to pay double for maca, compared with last year’s prices.

Mr. Arment, who sells up to 5,500 pounds of maca a month, said he can no longer find any black maca. Meanwhile he raised the price for sought-after red maca to $125 a pound in September, up from $84 a pound and expects to raise the price for red maca again this month to about $159 a pound.

“We’ll see what the market bears,” he said.

It is bad timing for Gaia Herbs, based in Brevard, N.C., which supplies maca products toWhole Foods Market Inc., Vitamin Shoppe Inc. and smaller retailers. It rolled out four new maca products in mid-October. Sales of two of Gaia’s maca products rose fourfold over the past year. Whole Foods and Vitamin Shoppe didn’t respond to requests for comment.

Bill Chioffi, vice president of global sourcing at Gaia, said the company had planned to buy about 120,000 pounds of maca this year from Peruvian growers, up from its initial purchase of 20,000 pounds in 2010, but says the latest prices are unsustainable.

Peruvian maca exporters are also wondering if they will be able to stay in business.

“Of all the suppliers we had long-term contracts with, none of them complied this year. The contracts are dead,” said Carlo Paulet, managing director of Natural Peru, the largest exporter of maca from Peru. He said the company expects to be able to buy about a third of the maca it wants this year.

“It is sad to see how this market that was built up over many years, and which was able to penetrate new markets, won’t be able to be sold in places like Wal-Mart or by other companies—if we don’t have the same access to supplies that we did before,” he added.

A 2010 literature review noted that maca was shown to enhance sexual function in some animal studies. In human trials, one study found that maca had no effect on male hormone levels, while another study found that it increased male sexual arousal.

Mark Blumenthal, executive director of the American Botanical Council, a nonprofit research organization, said there is a presumption that maca is safe to consume because it is a common food in the Andes.

In Carhuamayo, where generations of people have eaten the root, houses sport handwritten signs advertising maca, as there is no centralized market. With no banks in town, sales are conducted with cash.

Traditionally the maca grown in Junin is organic because insecticides aren’t used in the cold climate and chemical fertilizers are said to produce deformed roots—but that may change.

“What many people are looking for now is quantity, volume and not quality. They are using synthetic fertilizers,” said Dora Huaman Azapano, a grower in Junin.

Write to Kris Maher at kris.maher@wsj.com and Robert Kozak at robert.kozak@wsj.com