Coffee plantation owners in Vietnam, the world's top producer of robusta beans used in blends and instant coffee, are devoting more land to pepper cultivation with prices of the spice rising eight times faster than those of coffee.

Pepper prices in Vietnam, already the world's largest producer and exporter of black pepper, have climbed 16 percent to 184,000 dong ($8.45) per kilogram since the start of 2014 as global output lagged demand, outpacing the 2 percent gain in coffee prices during the same period.

Pepper will strike at the heart of Vietnam's coffee cultivation zone in the Central Highlandsas pepper vines thrive in the same soil conditions conducive to the growing of coffee trees.

Both crops equally take three years to become productive.

Pepper vines planted last year will bear fruit in 2017.

"Many coffee farms have started growing pepper, and it is obvious from distance, because of the high pole that supports the vines," said a coffee trader at a foreign firm in Ho Chi Minh City.

Pepper cultivation in Daklak, the country's largest coffee-growing province, has doubled to 16,000 hectares (39,500 acres) since 2012.

That exceeded the province's plan to plant 15,000 hectares of pepper vines by 2020, and accounted for 20 percent of the country's total pepper cultivation area last year.

Vietnam exports black pepper to the United States, Singapore as well as India, another major producer of the spice.

As the pepper fever catches on, coffee growing in Gia Lai, a major producing province, has fallen to 81,400 hectares this year from 83,200 hectares in 2014.

Vietnam's overall coffee output will total 28.67 million bags in the next 2015/2016 crop year, up 1.8 percent from the previous season but below the record 29.83 million bags in the 2013/2014 season, the U.S. attache said in a report in May.