Organic vegetables are seen on growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018. REUTERS/Loriene Perera

Rows of organic lettuces sprout from growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

Organic red spinach seedlings sprout from growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

Organic cilantro seedlings sprout from growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

An aerial view shows Citiponics' urban farm located on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

Organic lettuces are seen on rows of growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

Citiponics Chairman Teo Kwa Kok tends to a lettuce seedling on a growing tower that is primarily made out of polyvinyl chloride (PVC) pipes at an urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

Citiponics Chairman Teo Hwa Kok demonstrates how a lettuce seedling can easily be transported between growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at an urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

An apartment building is seen amid organic lettuces springing out of growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.

Organic lettuces sprout from growing towers that are primarily made out of polyvinyl chloride (PVC) pipes at Citiponics' urban farm on the rooftop of a multi-storey carpark in a public housing estate in western Singapore April 17, 2018.