More than 20 plants were spotted in six planters by local shop owner Dean Beddis and friend Steve Reynolds who informed a journalist friend. By the time police officers got to the Newport street the plants had disappeared.
Beddis, who runs Kriminal Records in the town told Wales Online: "I had never seen cannabis growing in the wild before so it was crazy to see it.
"We thought Newport was trying to be a new Amsterdam and doing its best to win City in Bloom."
The pots were the responsibility of Newport City Council’s internal gardeners.
A Newport City Council spokeswoman said the council did not have any knowledge of cannabis plants in any of its planters and launched an investigation after being contacted by a journalist.
"The council made an inspection of all its planters on 1 August and did not find any evidence of cannabis plants. The planters also did not look as though they had been disturbed," she said.
"The photographs provided by the report appear to show mature plants added to a council planter.
Although this could be a hoax, it is a serious issue and Newport City Council will be informing the police, and checking its CCTV cameras for additional evidence. If anyone has any information about this the council hopes they would contact the police."
The spokeswoman added that there had been no previous reports of cannabis growing in council planters.