Visitors are being asked to check with staff before bringing flowers onto wards in some south Wales hospitals, to help prevent the MRSA infection.
Some wards in the Royal Gwent Hospital are affected
The move by the Gwent NHS Trust has been applied across all of its hospitals to curb the spread of infections to patients.
Hospital managers said they are not prepared to compromise on infection control.
It follows similar policies in some hospitals in England.
Julian Hayman, from the Gwent NHS Trust, said: "We do not allow flowers in high risk infection areas such as intensive care, high dependency, haematology and some surgical wards as they pose an infection risk.
"Control of infection is a priority that we cannot compromise on.
"In other areas the decision whether or not to allow flowers on a ward is made by the nurse in charge of that ward.
"Besides infection risks there are other reasons that wards may decide not to allow flowers such as insufficient space, health and safety issues such as water in close proximity to electrical equipment and the need to ensure water is changed regularly to prevent it from becoming stagnant."
He said it was well documented that flowers can increase the risk of passing on infections and the decision had been taken to aid patient recovery.
"Although we'd like to be able to allow flowers on every ward in most cases it really is not practical," he said.
"We would ask patients and relatives to help us by checking with the nurse in charge before bringing flowers onto any ward."
There is a fear of potential infection risk from flowers
A ban on bringing flowers into intensive care and other high dependency wards is a common rule in many hospitals around the country.
As well as MRSA, there are fears that vulnerable patients could be affected by the pseudomonas bacterium which develops when vegetation rots.
A spokeswoman for the Welsh Assembly Government said there was no guidance about bringing flowers onto hospital wards.
"The assembly government has not issued central guidance relating to flowers on wards, the recognised issues concerning flowers on wards and control of infection.
"It is the responsibility of trusts to investigate local guidance and practices taking into account the specific circumstances within the trust, hospital or ward," she said.