Richard Pember has lived in Hopkinstown for 11 years, but he now worries it could be worth a fraction of the price after the plant grew more than 20 feet since he moved in.
TfW owns the nearby land where the weed has spread from and did not do enough to stop the spread into Richards's garden.
Fallopia Japonica, better known as Japanese knotweed, is extremely prolific and can grow up to 20cm in just one day.
The plant is non-native and invasive, meaning that it completely takes over.
Richard first noticed the growth in 2016. He said: “It started off so small that I didn’t think anything of it.
"Then it just grew from nowhere to a point where it was almost coming inside the property.
"It has massively devalued the house in my opinion because nobody wants to move into a property that has a Japanese knotweed infestation. It’s like living with a monster that always comes back", he added.
Earlier this year, Pontypridd County Court heard how TfW did not do enough to prevent the spread of the weed into Mr Pember's property.
He said dealing with the infestation was like dealing with a "never-ending monster."
The rail authority insisted that it has "categorically not failed" in treating the Japanese knotweed on its land and has a "thorough and effective" plan across its core Valleys lines network.
However, on 25 May the judge granted damages to be paid to Richard by Transport for Wales.
In a statement TfW said "[it] is naturally disappointed with this court result given the facts as presented to the court.
"TfW has categorically not failed in treating the Japanese knotweed on its land and has a thorough and effective treatment programme in place across the whole of the core Valley lines network where this property neighbours.
“In this instance the court found for the claimant on the tenuous basis of a single missing documentary record.
"Neither TfW nor Network Rail (from whom Transport for Wales purchased the core Valley lines on 28 March 2020) accept that a treatment itself was missed.
"An appeal was considered but on this occasion was not pursued.”
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