Santander was forced to justify itself after Dave Williams made news headlines for being refused an £83,000 loan following a survey.
Williams said officials told him that knotweed posed a risk to the structure of the building and could make it hard to sell the home or even unsaleable.
"It's crazy," he was reported as saying. "We should use some common sense. It is an intensive weed but all I've got is half-a-dozen sticks barely two-foot high.
"If I had a survey done in the winter I would have been OK because it only comes up in the spring and summer," added Williams, based in St Austell, Cornwall.
A Santander spokeswoman said: "The refusal of mortgage applicants if knotweed is deemed a threat to the property is standard across the industry and not only applicable to Santander."
However, a representative from the RHS was quoted as saying that knotweed is a nuisance, not a problem. "Japanese Knotweed might be a problem on land being developed, but in an ordinary domestic residence it is more of a nuisance than a real problem," they confirmed.