Japanese knotweed is considered to be a very invasive species of plant.
Once this plant is established, it is very difficult to get rid of. The strong growth of the Japanese knotweed allows it to displace many native plant species while its invasive root system and strong stems can cause damage to buildings, pipes and roads. its drive to expand, Japanese knotweed will exploit existing weaknesses in property underground services, walls, concrete, foundations, fences, out-buildings and conservatories.
As it grows and expands, it will crack walls, ruin garden fences and it can even break through into buildings. It is also difficult to eradicate so if the property has it, I would leave it alone.
However, the existence of Japanese Knotweed does not necessarily mean that you shouldn't buy the property. If there is an approved knotweed management and treatment plan in place, then there should not be any problem in getting a mortgage.