The highly-invasive plant has been identified by Watford Borough Council, which is now seeking contractors to bring the species under control.
The weed was found in Riverside Park, Oxhey Park, Laurence Haines School, Waterfields Rec, Radlett Road Playing Fields, Knutsford Playing Fields, Munden Drive, Cassiobury Park local nature reserve, Cardiff Road, Aldenham Road and Watford General Hospital.
The authority said the areas affected will be fenced off so treatment work can begin in Spring of next year.
The weed is a tall fast-growing Japanese plant of the dock family, with bamboo-like stems and small white flowers, grown as an ornamental but tends to become aggressive and invasive spreading quickly.
The invasive root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites.
The sites are a mixture of council-owned and private premises.
The council will be treating the weed by herbicide application, consisting of several visits through the growing season for two to three seasons.
Elected Mayor, Dorothy Thornhill, said: "As all gardeners will recognise, Japanese knotweed is horrible stuff. This treatment programme will ensure we are as weed-free as we can be for the next 10-15 years."