The most common and preferred treatment method is a herbicide treatment. The best time to spay the Japanese knotweed is when the plant is actively growing. This is because the herbicide we use is a systemic translocating herbicide. The herbicide treatment is most efficient on a warm, sunny day. We use various chemicals to treat Japanese knotweed. However, one is significantly stronger than what is available to the general public and one is not available to the public in any form.