The most common method of treatment is a herbicide treatment. Which, is also, typically the most cost effective for our clients. The best time to spay the Japanese knotweed is when the plant is actively growing. The herbicide we use is a systemic translocating herbicide which is most efficient on a warm, sunny day during the growing season. We use two different chemicals to treat Japanese knotweed, one is significantly stronger than what is available to the general public.