Knotweed stems emerge in late-March to mid-April, depending on soil temperatures, and begin a burst of rapid growth. In a warm spring, knotweed can be 6 feet tall before May 1. Flowering usually occurs in July, and the seeds mature in August and September.

What is the growing season for Japanese Knotweed?

When is it Japanese Knotweed growing season? Usually the plant starts growing in early spring and by June it can reach 3 metres in height. It will carry on growing until autumn before dying off in the winter. The growing season can be extended with a mild winter and a warm, damp summer.

How does Japanese Knotweed start?

Instead, Japanese Knotweed typically spreads through deliberate or unintentional movements of the plants chopped stems or fragments of rhizomes (roots). Even the smallest part of the rhizomes or cut stem (a finger nail size) can start a new growth of Japanese Knotweed.