Fallopia baldschuanica commonly known as Russian vine (or Bukhara fleeceflower)
Russian vine is very similar to Japanese knotweed with it being in the same Fallopia family of plants. It flowers in August and September and these flowers are also small and white, which is like Japanese Knotweed.
It is also fast growing and will choke anything that is growing in its path, meaning it will overtake your garden.
However, the leaves of the Russian Vine are pointed nearly triangular, which can grow up to lengths of about 10 centimetres.
To learn more about commonly misidentified plants, which are mistaken for Japanese Knotweed click here.