[6] The flowers are small, cream or white, produced in erect racemes 6–15 cm (2+1⁄2–6 in) long in late summer and early autumn.

While stems may reach a maximum height of 3–4 m (10–13 ft) each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down.

Dogwood, lilac, houttuynia (Houttuynia cordata), ornamental bistorts such as red bistort (Persicaria amplexicaulis), lesser knotweed (Koenigia campanulata), Himalayan balsam (Impatiens glandulifera), broad-leaved dock (Rumex obtusifolius), bindweed (Convolvulus arvensis), bamboo, Himalayan honeysuckle (Leycesteria formosa), and Russian vine (Fallopia baldschuanica) have been suspected of being Reynoutria japonica.

[16] Rhizome of R. japonica is the source of lactoperoxidase peroxidation cycle substrates, which can act as activators and inhibitors of the antimicrobial properties of that system.

[17] It grows widely throughout Japan and is foraged as a wild edible vegetable (sansai), though not in sufficient quantities to be included in statistics.

They are extremely sour; the fibrous outer skin must be peeled, soaked in water for half a day raw or after parboiling, before being cooked.

It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States.

[23] Japanese knotweed has a strong and extensive root system that can penetrate through the cracks of hard surfaces such as concrete, asphalt, and brick.

As the plant grows, it can cause significant damage to building foundations, walls, and drainage systems, which can be very costly to repair.

Its invasive roots can also damage concrete foundations, buildings, roads, paving, retaining walls, and architectural sites.

[25] Japanese knotweed shades out other vegetation, grows over buildings and other structures, encourages fire, and damages paved surfaces.

All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch.

[27] Trials in Haida Gwaii, British Columbia, using sea water sprayed on the foliage, have not demonstrated promising results.

[31][2][4] In 2020 Amsterdam imported and released 5,000 Japanese Aphalara itadori leaf fleas, exempting them from a strict ban on the introduction of alien species, as one of the measures to contain the knotweed.

[38] In the UK, Japanese knotweed is established in the wild in many parts of the country and creates problems due to the impact on biodiversity, flooding management and damage to property.

[40][39] Defra's Review of Non-native Species Policy states that a national eradication programme would be prohibitively expensive at £1.56 billion.

[41] The Centre for Ecology and Hydrology has been using citizen science to develop a system that gives a knotweed risk rating throughout Britain.

[48] In the United Kingdom, Japanese knotweed has received a lot of attention in the press as a result of very restrictive lending policies by banks and other mortgage companies.

[49] The Royal Institution of Chartered Surveyors published a report in 2012 in response to lenders refusing to lend "despite [knotweed] being treatable and rarely causing severe damage to the property".

Without actual advice and guidance, surveyors have been unsure of how to assess the risk of Japanese knotweed, which can result in inconsistent reporting of the plant in mortgage valuations.

As recently as 2012, the policy at the Woolwich (part of Barclays plc) was "if Japanese knotweed is found on or near the property then a case will be declined due to the invasive nature of the plant.

Treatment must be covered by a minimum 10-year insurance-backed guarantee, which is property specific and transferable to subsequent owners and any mortgagee in possession.

"[40] At Mission Point Park in Davis Bay, British Columbia, municipal crews attempted to eradicate it by digging out the plant to a depth of about three metres (10 feet) with an excavator.

[40] To avoid an epidemic as in the United Kingdom, some provinces in Canada are pushing for relaxation of provincial limits on the use of herbicides close to waterways so knotweed can be aggressively managed with strong chemicals.