The fast-growing plant, originally from east Asia and brought to the UK in the 1800s, can grow up to four inches a day.
It spreads quickly, smothers other plants and can even damage buildings.
Experts at Environet describe it as a pest that outgrows and harms native plants.
We used Environet’s online map to find the average number of Knotweed sightings in Croydon.
To do this, we looked at five postcodes in each area and worked out the average number of reports.
Thornton Heath (CR7) had the highest number of sightings this year with an average of 857 in each CR7 neighbourhood, followed by Central Croydon (CR0) which averaged 505 sightings per neighbourhood.
South Croydon (CR2) had an average of 362 sightings.
Purley and Kenley (CR8) registered 196 average sightings.
The CR5 postcode, which covers Coulsdon, had the lowest number of registered sightings, with 161.
Environet director Emily Grant said: “By mapping verified sightings, we’re helping people understand the level of risk in their local area and encouraging early identification and treatment.
“Vigilance is the best way to protect your property from Japanese Knotweed, making sure you tackle it as early as possible before it becomes well established.
“At this time of year, knotweed is in full growth mode and easy to spot with its bright green heart-shaped leaves, which grow in a zigzag pattern up the stems.”
“Mature plants flower around August, when they produce clusters of tiny white blooms.
“If it is knotweed, a survey will determine the extent of the infestation and the best way to treat it, whether that’s digging it out of the ground, herbicide treating it or a combination of both.
“Public engagement is key in the fight against knotweed, and by reporting new sightings, homeowners can play an important role in controlling this invasive plant and protecting property values.”
