Ecological survey calendar › Invasive plants
When to survey and treat invasive plants in Wales and England
Updated November 2026. Based on current Natural Resources Wales guidance on Japanese knotweed.
Japanese knotweed is easiest to map from May to September and should be treated with herbicide from late summer into autumn, after flowering. Himalayan balsam should be pulled or cut between May and July, before its seed pods form. Large knotweed stands need two or three years of treatment, so start early.
Survey windows, sensitive periods and works timing
| Activity | Type | When | Notes |
|---|---|---|---|
| Japanese knotweed survey | Survey | Apr (not ideal); May to Sep; Oct (not ideal) | Dead canes can be found in winter, but the extent is easiest to map in growth. |
| Himalayan balsam survey | Survey | Jun to Aug | Easiest to find in flower. |
| Japanese knotweed herbicide treatment | Works window | Aug to Oct | Glyphosate works only from late summer into autumn, after flowering. Large stands need two or three years of treatment. |
| Himalayan balsam pulling | Works window | May to Jul | Pull or cut before seed pods form, usually from July. Repeat visits through the season and for at least two years. |
Optimal survey Possible, not ideal Sensitive period Works window
Timing depends on the site, the weather in a given year and what the planning authority and licensing body need. Upland and northern sites can run later. See every species group side by side on the ecological survey calendar.
About invasive plants
Invasive plants found late can stop a construction programme and add cost. Mapping them early means treatment can start in the right season, often a year or more before works.
Legal protection
It is an offence to plant these species or cause them to grow in the wild (Wildlife and Countryside Act 1981, section 14 and Schedule 9). Soil containing knotweed is controlled waste.
Licensing
Wales: No licence is needed, but on-site burial, movement and disposal must follow Natural Resources Wales guidance and waste law.
England: No licence is needed, but on-site burial, movement and disposal must follow Environment Agency guidance and waste law.
Practical tips
- Map knotweed in summer and treat it from late summer into autumn, after flowering.
- Never strim or flail knotweed. Small fragments of stem and root can grow into new plants.
Common questions
When is the best time to treat Japanese knotweed?
Late summer into autumn, after the plant has flowered. Glyphosate applied earlier may stunt growth but will not kill the plant (Natural Resources Wales).
How long does knotweed treatment take?
Large mature stands normally need treating once a year for two or three years.
When should Himalayan balsam be pulled?
May to July, before the seed pods form. Repeat visits through the season and for at least two years, as seed in the soil keeps germinating.
Is it illegal to have knotweed on site?
It is an offence to plant it or cause it to grow in the wild, and soil containing knotweed is controlled waste, so it must be managed and disposed of properly.
Need a survey or advice on timing? Ecoleg's licensed ecologists programme surveys across Wales and England so the right work happens in the right season. Call 07356 123706.
Request a quoteSee the full calendarSources
- Environment Agency and Natural Resources Wales guidance on managing Japanese knotweed and other invasive non-native plants (current).
- Natural Resources Wales, Japanese knotweed: what you need to know (accessed November 2026).
- Common professional practice; confirm for each site.
Updated November 2026. General guidance only: confirm timing for your site with a suitably qualified ecologist.