Pictures of invasive plant infestations
By Jennifer Holmes

Pictures of invasive plant infestations

As the growing season picks up the pace we asked our INNS experts “how bad can invasive plants get?”

In spring 2024 we spoke to our specialist advisers and surveyors about the extent of infestations they have seen over the past 12 months, and the impact it has on the environment.

You’re about to see just how bad invasive plants and water weeds can get!

A canopy of overgrown knotweed

We’ll start with the most commonly talked about species. The image above shows a typical Japanese knotweed canopy – the result of neglect for just one season – this is how knotweed can get out of hand. Our lead surveyor told us “when it gets this bad it’s difficult to navigate through the thicket when surveying or treating this plant, and really excavation is the best solution.”

But invasive plants do not just infest land, many water weeds can completely clog up waterways, reservoirs and irrigation systems, causing havoc for UK businesses such as utilities, environmental organisations, River Trusts and the leisure industry.


Below is a great example of what an infestation of Parrot’s feather looks like, when our specialist advisory manager was called out to investigate the extent of an infestation. It is smothering the water’s surface meaning many species will not be able to survive beneath this extensive colony. As he pointed out “it can be tricky to eradicate but we will always recommend the best option for protection of the habitat and its ecosystem, and this may involve using more than one method. We’ve been asked to survey more waterways recently due to extensive colonisation of aquatic weeds such as Nuttall’s water weed and Azolla, and the recent flooding has only served to help these invasive plants spread further afield. Conditions are not ideal for surveying these habitats, it’s always a 2-man job, but we know how important it is to get these infestations under control.”

The water weed Parrot's feather infesting a river


Other invasive plants, such as Himalayan balsam are more adaptable to changing climates, and as you can see from the image below this tall plant grows so densely that it completely blocks any pathway through. Our excavation team told us “Himalayan balsam needs to be excavated when it gets this bad, and especially when it’s covering land deemed for development.”

Himalayan balsam infestations such as these create a significant ecological impact especially in riparian environments since it grows in dense stands that have the capacity to completely suppress native grasses and other flora. In the autumn the plants die off leaving riverbanks bare and therefore highly susceptible to erosion – which can exacerbate flooding.

The invasive plant Himalayan balsam in woodland

Backlit image of a row of trees against the sky. The photo was taken in a Dutch nature reserve. The terrain is overgrown with the yellow discolored invasive swamp stonecrop or Crassula helmsii plants.stonecrop or Crassula helmsii plants.


Above shows the extent of damage Crassula helmsii can cause to what should be a free flowing river. Look closely and you can see it has encroached onto the river bank to the point where the boundary is diminishing.

It’s easy to see how other plants in this ecosystem simply cannot compete for vital nutrients and die out, which then threatens the survival of species (such as insects and birds) who thrive on the affected plants. The lack of oxygenated water also impacts the survival of fish and amphibians.

Japanese knotweed and Giant hogweed


A health and safety threat – the above image is Giant hogweed in full bloom last summer (among knotweed growth). At the moment it’s looking more like the image below, but this impressive plant, come June or July can reach 5m in height – and you can see last year’s growth in the picture below as dry brown canes with the symbolic umbrella shaped seed head (which is a mass of small white flowers in summer).

Giant hogweed early growth and last year's dry stems

Important to note this is one the UK’s toxic species, we urge not to touch! The toxicity of its sap will cause severe skin blistering which will reoccur when skin is exposed to sunlight – a condition that can last for years.

Smothering, colonising, encroaching and (in some species) toxic – invasive plant infestations must be controlled

Across the UK our teams are reporting infestations earlier in the year than ever before – which scientists have already attributed to climate change.

But the facts remain the same no matter what or when the infestations occur:

  • Invasive species are a threat to our native species which are already dwindling
  • Control can be achieved by a professional treatment plan or complete excavation/removal
  • Some invasive plants are toxic and need to be handled by specialists invasive weed contractors

Our advice – be vigilant when out and about, whether for work or leisure, and report any sightings of infestations to a reputable contractor.

Last word – has to come from our advisory manager: “infestations out-compete native species to the extent that we need to be more aggressive and clever in our approach to controlling them. Looking at alternative methods is becoming more important as species adapt and evolve to our changing climate.”

To speak to our specialists or to book a survey contact us below: