Pests of the Week

12th March 2020

Pests of the Week

Invasive non-native plants on the Isle of Bute are hazardous to both humans, wildlife and the native species across the island. Here, we’ve put the facts together for the four most prominent invasive species across the island.

If you see one of these plants, as highlighted below, or need more information, please email us here, providing the location and date.

Invasive species are capable of causing extinctions of native plants and animals, reducing biodiversity, as they compete with native organisms for limited resources. This can alter habitats, resulting in huge economic impacts and fundamental disruptions of coastal and island ecosystems

*All listed plants are listed under schedule 9 of the Wildlife and Countryside Act 1981 – it is an offence to plant or cause this species to grow in the wild.

1. Rhododendron Ponticum

Introduced to Britain around 1763 as a cultivated flowering plant it became especially popular on country estates during Victorian times. It is now widespread around the Argyll and Bute countryside.

Why is it a problem?

It is an aggressive coloniser that out competes native plants reducing the biodiversity value of a site, obstructs regeneration woodlands and once established is difficult and costly to remove. Mature bushes also act as a prolific seed source for invasion of adjacent areas, and are a continued source of new plant material into areas successfully cleared.

The safest and most efficient methods of controlling rhododendron depend on the size, life stage and accessibility of the target bush. In general stem treatment is the most effective and efficient method of killing large rhododendron bushes. Where there is no access to treat the stems, an overall foliar spray is the next option. Small bushes are easily treated, but foliar sprays cannot be applied safely to larger bushes, which must therefore be reduced in size before a safe application of herbicide can be given. There are two main ways to reduce bush size: using a mechanical flail or by manual cutting.

More recently, R. ponticum has become a host to two new plant pathogens; Phytophthora ramorum and P. kernoviae. These pathogens spread from infected plants to our native tree species eventually leading to the death of the tree.

2. Japanese Knotweed

Introduced to Britain in 1825 as a popular garden plant, it is an extremely aggressive plant which is very difficult to control once established. Knotweed has spread rapidly since and is now found in gardens, by roads, waterways, woodlands and waste land.

Why is it a problem?

This species out grows and eventually replaces native flora in all of the above environments. Along riverbanks when the plant dies back in the winter it leaves bare areas which are more susceptible to erosion which can lead to large sections being eroded.

Both roots and stems can grow through weaknesses in asphalt and concrete, damaging structures affecting both property values and the ability to get a mortgage.

Eradication of Japanese knotweed is very difficult for the general public to attempt. Non-chemical methods can take up to 10 years to kill the weed. Chemical control is the preferred method which is best undertaken by a contractor. This is most effective when applied to the Knotweed in late summer from July to October.


Please DO NOT dispose of Japanese knotweed at refuse tips, in the countryside or ‘over the hedge’. A piece of stem or root the size of a fingernail can root like a cutting and create a new plant.

3. Himalayan Balsam

Introduced in the 19th century, Himalayan Balsam has rapidly colonised rivers, ditches, areas of woodland and the coastline of Argyll and Bute.

Why is it a problem?

A fast growing plant which forms dense stands suppressing native flora along riverbanks, parks and woodlands. In autumn the balsam dies back, leaving the soil bare of vegetation, and therefore liable to erosion. The plant produces a good pollen supply for insects, over a long period of time leading to concerns that native flora will be overlooked in favour of this species, further threatening native biodiversity.

Control methods aim to prevent the flowering of the plant and should be carried out before July by either manual pulling of the plants or chemical control.


The plant sets seed from mid-July onward with seed capsules exploding at the slightest touch when ripe.

4. Giant Hogweed

First introduced to Britain in the 19th century as a garden curiosity from the Caucasus it is a highly invasive plant that can grow to 5m tall.

Large monocarpic (Grows ,Flowers, produces seed and dies) is a herbaceous plant, growing to over 3m when in flower. A member of the cow parsley family, its white flowers form large umbels, up to 80cm across, and are produced during July and August. The largest leaves can be over 1m in size.

Why is it a problem?

Giant hogweed is very competitive due to its size and rapid growth rate, out-competing native flora along river banks and coastline. When the plant dies back in the winter it leaves bare areas which are more susceptible to erosion, this can lead to large sections of river bank being eroded. Each plant can produce around 50,000 seeds each year which can remain viable for up to 15 years. There are several options for control both manual and chemical. The simplest method to prevent seeding involves cutting off the flower heads after the seeds have formed but before they start falling off the head (late August/September).


Hogweed is hazardous to human health. Contact with sap from this species can cause blistering of the skin when exposed to sunlight.

With thanks to our partners at: Scottish Natural Heritage, Bute Conservation Trust, Argyll and Bute Council, and Martin's Gardens.

Need to report Invasive Species outside of Bute? Contact Scotland's Environment.