Other invasive plants

There are many invasive non-native plant species present in Scotland and within our project area – we are able to control only some of them. The species below are ones that can be common in Scotland or which we quite often come across during our work. Whilst they are not on our project priority ‘hit-list’ and we don’t actively survey for their presence we are often asked about them.

Some – like Gunnera – we may control if we come across plants while out controlling other species, whilst others – like coneflower – can be locally invasive and the local fishery trust or community group may be implementing local control.

White butterbur

White butterbur is native to northern and eastern mainland Europe and to temperate Asia. It was first imported as an ornamental garden plant in 1683 and found in the wild in West Yorkshire in 1843. White butterbur is not particularly widespread in Great Britain – with much of its growth concentrated in north east Scotland, and particularly in the River Spey catchment.

Meet the plant

White butterbur (Petasites albus) flowers appear early in the year (February – May) and the spiked white flower heads can dominate damp riverside areas. The leaves appear after the flowers and are small during flowering, but later become much larger, up to 30cm across. They resemble rhubarb leaves, but are more heart-shaped and grow low, forming dense carpets completely dominating the ground.

White butterbur has a rhizome root and spreads readily in damp ground along rivers and road verges. It can regenerate from rhizome fragments, which can be carried along river corridors by water and making disturbed flood-prone ground particularly prone to infestation.
White vs. Common butterbur

White butterbur is similar to the native common butterbur (Petasites hybridus) with the flowers being the main distinguishing feature. White butterbur flowers are all-white, whereas common butterbur has mauve to purplish flowers. White butterbur flowers have five narrow white petals, narrower than those of the common butterbur, with long pale-green sepals, whereas in common butterbur sepals are shorter.

White butterbur leaves can be more elongated with a white felt underneath, but are difficult to separate from common butterbur leaves.

Species identification is best made when species are in flower.

Read more about White Butterbur
Impacts

White butterbur comes into leaf early in the year and forms a dense overlapping canopy of leaves through which light cannot penetrate. Growth patches can be many metres across, suppress native vegetation growth and reduce native wildlife present.

White butterbur rhizomes are not as strong and branched as native plant roots and so infested riverbanks can be less stable, prone to erosion (leading to increased sediment loads being introduced to the water column) and at increased risk of flooding.
Management of white butterbur

White butterbur is not particularly widespread in Great Britain and so there is relatively little information on its control. We started a COVID interrupted trial on control which generated limited findings.

We looked at different control methods – herbicide application at various concentrations, hand digging and strimming and native tree and wildflower planting to see if these plantings help suppress re-growth. Chemical application was the most effective control method. Hand digging was time consuming and laborious, strimming may progressively reduce regrowth strength and trees and wildflower growth was not sufficient to determine any shading impact.

Read the associated blog: Buttery Business

Gunnera – giant rhubarb

Gunnera (Gunnera tinctoria) is a fast growing perennial growing up to 2m in height with huge leathery leaves with short bristles and spines. It is a popular ornamental garden plant that has become well established locally – particularly on the Scottish west coast and islands. It produces vast quantities of seed which are spread readily and can regenerate from small rhizome fragments.

When established it can displace native vegetation and impact local ecosystems. It has also contributed to erosion on soft coastal cliffs and blocked drains and streams contributing to flooding.

Gunnera species identification guide

Coneflower

Cut-leaved coneflower (Rudbeckia laciniata) is a popular garden plant, but in a few locations it has and spread into the wild. A member of the aster family, it has large yellow daisy-like flowers growing to a height of 2-3m. It prefers wet sites along stream banks and wet woodlands so can readily colonise river banks and other wet habitats.

Coneflower is present specifically in the Cromarty Firth Fishery Trust area on the Garrie Island in the River Conon.

Giant butterbur

Giant butterbur (Petasites japonicus) can reach up to 2m high and has a stout flowering stem with many creamy white flowers which appear before the leaves. The enormous kidney-shaped leaves – 30cm to 1m across – can block out light and suppress native vegetation. Underground is a tough rhizome which grows rapidly enabling single-clone populations to establish quickly and significantly alter existing ecosystems.

Scattered populations are found in our project area, particularly around the Moray – Inverness areas. Control is by application of pesticide to the leaves.

Rhododendron

Rhododendron (Rhododendron ponticum) is a dense, evergreen, branched shrub which can grow to 5m tall. The flowers are pink to purple and present in clusters of 10-15, has solid stems forming a trunk when mature and leathery dark green leaves. Growing in dense thickets it shades out native plants, prevents natural regeneration of trees in woodlands and impacts ground vegetation. Its extensive root system and leaf litter is toxic to many other plants and it can harbour Phytophthora, a fungus-like pathogen, that affects many other trees and plants.

This is an invasive plant we are often asked about but it is not a target species. In Great Britain it occupies moorland, woods, screes, rocky banks, derelict gardens and stream sides and grows best in wet and mild areas in the west of Scotland. Control is by physical removal (by hand or using machinery), chemical application and stem injection with follow treatment necessary when stumps regrow or seedlings take root.

Large rhododendron control programmes are undertaken by bodies such as Forestry and Land Scotland, the National Trust for Scotland and the John Muir Trust, by partnerships such as the Alliance for Scotland’s Rainforest and by numerous estates and land managers.