Invasive Non-Native Species

Rhododendron ponticum

Why is Rhododendron ponticum so successful?

Like many IAS, Rhododendron ponticum was first introduced to the British Isles during the Victorian era as an ornamental shrub. It was planted in the gardens of many larges houses where it grew very successfully. It thrives in areas of high humidity with damp substrates and acidic soils, making many of the naturally occurring habitats of Wales ideal for its growth. A single Rhododendron flower is capable of producing many thousands of seeds each year, which are primarily dispersed by wind, enabling the plants to spread widely. Initially, the plant was largely confined to well managed gardens, where any seedling growth was removed. However, a reduction in large estates and their staff has seen Rhododendron ponticum spread unchecked into the wider environment during the 20^th century.

Why is it a problem?

Rhododendron ponticum…or pied flycatchers, bluebells, hazel bushes, shrews, wood sorrel, wood mice, woodpeckers, ferns, mosses, toadstools and oak saplings? We can’t have both.

Given the right conditions, Rhododendron ponticum can form an extensive and continuous canopy layer which casts a dense shade over existing ground flora. Thus flowering plants, grasses, ferns, bryophytes and lichens are all outcompeted, and tree seedlings are unable to become established. This is particularly worrying for woodland succession, and for many lower plants who are dependent on mature native trees as host species.

Rhododendron ponticum, particularly the leaves of young plants, is not suited to most mammals, birds and insects. Consequently, land infested with Rhododendron can become unsuitable for browsing animals, which are an essential part of a healthy functioning woodland ecosystem.

When combined, the above can have drastic adverse impacts on the dynamics of a natural woodland, meaning that you can’t have a healthy native woodland ecosystem with Rhododendron ponticum present.

Treatment

The Celtic Rainforests Wales project endorses a 3-stage approach to controlling Rhododendron ponticum, as follows:

Stage 1: the ‘Attack phase’

The attack phase is in almost all cases the most expensive phase as it is very labour intensive. Methods of control during this stage varies depending on such factors as site characteristics, density of stands, or landowner requirements, but the most common methods used include stem injection with systemic herbicide, cutting the plant and then either stacking, burning or chipping it, foliar spraying with an approved herbicide, or manual uprooting of seedlings

Stem injection will usually be the first line of treatment. Simply put, you drill a hole down into the stem with a normal cordless drill and fill it with an approved herbicide. The chemical is then absorbed and transported throughout the plant, which when done correctly results in the complete death of a plant. This method uses less herbicide, and as it is injected directly into the plant, reduces the amount of chemicals entering the wider environment. If done well, this method should also remove the need for follow up treatments due to achieving a whole plant kill. The potential down side to this method is that it leaves you with large areas of standing dead timber, which should be left for at least a year before removal in order to limit re-growth. After this time, the dead timber can be felled and removed in order to improve the appearance or accessibility of the site, or be left to rot naturally.

Stage 2: the follow-up phase

Usually undertaken two years after the attack phase, follow up action is always necessary. Even with high mortality stem injection, there is always the chance that a few bushes or branches have been missed, thus it is essential to complete a stage 2 treatment on any site. As with the attack phase, the method used during stage 2 will depend on the amount and size of re-growth, in conjunction with site characteristics, but usually consists of foliar (leaf) spraying, manual uprooting, and in some instances further stem injection or cutting and burning / chipping / stacking may be used.

Stage 3: the maintenance phase

The maintenance phase is reached when Rhododendron ponticum is reduced to levels where it no longer has significant adverse impacts on conservation interests or on land management practices. However, if left untreated at this point, significant recolonisation would happen over subsequent years. By this stage, all mature bushes should have been successfully treated, with no more than a few occasional seedlings to be found scattered across a site. Flowering plants should be absent, or present in very small quantities. However, it must be noted that the maintenance phase will never be reached if significant seed source remains on areas of adjacent land which is why it’s important to tackle the plant on a landscape scale to ensure success.

It is imperative that work is carried out over a 5-year period, as it can take up to five years for new plants to emerge and be visible from propagated seeds. Even beyond this period, continuous monitoring of those sites treated should continue to ensure no further recolonisation. This is why building good, long-term working relationships with landowners is essential if we are to achieve our ultimate aim of eradicating Rhododendron ponticum from within the Celtic Rainforests in Wales.

The hybrid Montbretia, which is the main species to have spread into the wild in the British Isles, originates from South Africa and was first introduced from France in the 1880’s as a garden plant. Since then, it has become widely established in the wild, especially in western areas. Once established it can spread rapidly, giving it the ability to quickly outcompete native flora. Although viable seed is produced, most reproduction is vegetative from underground corms and long creeping rhizomes (underground stems), with even the smallest fragments of root readily able to establish new stands.

Montbretia is listed as a Schedule 9 species under the Wildlife and Countryside Act 1981, meaning it is an offence to plant the species or otherwise cause it to grow in the the wild.

Mechanical control

Plants can be dug out but it is essential that all the plant material and corms are removed. If corms are broken up or accidentally left they can produce new plants potentially making the problem worse.

Due to its classification as a Section 9 species, any soil containing the plant waste of Montbretia is classified as controlled waste and should be disposed of at a licensed landfill site.

Chemical control

Stands can be effectively treated with herbicide whilst the plants are actively growing. When treating large areas, a suitable grass and forb mix should be sown to prevent bare ground and colonization of other unwanted species.

The flowers of skunk cabbage are bright yellow in colour, and can reach up to 40cm in height, whilst the leaves, which form after the flower, can reach up to 1.5m in height. The plant dies down over winter leaving vast areas of bare soil which can be susceptible to erosion. The flowers have a very strong fragrance similar to that of the skunk, hence its common name. It prefers wet conditions to grow in, but has no specific soil requirements and will grow well on most sites.

Most plants in the wild probably arise from garden material being disposed of as colonies quickly outgrow their intended space within domestic gardens. It is also likely that some populations in the wild have become established from seed dispersed from nearby dwellings. Once established, the plant is very invasive – its very large leaves and rapid rate of spread allows it to out-compete native plants and cause extensive damage locally. It can spread via its rhizome or by seed, and is easily distributed by water, birds or mammals.

Mechanical control

Plants, new seedlings and rhizomes can be dug up and removed. However, if fragments of rhizome remain, these can regenerate and new plants will grow.

Chemical control

The leaves can be effectively treated with herbicide but there are limitations to which herbicides are permitted to be used near water. The plant should be treated between June and October when the leaves are fully grown and ground conditions are drier. Multiple applications may be required over a number of seasons in order to achieve full eradication.