Japanese Knotweed and Other INVASIVE PLANT SPECIES

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Japanese Knotweed (Botanical name: Fallopia Japonica)

Reynoutria Japonica, synonyms: Fallopia Japonica and Polygonum cuspidatum, is a large species of herbaceous perennial plant of the knotweed and buckwheat family Polygonaceae. It is commonly known as Asian knotweed or Japanese knotweed. It is native to East Asia in Japan, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats and is classified as an invasive species in several countries.
 

Related species include: 

• Giant Knotweed 

  - (Fallopia Sachalinensis) 

• Bohemian Knotweed 

  - (Fallopia Bohemica) 

• Himalayan Knotweed 

  - (Persicaria Wallichii)


• Russian Vine 

  - (Fallopia baldschuanica, Polygonum baldschuanicum).
 

Japanese Knotweed is probably the most commonly well known invasive plant species in the UK. The very thought of it near a home owners property will strike fear into people due to the fact it is very hard to eradicate. Its presence even in a neighbouring garden can be enough to cut a property's value massively, or prevent a mortgage lender approving a loan. Many prospective house purchasers often won’t even entertain the idea of purchasing a property that has this issue.

It is believed that in the nineteenth century, when Victorian engineers were creating the latest form of transport, Japanese knotweed sounded like a very clever idea. It is a plant that typically colonised volcanoes in Japan and Eastern China, it was imported to Britain to help hide, or possibly even stabilise, railway embankments. Since then its spread has caused much unhappiness amongst home-owners and prospective house purchasers.

As an invasive species it is not only very difficult to get rid of it is also very damaging. It can crack tarmac, block drains, lift patios, undermine foundations, invade homes and has even been known to work its way through concrete. 

The problem with Japanese Knotweed, is that despite the fact it dies back throughout the winter, its root system is both extensive and very strong. It is believed that its roots can lay dormant for many years before it suddenly gets active. The roots are the only way in which the plant can spread, as it doesn't produce seeds. It also also possible for the plant to go in to a form of shock when being treated which again can make it go dormant for a number of years before it comes back again.

This vigorous characteristic has resulted in it being an offence against the 1981 Wildlife and Countryside Act to grow it, and if it isn't controlled by those who have it in their garden, a community protection notice can even be given.

How to recognise Japanese Knotweed:

• Dense thickets of green, purple-speckled, bamboo-like stems up to three metres tall

• Heart or shield-shaped leaves

• Alternate leafing pattern along stems

• Japanese Knotweed has hollow stems that can be snapped easily with distinct raised nodes that give it the appearance of bamboo, though it is not related.
• While stems may reach a maximum height of 3–4 m (9.8–13.1 ft) each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down.
The leaves are broad oval with a truncated base, 7–14 cm (2.8–5.5 in) long and 5–12 cm (2.0–4.7 in) broad, with an entire margin.
• The flowers are small, cream or white, produced in erect racemes 6–15 cm (2.4–5.9 in) long in late summer and early autumn (August to October).

There are problems with Japanese Knotweed concentrations across the whole of the UK and specific areas that have been highlighted in South Wales, the Midlands, London, Scotland's central belt and Cornwall - where the plant was also originally introduced in ornamental gardens by the Victorians.
 

In South & Mid Wales section knotweed is easily spread along the course of rivers and canals as tiny fragments of knotweed float downstream, and quickly establish themselves elsewhere.
 

In terms of controlling Japanese knotweed on your property, it is extremely difficult and should be treated by someone who is suitably qualified. Digging the weed out is the most environmentally friendly method; however it is back breaking work considering the roots can travel so far and that the smallest rhizome can produce an entirely new plant.

(Rhizome: a rhizome is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow horizontally. The rhizome also retains the ability to allow new shoots to grow upwards.)

Japanese knotweed has a large underground network of roots (rhizomes). To eradicate the plant the roots need to be killed. All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch. Picking the right herbicide is essential, as it must travel through the plant and into the root system below Ground.

In terms of chemical sprays, Glyphosate is the most viable option. Applied to the leaves, it is absorbed by the entire plant and is capable, after a few years, along with multiple treatments of killing this troublesome weed.
 

The most effective method of control is by herbicide application close to the flowering stage in late summer or Autumn. In some cases, it is possible to eradicate Japanese knotweed in one growing season using only herbicides. Glyphosate is widely used as it is non-persistent, and certain formulations may be used in or near water
 

Method of applying a suitable and effective weed killer is through the following methods:
- Spraying

- Stem Injection

- Weed Wiping

Another option is to stem inject a herbicide direct into the stem of the plant. It should be noted that the plant can go into shock when being treated and parts of its roots system can go into a dormant state for a few years. This is why it is a very difficult plant to eradicate and it will take between 3-5 years of continuous treatments.
 

A treatment program will involve:
- Multiple site visits over a number of years. 

- Multiple treatments.

- Monitor the effects of the treatments.
- Re-Treatment of the annual regrowth / regeneration until it is permanently interrupted.
- Even if the problem Knotweed has been killed and there is no regrowth the site will need to have continued monitoring to ensure the root system has not traveled through the sub-terrain and start sprouting fresh shoots in a new area. 

- It should be noted that the root system can go into shock and lay dormant before re-appearing.
 

A relatively new option however is biological control using psyllid (plant sucking bugs). Currently on trial, if this method proves successful, hopefully it will be offered to gardeners up and down the UK.

What To Do If You Think You Have Japanese Knotweed On or Near Your Property:

Japanese Knotweed Do’s

- Contact a professional who is qualified to treat and manage Invasive Weed Control and can deliver efficient, effective and reliable treatment.
- Do not ignore Japanese knotweed when you see it in your garden, building plot, a neighbours garden or adjoining land.
- It can grow very quickly at a rate of 10cm a day and the costs will grow to treat the problem as the plant does.

Japanese Knotweed – Don’ts

- Don’t cut it down / flail Japanese Knotweed as this could cause it to spread especially when the stem is growing as it can stimulate the root system to grow instead.
- When removing treated dead plants, cutting with sharp hooks, blades, slashers etc. or hand pulling is recommended to avoid any dispersal of cut fragments. If you cut down Japanese knotweed, it is best to dispose of it on site by burning it.
- Don’t cause the spread of Japanese knotweed stem and crowns. If you cut down Japanese knotweed, it is best to dispose of it on site.
- Material taken off site is classified as waste and must be safely contained and disposed of at a licensed disposal site. Not the normal Green Waste Recycling Facility.

- Don’t try to dig up Japanese Knotweed as this will lead to a significant increase in stem density. Even a tiny fragment of the cut rhizome is capable of regeneration and when you are digging it up you run the risk of creating multiple new plants that will reappear with vengeance.
- Any soil that is obtained from ground within 7 m horizontally and 3 m deep of a Japanese knotweed plant could contain rhizome. Don’t spread soil contaminated with Japanese knotweed rhizome in as this will create a whole new problem.
- The rhizome is highly regenerative and will readily grow into new plants.
- Don’t machine Chip/Shred Japanese Knotweed material. Mechanical chippers will not kill Japanese Knotweed. If you spread the chipped material on soil, it is highly likely that Japanese knotweed could regrow.
- It is important that you don’t dump garden waste contaminated with Japanese knotweed in the countryside – you will be breaking the law under The Wildlife and Countryside Act 1981 (as amended).
- It is important that you don’t take Japanese knotweed to recycling centres that receive garden waste as it will contaminate the compost. Don’t break the law. Remember, if you cause Japanese knotweed to spread you are guilty of an offence under the Wildlife and Countryside Act (1981)

Important Note: Don’t add Japanese Knotweed to compost. You will need to compost it separately (preferably on plastic sheeting to prevent rooting) so that you can be sure it is dead. 

Other Interesting Japanese Knotweed Information:

In the far east the young stems are edible as a spring vegetable, with a flavour similar to extremely sour rhubarb. In some locations, semi-cultivating Japanese Knotweed for food has been used as a means of controlling knotweed populations that invade sensitive wetland areas and drive out the native vegetation. It is eaten in Japan as 'Sansai' or wild foraged vegetable.
 

Japanese Knotweed is also used in traditional Chinese and Japanese medicine to treat various disorders through the actions of resveratrol, although there is no high-quality evidence from clinical research for any medical efficacy.
 

Legal:
 

In the UK, Japanese Knotweed is established in the wild in many parts of the country and creates problems due to the impact on biodiversity, flooding management and damage to property. 

You should be aware that it is an offence under 'section 14(2)' of the 'The Wildlife and Countryside Act 1981 (as amended)' to "plant or otherwise cause to grow in the wild" any plant listed in Schedule nine, Part II to the Act, which includes Japanese Knotweed. Within towns householders and landlords in 2014 who did not control the plant in their gardens could receive an on-the-spot fine or be prosecuted. It is also classed as "controlled waste" in Britain under part 2 of the Environmental Protection Act 1990. This requires disposal at licensed landfill sites.

Some home owners in the United Kingdom are unable to sell their homes if there is any evidence of knotweed on the property. The species is expensive to remove. According to the UK government, the cost of controlling knotweed had hit £1.25 billion in 2014.

It is estimated that it cost about £70 million to eradicate knotweed from 10 acres of the London 2012 Olympic Games velodrome and aquatic centre. 

The decision was taken on 9 March 2010 in the UK to release into the wild a Japanese psyllid insect, Aphalara itadori. Its diet is highly specific to Japanese knotweed and shows good potential for its control. Controlled release trials began in South Wales in 2016.

In Scotland, the Wildlife and Natural Environment (Scotland) Act 2011 came into force in July 2012 that superseded The Wildlife and Countryside Act 1981 (as amended). This act states that is an offence to spread intentionally or unintentionally Japanese knotweed (or other non-native invasive species).

If you have an 'Invasive Plant Species' problem on your property. Don't delay getting in touch, because you need a treatment plan putting into place. 

Treating these problems is not a quick process, but it doesn't need to be that expensive if you act sooner, rather than later.

Jamie's Garden Service is fully NPTC Qualified and Insured to supply Invasive Plant Species Weed Control Services to Commercial and Residential Sites. 

Giant Knotweed is an invasive, non-native plant. Very similar in appearance Japanese Knotweed in looks and they flourish in similar habitats. However, as the name 'Giant' suggests, this version of knotweed grows much taller, 4-5 metres in the right growing conditions and it has much larger, elongated leaves.

Like Japanese knotweed, it was introduced as a garden plant in Victorian times but was not as widely planted because of its huge size. Hybrids between the two species, the Bohemian Knotweed are also spreading rapidly.

It is increasing in the wild, especially along riverbanks and around sea-loch shores and has similar impacts to Japanese Knotweed. Giant knotweed impacts on habitats in the same way as Japanese knotweed. It forms dense stands which obliterate virtually all other plant species

Giant Knotweed's original country of origin where it is native to the islands of Sakhalin (Russia), the southern Kuril chain, and Hokkaido and Honshu (Japan). 

There is no directly adverse Human impacts, except the damage to habitats and gardens where the plant rapidly gets out of hand. Knotweed of all types can cause damage to buildings and other structures and therefore can have economic as well as environmental implications. 

Japanese, Bohemian, Himalayan, and Giant knotweed are listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild.

For more in-depth information please read the Japanese Knotweed page.

If you have an 'Invasive Plant Species' problem on your property. Don't delay getting in touch, because you need a treatment plan putting into place. 

Treating these problems is not a quick process, but it doesn't need to be that expensive if you act sooner, rather than later.

Jamie's Garden Service is fully NPTC Qualified and Insured to supply Invasive Plant Species Weed Control Services to Commercial and Residential Sites. 

Bohemian Knotweed   - (Fallopia Bohemica)
 

(Picture Shows a fresh spring shoot)

Bohemian Knotweed is a close relative of the Japanese Knotweed plant, that is probably the most commonly well known invasive plant species in the UK. 

It  can grow to 2-3 metre in height.  The leaves are larger than Japanese Knotweed but smaller than Giant Knotweed, and they can vary in shape from square ended to heart shaped.  Bohemian Knotweed  is a hybrid of Japanese Knotweed and Giant Knotweed. Its features are in between those of the parent species.  Both variations of the leaf can appear on the same plant. 

This hybrid is considered to be more vigorous than either of its parents, it spreads quicker and could potentially be harder to eradicate.

For all other aspects of this plant and how to treat it, please refer to the Japanese Knotweed page.

If you have an 'Invasive Plant Species' problem on your property. Don't delay getting in touch, because you need a treatment plan putting into place. 

Treating these problems is not a quick process, but it doesn't need to be that expensive if you act sooner, rather than later.

Jamie's Garden Service is fully NPTC Qualified and Insured to supply Invasive Plant Species Weed Control Services to Commercial and Residential Sites. 

Himalayan Knotweed (Persicaria Wallichii)

Himalayan Knotweed is a close relative of Japanese Knotweed that is very similar appearance. However the leaves are much larger in appearance (very narrow and long). Himalayan Knotweed will grow in a similar fashion to Japanese knotweed throughout the year and is sometimes referred to as cultivated knotweed.

Himalayan Knotweed will start with reddish shoots in the Spring and will quickly develop into thick foliage. Generally it is not as tall as its Japanese relative although it can reach heights of up to 2m by the Summer.

This picture shows how dense 2m tall Knotweed becomes if untreated. I had to battle my way into and through this dense infestation to be able to spray herbicide over the whole crop ensuring that all the stems upper and lower leaves were treated.

In the Autumn, the foliage starts to wilt as the leafs turn yellow before the plant turns into pale hollow stalks by the Winter.

Caution when grass cutting in areas of Japanese or Himalayan Knotweed

If you have a Japanese or Himalayan Knotweed problem there is an extensive root system under the surface that can travel for quite a distance. New stems can start to shoot up and if they appear in your lawn it is important that you don't cut them down by mowing or strimming because the will further encourage and stimulate the root system.

You can also spread the infestation to new areas when collecting the cuttings in the mowers grass catcher box and then when disposing of these cuttings start a whole new problem in the area where you dump the cuttings.

The other reason for not cutting these new stems down is because you can only treat the part of the plant that is above the ground with herbicide.

The problem is Himalayan Knotweed is just as capable of damaging and reducing the value of a property as Japanese Knotweed. This means a specialist contractor should be contacted as soon as Himalayan knotweed is identified.

If you become aware that you have Himalayan Knotweed on your property, a treatment and monitoring program needs to be put into place as soon as possible. It is treatable in the same way as Japanese knotweed, and various techniques can be used to removal and control of Himalayan knotweed.

If you have a Himalayan Knotweed problem contact Mid Wales Invasive Plant Species Weed Control to arrange a treatment programme.

For further reading you might want to have a read of my Japanese Knotweed Information Page... 

There are a number of plants that look similar to Himalayan Knotweed:

Lesser knotweed (Persicaria campanulata)

The Similarities:

Lesser knotweed is relatively common ornamental Persicaria species that is closely related to Himalayan knotweed (Persicaria wallichii). As with other knotweed species, Lesser Knotweed has the same, bamboo-like, hollow stems with alternately arranged leaves.

It is often mistaken for the Himalayan Knotweed species or for Japanese knotweed.

The Differences:

The leaves are long, thin (which is similar to Himalayan Knotweed species) and ovate (i.e. an elongated ellipse-shape) But they have clearly marked parallel veins, unlike Himalayan & Japanese knotweed.

Lesser knotweed is shorter than Japanese knotweed, growing approximately 1.5m tall.

Individual flowers are much bigger than those of Japanese knotweed and are clearly bell-shaped. They range in colour from pale to bright pink.

Ornamental Bistorts (Red Bistort – Persicaria amplexicaulis)

The Similarities:

Ornamental bistorts are commonly planted decorative garden species. They are closely related to Japanese knotweed and are in the same genus as Himalayan knotweed (Persicaria wallichii).

Red bistort is probably the most common. It and many other ornamental bistorts have leaves and stems that are very similar to Himalayan knotweed species, and when not in flower they can easily be mistaken for them.

Leaves are alternately arranged along stems, like knotweed.

Stems are hollow and separated into nodes like knotweed.

The Differences:

Ornamental bistorts are usually planted on purpose and don’t spread widely. Therefore, they are usually located in planted borders and areas of landscaping.

Flowers appear in summer and early autumn and are very different to those of Japanese knotweed. They form small clusters of pale pink/white to bright red/purple ‘lollipops’ on tall (10cm) straight ‘sticks’.

Leaves are longer than those of Japanese knotweed, appearing more like those of Himalayan knotweed, with marked lobes that overlap slightly around the stems.

Bistorts have very long, semi-translucent, leaf sheaths that envelop the stem nodes (bamboo-like rings from where leaves sprout) for almost the entire length of the stem internodes (the smooth, straight bits of stem between the nodes). These sheaths are absent on Japanese knotweed and are generally shorter on Himalayan knotweed.

Stems are much thinner and shorter than knotweed, generally growing to around 1m tall and less than 1cm in diameter.

Some varieties and species of ornamental bistort have dark, triangular, arrow-shaped blotches across the central midribs of the leaves.

If you have an 'Invasive Plant Species' problem on your property. Don't delay getting in touch, because you need a treatment plan putting into place. 

Treating these problems is not a quick process, but it doesn't need to be that expensive if you act sooner, rather than later.

Jamie's Garden Service is fully NPTC Qualified and Insured to supply Invasive Plant Species Weed Control Services to Commercial and Residential Sites. 

Himalayan Balsam (Impatiens glandulifera)

Despite a display of rather pretty pink flowers, Himalayan Balsam can be extremely difficult to get rid of if it takes hold in your garden.

Compared to many of the deep rooted invasive plant species, Himalayan Balsam is exceptionally shallow rooted. 

Himalayan Balsam is a tall growing annual, 2-3m (6-10ft) in height. Between June and October it produces clusters of purplish pink (or rarely white) helmet-shaped flowers.

The flowers are followed by seed pods that open explosively when ripe. This causes it  to spread rapidly, especially when the seed pods float down river.

Producing 800 seeds every year, which can survive for up to two years whilst floating in rivers or streams, and extremely tolerant of shady positions, Himalayan balsam is extremely invasive.

Similarly to Japanese Knotweed, Glyphosate can be used for chemical control; however simply digging it out prior to it setting seed is perhaps the most fail-safe and environmentally friendly way of control.Himalayan Balsam (Impatiens Glandulifera) 

Himalayan Balsam (impatiens glandulifera) was introduced to the UK in 1839 from Northern India, Himalayan or Indian balsam is most commonly found on river banks and damp areas, though it is capable of thriving in many other habitats.The dense stands on river banks impede the flow in flood conditions exacerbating flooding. They also shade out native plant species.

Himalayan balsam also causes a less obvious problem for native species. Like many flowering plants, Himalayan balsam produces a sugary nectar to attract insects. However the flowers produce more nectar than any other native European species making it more attractive to bees and other insects, luring them away from pollinating our native flowers.

A structured treatment program using both herbicides and cultural control methods such as hand pulling can provide effective Himalayan balsam control. Annual treatments are needed, focusing on early control to kill plants before they seed.

Treatments with herbicides: This is best done prior to it coming into flower to prevent it getting to the point where the seed pods are ready to fire it contents that causes future years regenerations. Also if you are near a water course (River, stream or brook), you will need to ensure that aquatic friendly herbicides are used. And more importantly ensure that the water is not used for drinking for humans or live stock. Also you need to ensure that the water is not used to water crops as well. If there are issues of Drinking Water, Fisheries, Live Stock, Farming or Organic Farms in the area or down stream, it will pose multiple issue in using an aquatic friendly herbicide. Permissions will need to be gained from the local Water Authority and people who could be affected further down stream. 

Hand Pulling Himalayan balsam:

With the issues of using herbicides near a water course or the wish to pursue a non herbicide course of eradication. This may mean that your only course of action to control the spread of Himalayan balsam will be to pull it up by hand. This is easily done because the roots are so shallow. 

By using the hand pulling method, this will need to be done annually before the plant comes into flower and forms seed pod. This will eventually interrupt the regeneration cycle, because the plant relies on spreading it's seeds to continue to survive and regenerate. 

Seeds spread in one year will germinate over the next few years. So it won't be a problem solved quickly. But you will eventually exhaust the the seeds ability to regenerate new stems.

All plants that have been hand pulled should be burnt on site. This is to prevent the spread of the plant to new sites.

If you have an 'Invasive Plant Species' problem on your property. Don't delay getting in touch, because you need a treatment plan putting into place. 

Treating these problems is not a quick process, but it doesn't need to be that expensive if you act sooner, rather than later.

Jamie's Garden Service is fully NPTC Qualified and Insured to supply Invasive Plant Species Weed Control Services to Commercial and Residential Sites. 

Rhododendron

(Pictured: Rhododendron Ponticum which is one of the most common varieties in the UK).

Rhododendron (from Ancient Greek: Rhódon "rose" and Déndron "tree") is a non-indigenous genus of various species of woody plants in the heath family (Ericaceae), either evergreen or deciduous, and found mainly in Asia, although it is also widespread throughout the highlands of the Appalachian Mountains of North America. It is also the national flower of Nepal as well as the state flower of West Virginia and Washington in United States, and state tree of Sikkim and Uttarakhand in India .

Rhododendron has become a very popular plant for people to have in their garden due to its evergreen nature and brightly coloured flowers that can bloom from late winter through to early summer. There are many different varieties of Rhododendron and a wide range of different coloured flowers (Reds, Pinks, Orange, Yellows, Purples, Blues, Cream, White etc) . There are hundreds of different varieties of Rhododendron and it would be impossible to cover all of them here.

A brief description of Rhododendron is a genus of shrubs varying from small to (rarely) large trees. The smallest species growing to 10–100 cm (4–40 in) tall, with the largest species Rhododendron Protistum var. giganteum, reported to grow to 30 m (100 ft) tall.

The leaves are spirally arranged; leaf size can range from 1–2 cm (0.4–0.8 in) to over 50 cm (20 in), exceptionally 100 cm (40 in) in Rhododendron Sinogrande. They may be either evergreen or deciduous. In some species, the undersides of the leaves are covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. There are alpine species with small flowers and small leaves, and tropical species such as section Vireya that often grow as epiphytes. Species in this genus may be part of the heath complex in oak-heath forests in eastern North America.

They have frequently been divided based on the presence or absence of scales on the abaxial (lower) leaf surface (lepidote or elepidote). These scales, unique to subgenus Rhododendron, are modified hairs consisting of a polygonal scale attached by a stalk.

Rhododendron are characterised by having inflorescences with scarious (dry) perulae, a chromosome number of x=13, fruit that has a septicidal capsule, an ovary that is superior (or nearly so), stamens that have no appendages, and agglutinate (clumped) pollen.

Both species and hybrid rhododendrons (including azaleas) are used extensively as ornamental plants in landscaping in many parts of the world, including both temperate and subtemperate regions. Many species and cultivars are grown commercially for the nursery trade.

Rhododendrons can be propagated by air layering or stem cuttings. They can self-propagate by sending up shoots from the roots. Sometimes an attached branch that has drooped to the ground will root in damp mulch, and the resulting rooted plant then can be cut off the parent rhododendron.

Many species of Rhododendrons are often valued in landscaping for their structure, size, flowers, and the fact that many of them are evergreen. Azaleas are frequently used around foundations and occasionally as hedges, and many larger-leafed rhododendrons lend themselves well to more informal plantings and woodland gardens, or as specimen plants. In some areas, larger rhododendrons can be pruned to encourage more tree-like form, with some species such as Rhododendron arboreum and Rhododendron Falconeri eventually growing to 10–15 m or more tall.

Rhododendrons flowering is a magnificent springtime spectacle – but they can be aggressively invasive to some of our finest and most precious countryside with catastrophic impacts on wild plants and animals.

One of the more common varieties Rhododendron Ponticum was first introduced to Britain, probably from Spain or Portugal. It was originally introduced by Conrad Loddiges as seed in 1763 to be used as a cultivated flowering plant in gardens, parks, and estates as a horticultural exhibit, but was also extensively planted in western Victorian hunting Estates under woodland canopies and on heathland areas to provide shelter for game species. They also used Rhododendron Ponticum as a rootstock for grafting scions from less hardy but more colourful stock from places such as China and the Himalayas.

But the shrub has often spread out of control with huge damage to many native woodlands, heaths and other wild places like the Snowdonia national park where they have spent millions trying to remove and eradicate it. It is estimated that the plant now covers 98,700 hectares, roughly 3.3 per cent of Britain’s total woodland, a report by the Forestry Commission found, and Scotland has been hit particularly hard, where it covers 53,000 hectares.

It has been determined through DNA analysis of naturalised ‘wild type’ Rhododendron ponticum, that most if not all of the invasive bushes in the British Isles originate from the Iberian Peninsula. A degree of backcrossing with the two species R. maximum and Rhododendron Catawbiense, proves that our invasive Rhododendron Ponticum plant in the British Isles is a hybrid species.

The main problem with Rhododendrons is that they grow into huge bushes with thick vegetation that blocks out sunlight and smothers most other wild plants and trees, stopping them from growing or regenerating. Its leaves are toxic to animals and repels wildlife from earthworms to birds. Many bushes have become infected with the highly pernicious tree disease called sudden oak death that threatens many types of trees and shrubs. Outbreaks of the disease in the UK, especially on larch trees, have often been linked to Rhododendron Ponticum.

The flowers of Rhododendron are very attractive to insects, particularly Bumble Bees. In the main flowering period of May/June, the exotic showy blooms monopolize the attentions of pollinating insects, virtually to the exclusion of all others. This means that the flowers of native plant species in the vicinity suffer from a lack of pollinating insects. As a result they may not successfully set seed. This is yet another way that Rhododendron may be detrimental to competing native vegetation.

All of this means that areas dominated by Rhododendron have an exceedingly impoverished fauna in comparison to native habitats, both in terms of species and of biomass. If there is little eating the Rhododendron, then it follows that there are few or no carnivores eating these herbivores and so also, few top carnivores. Song birds which feed on either seeds or invertebrates are reduced to trying to survive in smaller numbers by feeding in areas above or adjacent to the Rhododendron. Once the song bird populations decline so do species such as Sparrow Hawks which prey upon them.

Each plant can produce one million or more tiny seeds each year that spread in the wind, and it also spreads with massive tangles of branches rooting in the ground. The plant is incredibly difficult to get rid of by digging up or using herbicides. Snowdonia national park and several other sensitive areas have tried to destroy the invading rhododendron involving hundreds of people over many years digging up the plant. It’s expensive, time-consuming and takes years to completely eradicate.

Legislation:

Rhododendron Ponticum has now become classed as an invasive species of plant in the United Kingdom and Ireland. Rhododendron Ponticum is now covered by the Wildlife and Countryside Act 1981. It is listed under Schedule 9 of the Act and Section 14 of the Act states that it is an offence to plant or otherwise cause the species to grow in the wild.

Control Options:

Control of Rhododendron Ponticum as an invasive species in woodland is often the first operation in the restoration of native habitats. The choice of control or eradication method can influence the recovery of the site, and this should be considered when planning any control operations. The control options that can be undertaken include the complete or partial removal of Rhododendron Ponticum shoots before herbicide control will allow faster re-invasion of plants, especially if coupled with ground disturbance. The speed of recovery is also dependent on a local source of viable propagules.

Effect Control Measures:

The use of stem injection as a control method has been successfully trialled in Wales and Western Scotland.

The removal of the largest plants has until now posed the greatest challenge and expense to estate managers, their continued presence in the environment the greatest threat to the species’ long-term eradication, as mature bushes can produce up to a million seeds a season, continually re-invading surrounding habitats.

Injecting herbicide directly into the stems of large Rhododendron results in their death within six months. Not only is the dead material then easier to remove, but the application of the herbicide is more precisely targeted than in traditional methods, uses less product producing overall cost savings.

The National Trust for Scotland, Inverewe Gardens that are famous for their extensive Rhododendron collection, has recently been using the stem injection method to control seedlings from their hybrid Rhododendron Ponticum windbreaks invading surrounding woodlands.

Full Control Measures:

Current methods of management and disposal for Rhododendron Ponticum include the following:

Mechanical Control: Heavy digger machinery with long hydraulic arms are best for digging up the deep root material and crushing branches. However you do need to dig out all the root system to prevent re-growth. This is a very difficult operation because the roots will be very deep and can be wrapped around and under rocks. You will need to ensure that all root material is removed to prevent regrowth. (Good Operator knowledge of the machinery and habitat is key to ensure the heavy machinery does not compact valuable forest flora and fauna). Mechanical clearance must be followed by repeated herbicide applications for at least two years to prevent re-sprouting and recolonisation.

Chemical Control: spray cut stems with chemicals, although surrounding vegetation may be affected by this.

Disposal through production of biochar. This could be achieved by on-site charcoal production using modern portable reactors to reach optimum conditions. This can lead to a yield of 25-33% of the original material.

Biological Control: There are few natural enemies associated with Rhododendron Ponticum even in its native range. Research is currently being undertaken into biological control, including the proposed use of a rust fungus from Portugal. This is currently being evaluated in other countries and is a possible cheaper, simpler alternative approach to chemical and mechanical control.

Once Rhododendron Ponticum has been removed from an area, a study by Maclean et al (2017) has shown that understorey plant community composition does not return to the state pre-invasion, even decades after the removal of Rhododendron Ponticum. The overall conclusion from the study states that native plant communities showed no signs of returning to pre-invasion conditions up to thirty years after the removal of Rhododendron Ponticum. It is suggested that restoration should focus on aiding the arrival of forbs and grasses under these circumstances, rather than altering the condition of the soil.

Toxicity of Rhododendron:

Potentially toxic chemicals, particularly 'free' Phenols, and Diterpenes, occur in significant quantities in the tissues of plants of Rhododendron species. Diterpenes, known as Grayanotoxin, occur in the leaves, flowers and nectar of Rhododendrons. These differ from species to species. Not all species produce them, although Rhododendron ponticum does.

These toxins make Rhododendron unpalatable to most herbivores. Phenols are most concentrated in the young tissues, such as young emergent leaves and buds. This provides a primary defense against herbivores, before the tissues have acquired the added deterrent of physical toughness found in older tissues. Young emergent leaf buds have the additional protection of a sticky exudate which also contains phenols. This physically discourages small invertebrates from eating the buds, because they get stuck in the exudate. Its poisonous nature must act as a further discouragement.

People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers. Xenophon described the odd behaviour of Greek soldiers after having consumed honey in a village surrounded by Rhododendron ponticum during the march of the Ten Thousand in 401 BC. Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BC during the Third Mithridatic War. Later, it was recognized that honey resulting from these plants has a slightly hallucinogenic and laxative effect.

Cases of human poisoning are also known. Most are caused by the consumption of honey produced from Rhododendron flowers. This is known as 'Mad Honey Disease', or 'Honey Intoxication'. Cases of this have been recorded from as far back as 400 BC. It results in relatively short-lived intestinal and cardiac problems and is rarely fatal. The severity of symptoms depends on the amount of contaminated honey consumed. It is worth thinking carefully about the siting of bee hives if Rhododendron is a prominent feature of the area.

The suspect Rhododendrons are Rhododendron Ponticum and Rhododendron Luteum (formerly Azalea pontica), both found in northern Asia Minor. Eleven similar cases have been documented in Istanbul, Turkey during the 1980s. Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant, although most horses tend to avoid it if they have access to good forage. The effects of Rhododendron Ponticum was mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution. It was also mentioned in the third episode of Season 2 of BBC's Sherlock, speculated to have been a part of Sherlock's fake death scheme.

Grazing animals are discouraged from eating Rhododendron foliage because of its toughness and unpalatability. The unpalatability is learned and cases of poisoning may result in animals such as sheep and cattle if they ingest sufficient quantities because of extreme hunger or inexperience. The general toxicity of Rhododendron to herbivores means that it cannot generally be controlled by grazing.

In summary: Rhododendron is an introduced species. It is highly invasive. It destroys habitats and thus whole colonies of native plants and animals disappear. Because it is so expensive to control and physically prevents access, land has been abandoned. However such areas can be restored but reinfestation must be prevented.