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Japanese knotweed is no longer the only ‘invasive’ in the spotlight

For a long time, each summer invasive plant coverage in the press was devoted to one plant, Japanese knotweed. However, times have changed as people have started to recognise the problems caused by other non-native, and some native, plant species.

Surveyors, estate agents, landowners, and now many residential property owners have become more aware of the associated issues caused specifically by Japanese knotweed rhizome and roots. They really are at the heart of the problem. The means by which the knotweed propagates and spreads, causes damage to buildings, and the reason excavation costs are so high. Soil which contains knotweed rhizome is classified as ‘controlled waste’ and landfill charges and the tax on this waste are key drivers in the cost of Japanese knotweed removal.

Previously, perhaps the tendency was to look at other invasive plants and consider whether the associated problems were directly comparable with Japanese knotweed. There are some that are comparable, especially other knotweeds, however many have quite distinct characteristics that create different issues.

Other plants making headlines

Recently, we were contacted by The Sun newspaper to contribute to an article on Buddleia davidii, after the plant suddenly started to gain negative publicity on social platforms. PBA Solutions has long expressed the need for buddleia control, and are now contacted more than ever before about the plant. Buddleia is a prolific seed producer with seeds, in the thousands, able to germinate in the most inhospitable conditions. These abilities combined with the plant’s strong fibrous root system make it is easy to understand why buddleia should be considered a cause for concern.

In fact, the damage being caused buddleia is in plain sight, but often goes unnoticed. Look up at chimney stacks, the sides of buildings and railway arches – the plant you can see growing from them is probably buddleia. And, as all these examples exist at height, sometimes the solutions aren’t straightforward.

Invasive plants that propagate by seed, like buddleia, have an ability to spread further and more easily than the Japanese knotweed. In the UK, Japanese knotweed is dependent on rhizome to spread, as only a single sex of the plant is present. Buddleia is highly recognisable, many have the plant in their gardens, but another common seed-spreading invasive is less recognised.

Himalayan balsam has become a cause of concern for DEFRA, who is working with water companies to tackle the spread of the plant along waterways. Fewer are aware of this invasive species, there is frequently a failure to identify Himalayan balsam, and as a consequence it often goes unchecked. Given the time to do so, seeds disperse along the banks of waterways inflicting riparian habitat changes by impacting soil stability, leading to bank erosion and a negative impact on water quality.

The common issue to resolve when attempting to tackle Buddleia davidii and Himalayan balsam is the seedbank both plants leave. These seeds can remain viable for two years, and in the case of buddleia sometimes even longer. Whether the solution involves cutting, removal or herbicide treatment, consideration needs to be given to either scraping away the seedbank or returning for the appropriate number of years, prior to seed dispersal.

Himalayan balsam in flower

Unsurprisingly, Japanese knotweed and Himalayan balsam are listed as Schedule 9 non-native species in the Wildlife and Countryside Act [1981]. These are species where there is a responsibility not to ‘cause the plant to grow’ in the wild. Although native to China, Buddleia davidii is not currently listed amongst the Schedule 9 plants; however, DEFRA acknowledge the damage caused each year by the plant.

It’s not simply non-native plants causing problems

Some UK native plants have what could be considered ‘invasive properties’, Natural England call bracken an ‘aggressive competitor’, the term ‘invasive’ is typically reserved for non-native plants. However, Ragwort is another UK native that has these properties and more. Capable of producing 50-60 seeds per plant, Common ragwort can quickly cover a field or paddock creating problems for landowners or farmers with livestock.

When ingested ragwort is poisonous to horses and cattle, and can cause serious liver damage. DEFRA has a Code of practice on How to Prevent the Spread of Ragwort, which references the legal obligations of those responsible for land with the plant present.

Ironically, less advice is available for dealing with hemlock, or poison hemlock as it is sometimes known. This is a highly toxic UK native plant that can be fatal to humans and livestock when live (or dried) plant material is ingested. To add to matters, it is a plant often confused with others: cow parsley, chervil and parsnips amongst these.

What happened to one of our favourites?

Thankfully, the majority of the UK’s problematic plants aren’t poisonous, most create complications just by being aggressive and outcompeting native species, in short by being ‘invasive.’ Some species have had a change of fortune over the years. Japanese knotweed was once a new and exotic plant shown off in Victorian times, and now bamboo may yet become the fallen king of bamboo screening.

Running bamboo is usually considered ‘the’ problematic type of bamboo, able to rapidly spread across your garden onto your neighbour’s land. However, there are over 1,000 species of bamboo and closer to 2,000 if you include sub-species. Amongst these are certain clumping species that can also prove challenging to manage and become a source of dispute for neighbours.

Bamboo won’t respect boundaries, and when planting directly into the ground, we advise clients to use a bamboo barrier to block or contain the plant. This will ensure that later there isn’t the need for more drastic bamboo control and removal methods.

Taking steps to deal with invasive and aggressive plants

The above, provides a selection of some of the non-native and native plants that have risen in prominence, for the wrong reasons, in recent years. There are many others, but the solution for dealing with any aggressive or invasive plant starts with understanding how it spreads. Once this is understood, a management plan can be drawn up that takes measures to correctly deal with the particular plant in the given environment, and in line with the budget.

Frequently, the management plan for invasive plants, at least in part, involves the use of herbicides. It is now necessary for anyone using pesticides (or professional plant protection products) in a professional capacity to be registered with DEFRA. This new regulation, coupled with tightening regulation on herbicide use generally, heightens the need to use specialist professionals who invest in pesticide storage, training and record keeping.

By Jon Barton, MD of invasive weed specialists, PBA Solutions.

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