December has been exceptionally wet, and such damp conditions run the risk of spreading invasive weeds
UK homeowners are being urged to check their gardens for a potentially harmful weed following days of heavy rain and floods. The first half of December has been exceptionally wet, with some regions already receiving over half of their average monthly rainfall.
Following the wet weather conditions, experts at Environment Controls have issued an alert for invasive weeds. These conditions are ideal for invasive weeds to spread, which is a significant issue for homeowners, particularly those in high-flooding areas – but also for house developers, ground workers, and construction companies.
The alert has gone out for Japanese knotweed and Himalayan balsam. Floodwaters can transport fragments of Japanese knotweed rhizomes and the buoyant seeds of Himalayan balsam (which can remain viable for up to two years) over long distances, establishing new colonies far downstream and even in your back garden.
A single, small piece of Japanese knotweed rhizome is all it takes to establish a new, highly destructive colony, which can rapidly undermine property foundations and severely impact house sales. If left unchecked, removing invasive weeds can become quite costly.
“Winter weather is increasingly becoming one of nature’s threats as high winds and overflowing watercourses enable invasive plants to spread far beyond their local habitat,” Jennifer Holmes from Environment Controls says. “It’s vital for homeowners to know how to identify Japanese knotweed.”
She says the weed not only impacts house sales, but also those of commercial sectors, land investors, and construction companies. “The issues that invasive plants cause have a bigger financial impact as projects have to grind to an immediate halt,” she added.
“The key is to have land professionally surveyed so that informed decisions are made more easily. Often, more invasive plants than originally suspected show up on a survey – and this can be due to seed and plant drift from storms and floods.”
Japanese knotweed can grow back from tiny pieces of its root that weigh just 0.7 grams. When you pull it out, you might leave behind these little bits, which can cause it to grow back quickly.
In the UK, Japanese knotweed is considered “controlled waste” according to the Environmental Protection Act 1990. It’s against the law to throw it away with regular garden waste or to dump it illegally, and doing so can result in big fines or even legal action.
What does Japanese knotweed look like, and why is it so bad?
Japanese knotweed has tall, hollow stems that resemble bamboo and feature purple spots. They grow in a zig-zag way and have spade-shaped green leaves that change to yellow in the fall. In late summer, it produces small clusters of creamy white flowers, and in winter, it leaves behind brittle, brown canes. All of this originates from unique underground roots, known as rhizomes, which are orange-tinged.
Shoots can grow as much as 10cm each day. Roots can extend several meters in depth and width. Even small pieces of stem or root can regrow, which means that digging them up might actually exacerbate the issue.
Japanese knotweed is harmful because it is a highly invasive plant that can cause significant damage to buildings, roads, and flood defences. It does this by taking advantage of cracks in concrete and tarmac with its strong root system.
It destroys native plants, harms wildlife habitats, and creates significant problems for property sales because it is difficult to remove. Removing it often costs a significant amount of money and requires specialised expertise.
What about Himalayan balsam?
Himalayan balsam is a tall, quick-growing plant that can reach heights of up to three meters. It has unique hollow stems that are soft and often have a reddish colour, which can be brittle.
From June to October, it produces bright, helmet-shaped flowers that are either pink or purple, and they smell sweet. The leaves have jagged edges and red veins, and they end in seed pods that pop open when you touch them.
Himalayan Balsam can be a significant problem because it’s an invasive weed that takes over native plants, especially near water, by forming dense stands that block sunlight and reduce biodiversity. It spreads quickly through its explosive seed pods, known as “touch-me-not,” attracts insects away from local flowers with its extra nectar, and its shallow roots lead to riverbank erosion when the plants die in winter, leaving the soil exposed and unstable.
