NON-native invasive plants are to feature in a show garden at this year’s RHS Chelsea Flower Show – so what are they and how can we control them?
When we think of invasive plants, alarm bells might ring at the mention of Japanese knotweed or giant hogweed - but can the same be said about buddleia, bamboo or montbretia?
Yet these – and other – common plants will be coming under the spotlight at this year’s RHS Chelsea Flower Show (May 22-26), in a garden of invasive plants, created to highlight non-native species that ‘escape’ from gardens up and down the UK and the damage they can do once let loose.
To raise awareness of the issue, experts from the Property Care Association’s Invasive Weed Control Group are developing the ‘Enemy Within’ garden to showcase 14 of the good, the bad and the ugly invasive non-native plants currently thriving in gardens across the country.
These include montbretia, false Virginia creeper, Japanese rose, three-cornered garlic, the garden lupin, buddleia and cotoneaster.
Professor Max Wade, chairman of the PCA’s Invasive Weed Control Group, says: “We know from studies of today’s invasive weeds that it can take decades to become a national problem after escaping from gardens.
“For instance, giant rhubarb was first seen outside of gardens in 1908 and it wasn’t until about the turn of the century that it became invasive, while Japanese knotweed took from 1886 to around 1940 to start its ascendancy.
“Based on this, we should consider that not only is tomorrow’s Japanese knotweed growing in gardens today, but we are busy planting the follow-on generation to perpetuate the process.”
Steve Hodgson, chief executive of the PCA, says: “We don’t have to allow invasive plants to become such a big problem if we act responsibly. Non-natives which find niches in habitats, tend to grow without competition.
“If we are responsible about how we manage what we’ve got, we should be able to avoid the problems we see with things like Himalayan balsam, giant hogweed and knotweed.”
Other plants that have the same potential include:
There’s been a lot of bamboo planted over the years, thanks to its popularity in gardening makeover shows and horticultural fashion generally, says Hodgson.
If they are left unchecked they can be just as damaging as anything else. “Watch it, manage it and if in doubt, remove it,” he advises. “There are well-established professional organisations that can help if you can’t do it yourself.”
“We have a massive problem with the butterfly bush,” he says. “It’s a real nightmare. Anecdotally, it causes more structural damage to buildings than anything else.
The fact is, if it is left close to buildings, the buddleia roots are so strong they will pull the masonry apart. It starts from a seed.” Plant a buddleia near to your home at your peril. If you see any seedlings which have taken next to the house, pull them out, he advises.
“Leave it alone it will spread like wildfire. In most domestic gardens it will out-compete the other plants in the flowerbed - and when you get it into a wild environment, it does exactly the same thing.”
You can dig the corms of montbretia up to keep it in check – but don’t dispose of it on your compost heap because it’s likely that wherever you spread the compost, the montbretia will follow. One solution is to burn the corms, he says.
Pennywort and water fern are problematic, he says. Reports of the invasive plant floating pennywort in rivers and lakes have reached a record high, officials are warning. “They often arrive because somebody’s cleaned out a fish tank.
They were imported and used in aquariums and fish tanks and people clean them out and chuck them into the nearest pond, and before you know it, they are filling up rivers and ponds. They do a huge amount of environmental damage.”
This is a massive problem, because if you disturb it and then leave fragments under new structures, it will grow back through an extension or a patio or into a conservatory.
The RHS has just extended its advice to include the role of professional firms to eradicate knotweed, warning that DIY attempts at treatment or removal may not be successful.
For keen gardeners who wish to attempt to treat or dig out Japanese knotweed themselves, the RHS warns the plant is very deeply penetrating and success is not assured.
Also, it is classed as ‘controlled waste’ and therefore requires careful disposal at licensed landfill sites.
How do you get rid of the corms, runners and rhizomes of invasive plants?
“The biggest issue is to be really sensible about disposal.
You can’t chuck this stuff in a skip because it will go to landfill and cause a problem somewhere else. Don’t put it into cold composting. You might have to burn it,” says Hodgson.
“The difficulty is the corms. If your garden waste bin goes to hot composting, that should be okay.
But if you don’t know where it goes, think it through. Most skip companies will do it for you, but it is very expensive.”
If you want these plants in the garden, put them in pots so they are contained, he advises.
“If you containerise plants which spread by runners or rhizomes, you will have a hugely restricting ability.”
He does not recommend strong weedkillers.
However, advice generally given about getting rid of bamboo and other invasive plants involves using a combination of digging and using of the strongest forms of glyphosate available to gardeners.
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