Ear to the Ground with Steve Blacksmith: Keeping a great invader at bay

Trying to keep it down: Japanese knotweed
Trying to keep it down: Japanese knotweed

I decided at the beginning of the summer to see what effect I could have on an area of Japanese Knotweed by the river.

This was after attending a talk by the Yorkshire Wildlife Trust about invasive plants.

Two of the worst in our area are the Knotweed and Himalayan Balsam. It occurred to me that I could seriously weaken the plant by regularly cutting it down to the ground. Then, I reasoned, the less vigorous plants could out-compete it.

Why we should feel a need to eradicate alien plants which appear to be crowding out native plants I will leave up to the philosophers.

I was doing well with it. I had broken off all the dead hollow stalks from previous years and piled them up in one place on the flat part of the bank. Then I cut down with the sickle the green shoots which had got to about 60cm (2 feet) high, collected them up and piled them on top of the dead stalk pile. It is illegal to move Japanese Knotweed without consultation with the waste disposal authority that can be reached through Calderdale Council. They have special sites to dispose of it.

So great is the nuisance value attached to it by banks that some are wary of lending mortgages for houses where surveyors have reported it in a garden.

Each week I called in to the riverside and cut, or merely snapped off with my feet the brittle new shoots that appeared. Any pieces that could have taken root I picked up and piled on the heap. I proved that a heap of shoots, even fresh ones, don’t root and start growing when piled on top of each other. Incidentally the young shoots, like fat asparagus, are eaten in Japan. It is sharp-tasting due to its oxalic acid content so probably best not eaten too much.

I had to take a break for two weeks. One week was a camping trip to the Mull of Kintyre, the other was the week before it when I had to work overtime in order to have the week away – one of the downsides of being self-employed. So the Knotweed was left to grow unchecked for two whole weeks. When I went down the plants had regrown to chest height, but they weren’t anywhere near as luxuriant as some nearby I had never touched. I am weakening it, I think, but it would have been better if I had been able to keep it to the “kicking off” stage all summer.

It is often stated that these aliens push out the native plants, but that isn’t always the case. Before my project I listed every other plant that was in my 14 metre x 14 metre square between the track and the Calder. There were 17 species; nothing rarer than Ground Ivy, with its huge Latin name and small blue flowers, and nothing really attractive except an odd plant of Red Campion, but still a good selection of plants. What the tall aliens do spoil is the view. At least now people can see the river as they walk past. I suppose my ideal outcome would be a patch of bluebells in spring giving way to ferns in summer, with shade-loving plants beneath.

A large, low branch of a Sycamore tree spreads across the middle. It is noticeable that the Knotweed doesn’t grow under this branch. Perhaps the solution to the problem is to plant more Sycamores. Not to everyone’s taste, though, with their own reputation as aliens, though they were introduced centuries ago, and first recorded as self-seeding in the wild in 1632. Other trees may be more acceptable, maybe Alders, which are adapted to riverside life. I notice the Knotweed can grow under some natives, for instance the Pussy Willows.