This pretty plant comes out in August. Pretty though it undoubtedly is, it is an invasive foreign plant that blocks the light from native species and completely takes over whole swathes of river bank – its favourite habitat. It needs to be cut down wherever it occurs. I first became aware of this weed in the early 1970s, when it had recently established itself around my local waterways, and before it was aggressively removed by the council from our river banks, which it loves to colonise.
The Himalayan balsam was introduced into England by the Victorians. It was unknown in Norfolk until 1927, and it took many years before it spread to the rivers around Norwich. I must admit I was rather taken with the plant when I first came across it. It was not the pretty pink flowers that attracted me so much as the fact that I could startle the unwary by inviting them to touch the seed pods. On taking up my suggestion, my companion would jump back in horror when the pod exploded in their hands! I had first come across the plant in the midlands, which gave me an advantage when playing this trick on my country cousins from Norfolk. One of its names is Touch-me-not, for obvious reasons.
The Himalayan balsam is my favourite invader of the plant world. Much less attractive is the giant hogweed. Himalayan balsam is just rather over-vigorous; giant hogweed is poisonous. It originates in the Caucasus region of central Asia and was introduced to England in the 19th century as an ornamental plant, although it is not a particularly attractive one. Like the Himalayan balsam it like river banks. The sap is highly toxic and any skin that comes in contact with it becomes inflamed when exposed to light. The effects can last for days. This is particularly unpleasant if the eyes are effected. Luckily is not quite such an invasive plant as the Himalayan balsam.
This plant is native to East Asia, and it has become established in many parts of the world. This plant is the most dreaded of these species, and it is for example illegal to have it growing on your property in Australia. It is particularly unpleasant because of its very strong roots. These can damage concrete foundations and block streams. Because of its extensive root system is impossible to control by cutting it down or digging it up, but it is possible to eradicate it by the use of herbicides. These chemicals can themselves have unwanted side effects, and some success has been achieved in British Columbia by spraying Japanese knotweed with salt water.
These are invasive species in Europe, but plants from our neck of the woods can be a nuisance in other parts of the world. The yellow toadflax is a charming and harmless European wildflower which I used to call ‘Golden Bells’ as a child, and pick them for my mother. It was introduced to North America in the 17th century, possibly to remind early settlers of home. In the USA it can crowd out native species. The same may be said of the teasel, a plant that was used in Europe in the cloth processing industry before modern methods were introduced. It may have been brought into the US for the same reason, but without its native predators and rivals it has become an invasive species there.