The first time I came across Impatiens glandulifera, I had no idea what it was. I didn’t know too many plants back then and it had appeared along both sides of a path near Cramond in Edinburgh since the last time I had walked along there. It was in full flower when I encountered it and the what struck me most strongly was the scent: walking between the plants on the path it was rather overwhelming, a sickly, cloying scent. The bright pink flowers, the large leaves and size of the plants were impressive too and it didn’t take me long to find out what it was.
Impatiens glandulifera, known as Himalayan balsam, Indian balsam, policeman’s helmet and jewelweed, belongs to the Balsaminaceae family: the touch-me-not family. It is not native to the UK and the species originates from the Himalayan areas of Pakistan, India and the Kashmir region. It was introduced, as so many non-native species have been, as an ornamental plant, in 1839. It escaped from cultivation and has since become a major invasive species, not only in the UK but in several other European countries. It grows particularly along riverbanks and damp and disturbed places and covers large areas of ground in dense blankets, making it hard for other plant species to grow. You will often see entire stretches of riverbank clothed with this plant.
Himalayan balsam is an annual plant and grows very large for an annual species: up to two metres high or even more. Despite its large size its root system is fairly shallow, only to about fifteen centimetres deep. This makes it relatively easy to pull up, an important factor for its control (more on that later). Adventitious roots tend to sprout from close to the base of the plant and on the leaf nodes and leaf stalks can be found red, stalked glands: extrafloral nectaries. The leaves themselves are lanceolate and either grow in whorls of three or in opposite pairs. The ridged stem is straight and slightly succulent, normally red-tinged. The midribs and teeth of the leaves can also be red-tinged, giving the plant a colourful look. The large flowers are commonly bright pink, with pale pink and purplish forms also (and occasionally white), and they have an interesting structure; have a closer look at some and you’ll see where the name ‘policeman’s helmet’ came from. The plants begin flowering around June and can continue into October. Once pollinated, the flowers form fruit capsules. These capsules are one of the reasons this species is so successful at spreading.
These seed capsules build up internal pressure as they ripen, and eventually a touch or a shake of wind causes them to explode open, throwing the seeds away from the parent plant. They can be flung several metres by the force of the pod splitting. This is a very effective method of seed dispersal, especially beside moving water that can carry the seed away from the parent. This explosive seed dispersal mechanism has earned the plants in the Balsaminaceae the name of ‘touch-me-not’.
Himalayan balsam’s invasive habit can impact on other vegetation through its ability to grow rapidly, colonise and shade out habitat. It is also thought that it may attract pollinating insects at the expense of native plants. This species is listed under Schedule 9 of the Wildlife and Countryside Act 1981, making it an offence in all of the UK countries to plant it deliberately or cause it to grow in the wild. There are many initiatives aimed at controlling its spread and eradicating from defined areas. One of the most effective methods of control is to pull the plants up before they flower and set seed (once the fruits are ripe, pulling the plants up will just result in assisting the seeds’ dispersal). There are a lot of local groups that organise ‘Balsam-bashing’ sessions where volunteers pull up balsam plants to aid in their control. Fortunately the seeds cannot survive for long in the soil – up to a year and a half or so – and if enough time and resources can be devoted to removing the plants this can be effective; however, seeds can come in from elsewhere and it can be difficult to get to all the plants.
A few years ago, a biological control method for use against Impatiens glandulifera was approved for release after much research. This is a rust fungus, named Puccinia komarovii var. glanduliferae, isolated from the plants’ native range in the Himalayas. The rust weakens the plants at various stages of their lifecycle and reduces the amount of seed they produce, thus reducing the populations over time. It is due to be released in Scotland this year, so keep an eye out for telltale dark spots on the underside on Himalayan balsam leaves…
– Heather Forbes
https://www.cabi.org/Uploads/projectsdb/documents/32944/2016-001.pdf [accessed 12/10/20]
https://www.cabi.org/isc/datasheet/28766 [accessed 12/10/20]
https://himalayanbalsam.cabi.org/ [accessed 12/10/20]