We have very few ‘problem’ plants at St Olaves, but one that is currently taking up time and energy is the Himalayan Balsam (Impatiens glandulifera). At first, when we moved in last August, we thought it was a charming plant – with its fragrant pink flowers and majestic stature (it can grow up to 2.5 metres tall). That was before we looked it up on-line. Himalayan Balsam comes originally from south Asia, and like so many invasive species, it was first introduced to gardens as an exotic ornamental by the Victorians. Today, it is a major problem in many parts of Britain, especially on riverbanks where it crowds out native plants, destroying the root systems that would normally bind the bank together. Being an annual (growing from seed each year), Himalayan Balsam dies back completely over the winter, leaving riverbanks (or in the case of St Olaves the hillside), exposed to the elements at their fiercest. It spreads by seeds; which are scattered widely from exploding seed pods in late summer and autumn. Since 2010 it has been an offence to plant it in the wild.
Last year it was already too late to do much to dent the Balsam’s progress at St Olaves, but we removed it wherever we could from the river’s edge and from the streams flowing down the hillside. This year we have made repeated sweeps of these vulnerable areas, pulling the Balsam out by hand. This is easier said than done as Himalayan Balsam is a relative of the Busy Lizzie (Impatiens walleriana), and has the same ability to sprout roots from anywhere along its stem. If it is growing through nettles or bracken it is necessary to follow the stem back to its origin point to avoid the plant being back as strong as ever within a few weeks.
Some people are more sanguine about Himalayan Balsam. The flowers are edible and can be used in salads or to make drinks. The hollow stems can also be used as straws to avoid the use of plastic. The seeds are also recommended as an ingredient in curry.
It is doubtful whether we will ever eradicate Balsam entirely at St Olaves, or manage to eat very much of it. Its seeds can survive 2-3 years before germinating, and it is also well-established on some neighbouring fields where there is currently no attempt at control. Our aim is to remove it from the most sensitive areas (the river and streams), and reduce its hold on the hillside. We would prefer not to use chemical sprays to control it, even on the hillside, because of the abundance of bulbs and biennial flowers such as foxglove – instead we are trying to mimic a grazing regime by strimming it back to about 5-10cm before it has had time to set seed. Many of the strimmed plants will regenerate, but it is hoped that a second strim in the autumn will be enough to prevent them re-flowering before the first frosts kill them off for good. Only over the next few summers will we know whether the Balsam is finally in retreat at St Olaves, or whether additional strategies may be necessary.