regnum-plantae:

Impatiens glandulifera, Balsaminaceae

Since moving to Glasgow I’ve become quite familiar with the beautiful and highly problematic Himalayan balsam, which is present in northern Italy too, but is far more widespread in the British Isles. Introduced in the first half of the XIX century, it was part of a group of exotic, novelty plants of large size, including Japanese knotweed (Fallopia japonica) and giant hogweed (Heracleum mantegazzianum), considered outstanding for their ornamental value. Unfortunately, they all turned out to be highly invasive in moist temperate areas and especially threatening to riparian ecosystems. 

This annual germinates quickly and grows very rapidly and tall -the tallest annual in Britain- forming dense colonies particularly in proximity of water, often smothering entirely any competition. The stems can develop adventitious and aerial roots from glandular nodes -they give the plant its name- anywhere along it, especially when damaged. This means that unlike many other annuals, it can sometimes recover easily even when badly ruined by strong winds and mechanical action. 

The individual, sometimes branched, hollow stems can grow to a considerable size, but the root system is particularly shallow and underdeveloped, generally extending only a few cm/inches below and around the plant, without a main tap root. This is a crucial characteristic: by forming thick colonies on the banks of streams and rivers and by preventing perennial competitors with more extensive root systems to develop, Himalayan balsam simply encourages erosion. Especially during the winter months, banks which are suddenly bare and not held together by roots give way under the pressure of rain and swollen waterways, with the resulting loss of original ecosystems. See the culprit in the photo below?

Its large and attractive lipped flowers, produced from late spring often until late fall, come in a variety of shades from pure white to dark pink. The bilateral symmetry, tunnel shape and ‘landing platform’ are all features which make them irresistible to bees, bumblebees and many hoverflies. Once I spent over an hour by a clump of Himalayan balsam just to take photos of the different visitors. It is an excellent plant to grow in your garden if your aim is to attract/observe highly efficient pollinators, as long as you remove the flowering stems as soon as the seed pods start to develop. 

Attracting huge numbers of pollinators ensures the plant can produce an incredible number of seeds. And here comes one further adaptation to make Himalayan balsam the perfect annual invasive weed: typical of the genus is a capsule which builds up pressure and explodes, projecting the seeds far and at great speed, generally when hit by raindrops or pushed against each other by the wind. For this reason the genus is called Impatiens, and commonly touch-me-not. When the plant grows near water, the seeds that fall in are carried away and end up germinating downriver, so its distribution can change rapidly year by year. 

When looking at non-native, ornamental plants coming from the other side of the world we often tend to forget they generally have a history of traditional uses in their native range, which we simply haven’t had the time to explore. Its abundant seeds are edible and can substitute nuts in many recipes and so are the raw flowers and the young shoots and leaves when cooked. Eating Himalayan balsam is certainly not the best way to control it though, so a few years ago a selective rust fungus, Puccinia komarovii var. glanduliferae, was introduced to research its effectiveness at reducing the population, and gave slow, positive results. The other alternative to biological control is the traditional ‘balsam bashing’, or mechanical destruction of the entire plant by hand. This isn’t necessarily the best option, unless a healthy succession of native perennial species is guaranteed to take over the otherwise bare ground, once again, to avoid erosion and the establishment of other invasive ones.  

It and Giant Hogweed are a known problem in London Ontario; growing along thee riversides choking out any and all other plantlife. Pretty but aggressive fella. Not all monsters are perennials.