Invasive plants: North American goldenrods

Among the various goldenrod species with invasive characteristics are the late or giant goldenrod (Solidago gigantea) and the Canadian goldenrod (Solidago canadensis). It is rather difficult to distinguish between these two invasive species originating from North America, especially as they readily hybridise with one another.

Canadian goldenrod can be distinguished from giant goldenrod mainly by its velvety leaves and its greater height (up to 2.5 m), contrary to what the name of the second species might suggest, which rarely exceeds 1.2 m. The inflorescences, composed of numerous golden-yellow flowers arranged in drooping clusters, are however difficult to tell apart. These two perennial plants share the same habitats (woodlands, abandoned land, wetlands, roadsides and railway embankments, slopes, meadows, etc.) and are highly nectar-producing.

Canadian goldenrod was one of the first plants imported into Europe, having been introduced into the botanical gardens of England as early as 1645. It then spread rapidly throughout the continent as an ornamental plant. Very common in Valais up to an altitude of 800 metres, it flowers from July to September, whereas its related species blooms from August to October and is much less widespread. Both species appear on the “black list” of plants to be controlled because of their dominant growth, which is detrimental to local flora.

In their native range, these two perennial herbaceous plants give rise to an important nectar flow, producing a monofloral honey with a distinctive flavour. In our regions, however, they must be controlled, especially as there exists a native goldenrod species, the common goldenrod (Solidago virgaurea), which is also nectar-rich but not invasive; smaller in size (10–80 cm), with larger and more widely spaced flowers, it is unlikely to be confused with the exotic species. Its abundant and late flowering (July to October) provides an interesting food source for bees at the end of the season and does not need to be restricted!

Sources

Infoflora.ch   

J. Piquée, Les plantes mellifères mois par mois, 2014

T. Silberfeld, C. Reeb, Les plantes mellifères, 2016