Invasive plant: horizontal cotoneaster

Horizontal or creeping cotoneaster (Cotoneaster horizontalis) is a shrub native to China that was introduced to Europe at the end of the 19th century. For a long time, it remained confined to parks and gardens, where it is appreciated for forming a dense ground cover that blankets rock gardens and stabilises slopes.

It is a hardy shrub of the Rosaceae family, whose prostrate branches ramify densely on either side of the main stem. The white-pink flowers bloom between April and June and develop into small red spheres 5–6 mm in diameter. These fruits, which are toxic to humans, are appreciated by birds and thus contribute to seed dispersal (3 stones per berry).

Cotoneaster is highly susceptible to fire blight, a disease that is dangerous for several botanical species of the Rosaceae family, particularly apple and pear trees. The responsible bacterium (Erwinia amylovora) is highly contagious and is easily transmitted from one plant to another via wind, rain, birds and especially insects. Bees, which are attracted to cotoneaster flowers because of their abundant nectar production, are therefore unintentionally involved in spreading the disease in orchards. For this reason, until last year, restrictions on the movement of hives were in force in Valais, as the canton was classified as a protected zone. Although these measures have been lifted since 2022, as the disease could not be eradicated, it is recommended, in order to avoid unnecessary contamination, to keep hives closed for two days prior to relocation, allowing time for the bacterium to be neutralised.

This already controversial shrub currently represents another threat, as it exhibits invasive characteristics: having escaped from gardens, it is beginning to spread into meadows and forests, thanks to both sexual reproduction (seeds dispersed by birds) and vegetative reproduction (creeping branches that can root and produce new independent plants). It is thus colonising dry grasslands, pastures and forest clearings up to elevations of 1000 m. Large, dense and impenetrable stands smother local flora in habitats of high ecological value.

Bees, unaware of all these drawbacks, continue to feed on cotoneaster nectar, which represents an excellent spring food source, particularly in urban environments. The honey derived from it is included in the composition of mixed-flower honeys.

Sources

Infoflora.ch

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

www.vs.ch/agriculture