Spiraea species: more information needed on their impact to the environment
Although North-American Spiraea species (Rosaceae) are rarely considered invasive, they are listed as invasive alien plants in Belgium. In this country, Spiraea alba, S. douglasii, S. tomentosa, and the hydrids (e.g. S. x billardii) were introduced for ornamental purposes during the 19th century and are still used for such purpose. They were first observed into the wild in Belgium in 1803. These Spiraea spp. are shrubs growing up to 2.5 m tall. Leaves are alternate, simples, dentate, lobate or entire. Flowers are white, rose or red, gathered in panicules, ombels or grappes. Fruits are narrow clusters of 5-parted dry capsules, they are persistent.
The distribution of these species in the EPPO region is as follows (indicative information from DAISIE and EPPO databases):
- Spiraea alba: Belgium, Denmark, Estonia, France, Germany, Ireland, Latvia (invasive), Lithuania (potentially invasive), United Kingdom.
- Spiraea douglasii: Belgium, Denmark (invasive), France, Germany, Ireland, Latvia (invasive), Slovenia, Sweden, United Kingdom.
- Spiraea tomentosa: Belgium, Germany
Flowering occurs in mid-summer. Flowering and fruit production depend on light availability. In Europe, seed production seems dependant upon climatic conditions and favoured by fire events. These species reproduce very well vegetatively. They form dense monospecific stands where they are planted; stems and rhizomes carried by water can regenerate. The plants are also spread by human activities through soil transport and garden wastes.
These species develop in the following habitats and conditions:
- Spiraea alba: originally a wet forest edges plant, it is found along river banks and ponds. The plant preferably naturalizes in alluvial conditions, on cool and well aerated soils. It resists temporary anoxy and winter floods.
- Spiraea douglasii: as S. x billardii, this plant is found on a wide range of habitats, on sandy soils.
- Spiraea tomentosa: it grows in peaty heaths, wet and acid sandy soils.
Impacts of these species on the indigenous flora need to be further documented. Spiraea spp. can form continuous monospecific stands over 1 ha, particularly where they have been present for many decades. On these sites, the structure of the ecosystem is changed since only tall species are found. In Belgium, it has been observed that S. douglasii was outcompeting Myrica gale (Myricaceae), Phragmites australis (Poaceae) and bryophytes. Spiraea spp. are likely to have a negative impact on regionally endangered species because they are often found in habitats of concern for nature conservation (heaths, wetlands, etc.).
Personnal communication Etienne Branquart, Belgian Biodiversity Platform. http://firstname.lastname@example.org
Delivering Alien Invasive Species Inventories for Europe (DAISIE). http://www.europe-aliens.org/
FloraWeb Website. http://www.floraweb.de/pflanzenarten/artenhome.xsql?suchnr=7043;
NOBANIS - Network on Invasive Alien Species. http://www.nobanis.org/