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© E. Delbart
© E. Delbart

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Spiraea douglasii

Identity

Common name: Hardhack steeplebush
Origin: North America
Plant type: Shrub
Life cycle: Perennial
Code of conduct: Annex II
Invasive status (ISEIA protocol): Black list
Main ornemental function: Ornamental shrub

Description

This factsheet is common for 3 species of North American Spiraea's : S. alba, S. douglasii and S. x bilardii. These are rhizomatous shrubs, with a height of 1 to 2 m, multi-stemmed, rising at the base. Forming single indivuduals or dense thickets. S. douglasii has small, dark pink flowers, grouped in dense conical panicles. Single leaves, toothed only along the superior half. Tomentose beneath.

Spread

In Belgium, reproduction modes of North American Spiraea's varies with species and environment. S. douglasii reproduces sexually (seeds) and vegetatively (rhizomes, sprouting, cutting). High lateral expansion abilities through rhizomes. Rhizomes can extend to several meters from the parent plant. Germination rate of S. douglasii seeds is low (between 8 and 15 %).

Habitat

North American spiraea's colonise riverbanks, wet woodlands, clearcuts, forest edges. They grow on sandy or loamy soils, rather moist and acidic. Also planted in hedges, garden and parks. S. douglasii colonises forests habitats on sandy and moist soils. Also found in bogs.

Impact

Species classified A2 in Belgium. Spiraea species are fast-growing rhizomatous species. They easily form dense monospecific thickets which outcompete native flora and may reduce plant diversity. S. douglasii can invade habitat of high conservation value like wetlands and bogs. There are invaded sites in nature reserves in Flanders (Northern Belgium). The ecology of S. douglasii seems similar to another Spiraea considered as invasive in Europe (Poland and Germany) : S. tomentosa, which invades wet heaths, wetlands, woodlands on sandy acidic soils. S. tomentosa is also present in Flanders. For more information about this species, click here

Recommendation

(1) Avoid planting this species near wet habitats, rivers and bogs, espacially in the vicinity of protected areas (natural reserves, Natura 2000 sites, etc.) ; (2) cut the flowers at the end of flowering and before fructification in order to avoid seed dispersal ; (3) put a rhizome barrier in order to limit lateral expansion.

Possible native alternative

Main ornemental function

Ornamental shrub

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