Northern Ireland's Priority Species

Scleranthus annuus – annual knawel


Distribution map

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Scleranthus annuus L.
Family: Caryophyllaceae

A species which has a wide natural range in Europe, and which competes successfully as an introduced weed in the Eastern United States (where it is known as ‘German knotweed’), but which appears to be losing its grip as a native species in Northern Ireland.

In brief

  • A species of open sandy soils now mainly found in sand pits, quarries and waste places
  • Declining throughout its range in Britain and Ireland
  • It has undergone a rapid decline
  • Usually self-pollinating
  • Has a long summer flowering season

Species description
It is a member of the campion family, which apart from carnations, pinks and campions, includes a number of smaller, weedier species. This species has a weak tap root from which the branched stems may spread along the ground or ascend to anything up to 20cm or more. The leaves occur in opposite pairs along the stems, have no leaf stalks, are linear to 2cm long but less than 1mm wide, and sharp pointed, so are rather grass-like. Each pair of leaves is joined across the stem by a fine transparent membrane. The flowers occur mainly in congested clusters in the leaf axils. They are numerous, but inconspicuous being small, and green. What appear to be petals are in fact the spreading sepals which are triangular, somewhat spiny to the touch and have a characteristic narrow membranous margin. True petals are entirely lacking.

Life cycle
A population of annual knawel is likely to be divided into plants with three different life strategies: winter annuals, which germinate in the autumn, survive the winter as small, grass-like seedlings, flower the following summer, then die in the late summer; summer annuals which will survive the winter as seeds, and a small proportion of biennials, which grow for two seasons before flowering. Regardless of strategy, flowering occurs from the end of May to the end of August. Even though this is a long flowering season and there would be plenty of time to attract insects to cross-pollinate plants, this species makes little effort to do so – petals are absent and little nectar is produced. Instead, flowers automatically self-pollinate. Seeds may persist for years in the soil. Self-pollination allows for distinct races to develop and several sub-species of annual knawel have been recognised, however their status in Ireland has not been investigated.

Similar species
Probably evicted from many of its former sites in arable fields by herbicide treatments. Its quarry and wasteland sites are often under threat from infilling, nutrient enrichment and conversion to alternative uses.

How to see this species
This is a species which prefers open vegetation on dry, sandy and slightly acidic soils, conditions which in Northern Ireland are most commonly found in quarries, sand pits, sandy arable fields and disturbed wastelands. Contemporary records exist from sand workings around Moyola, but its sites tend to be privately owned.

Current status
This plant was described in 1864 in Professor George Dickie’s Flora of Ulster as being ‘frequent throughout Ireland in fields and dry wastes’. Old records from all over the country seem to confirm this former status. In Northern Ireland, old records are especially from Counties Down and Antrim. It remains most frequently recorded from sand workings in the east of County Down and north of Lough Neagh.

Why is this species a priority in Northern Ireland?

  • Rapid decline (2 per cent per year)
  • It is scarce and Northern Ireland is a stronghold within Ireland.

The BSBI Atlas2000 project was revealing, with a significant decline noted in records of this species throughout its range, both in mainland Britain and Ireland, prompting the classification of this species in the Vascular Plant Red Data List for Great Britain as Endangered.

Threats/Causes of decline
Probably evicted from many of its former sites in arable fields by herbicide treatments. Its quarry and wasteland sites are often under threat from infilling, nutrient enrichment and conversion to alternative uses.

Conservation of this species

Current action

  • None known

Proposed objectives/actions

  • Maintain the number of viable populations of the species.

What you can do
The status of this plant in Northern Ireland is not entirely clear and more work is needed to pinpoint its status. Keep and eye out for this species in any suitable habitats, and if it is suspected, contact the Botany Department of the Ulster Museum or CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256 or email [at]

Further information

The Vascular Plant Red Data List for Great Britain

Smissen, R.D., and Garnock-Jones, P.J. (2002). Relationships, classification and evolution of Scleranthus (Caryophyllaceae) as inferred from analysis of morphological characters. Botanical Journal of the Linnean Society 140: 15-29.

Svensson, L. (1992). Estimates of hierarchical variation in flower morphology in natural populations of Scleranthus annuus (Caryophyllaceae), an inbreeding annual. Plant Systematics and Evolution 180(3-4): 157-180.

Text written by:
Shaun Wolfe-Murphy