''Scleranthus annuus'' is a species of flowering plant in the family

Caryophyllaceae Caryophyllaceae, commonly called the pink family or carnation family, is a family of flowering plants. It is included in the dicotyledon order Caryophyllales in the APG III system, alongside 33 other families, including Amaranthaceae, Cactacea ...

known by the common names German knotweed and annual knawel. It is native to Europe, Asia, and North Africa, and it is known throughout the rest of the temperate world as an

introduced species An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...

and a common

weed A weed is a plant considered undesirable in a particular situation, "a plant in the wrong place", or a plant growing where it is not wanted.Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24. ...

. It grows in many types of habitat, often in disturbed areas.

Description

''Scleranthus annuus'' is an annual or biennial herb with low, much-branched spreading stems up to long growing from a

taproot A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproo ...

. The leaves are needle-like or linear in shape with sharp, stiff points. They are oppositely arranged in pairs about the stem and are fused together at the bases. Flowers occur in pairs or small clusters of up to five. The flowers lack petals but have bell-shaped calyces of green petal-shaped

sepal A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 The term ''sepalum'' was coined b ...

Distribution and habitat

It grows in soil pockets among rocks, on bare places, on disturbed sandy soil on heaths, in arable fields, in sand pits, in quarries, and occasionally on shingle on the coast or beside rivers.

Life cycle

The plant seems to have three different life strategies; some seeds germinate in the autumn and overwinter as small seedlings; others overwinter as seeds and germinate in the spring, flowering the same year; and some germinate in the spring but do not flower until the following year. The flowering period for all three groups extends from late May to late August. The flowers make little effort to attract insects for pollination (no showy petals, little nectar), and most are self-pollinated.

Status

This plant is in decline in many of its previous habitats and is no longer present in many inland sites in Britain and Ireland. This may be due to changes in agriculture practices. It is listed in the Great Britain Vascular Plant Red Data List as endangered and is listed as a Northern Ireland Priority Species.

References

External links

USDA Plants Profile

Flora of North America

Washington Burke Museum

Jepson Manual Treatment: ssp. ''annuus''

Databases

* * * * {{Taxonbar, from=Q159192 Caryophyllaceae Plants described in 1753 Flora of Africa Flora of Asia Flora of Europe Taxa named by Carl Linnaeus