Feb

Stellaria media – 6th February 2023-Common Chickweed

Stellaria media, old painting attributed to Johann Georg Sturm and Jacob Sturm; published by Kurt Stüber., Public domain, via Wikimedia Commons. The species belongs to the plant Family of pinks and carnations, the Caryophyllaceae.

Chickweed is included in the book ‘The World’s Worst Weeds’. The authors provide a comprehensive description of this species, covering over five pages of text and diagrams. But does this plant really deserve the title of a ‘Worst Weed’?

It is certainly a very common weed, found all over the United Kingdom in gardens, arable fields, waysides, soil heaps, demolition sites, parklands and cemeteries, usually in fertile soils. It is considered native to Britain, having been first recorded in 1538 by the physician-naturalist William Turner (called “The Father of English Botany”). However, it is presumed to be native to much of Europe and Asia (judging from plant remains found in the stomachs of humans long-buried in peat bogs) but now it is one of the most cosmopolitan of species, found on all continents except Antarctica (but it is an invasive alien on the sub-Antarctic Macquarie Island).

S. media flowers. They are small (sepals 3-5 mm, petals somewhat shorter than sepals) with 5 deeply-divided petals, stamens 3-5 with red-violet anthers, styles 3. Photo: © Chris Jeffree.

Chickweed is a good example of what Philip Grime termed a ruderal species – those that rapidly colonise disturbed ground (Grime et al. 1988). In prehistoric times these ruderals would have been rather rare, although they must have flourished locally at the edges of retreating glaciers and around crumbling cliffs. But now, disturbed patches are frequent, especially in urban areas. They are rapidly colonised by the familiar ruderals: for example Groundsel, Shepherd’s Purse, Red Deadnettle, plantains and poppies. Humans are not always the disturbing agent: Chickweed is found in the tracks of sheep and deer, and naturally disturbed ground such as shingle banks by rivers. Sobey & Kenworthy (1979) described how Herring Gulls disturb the vegetation of their nesting grounds, which then become colonised by ruderal species including Stellaria media.

S. media flowers and fruits. The fruit capsule is slightly longer than the sepals and their stalks tend to point downwards. Photo: © Chris Jeffree.

What are its key credentials as a ‘weed’? Its weedy traits have been extensively reported (see Holm et al., 1977, Turkington et al., 1980, Sobey 1981, Grime et al., 1988), and here is a summary. Stellaria media is a sprawling annual plant (sometimes a short-lived perennial) which tolerates harsh winters, photosynthesises and grows at temperatures below 5 degrees, and even produces flowers and seeds in the coldest months. It doesn’t need insects for pollination, it is self-pollinated and cleistogamous (i.e. pollination occurs when the flowers are closed). The flowers do however open rather briefly, perhaps on only one day, and some insect pollination does occur. Seed production is in the range 2,000 to 13,000 per plant. There is no special mechanism of seed dispersal – the plant apparently relies heavily on the tendency of its seeds to stick to boots, hooves and birds’ feet. Birds and various mammals like to eat it, and thus some of the seeds may be dispersed in faeces. It may also ‘hitch lifts’ in the commercial seed mixtures sold to growers. They germinate in autumn or spring, and a ‘seed bank’ develops in the soil. Some of the seeds remain viable for many years others germinate readily when the temperature exceeds 10 degrees. The young plants have a very fast growth rate, especially when growing in a nitrogen-rich soil (as in a garden or arable field). Flowers and seeds are produced in about 5 weeks. The stem branches repeatedly and forms a mat (up to 1 metre in diameter) and continues to flower. Sometimes the prostrate stems form roots, accessing more nutrients for the rapidly expanding mat. The mat smothers seedlings of other plants and the roots may compete successfully with less aggressive species; there is also some evidence that the plant produces chemicals that inhibit the germination of competitors. There may be several generations within a year, some authors suggesting as many as five.

Upper: Insertion of upper leaves on the stem. The leaves are typically up to 25 mm long. Lower: the stems usually have a single line of hairs down each internode. Photo: © Chris Jeffree.

Seed longevity may be the key attribute to ensure the success of any annual weed. Reliable data on this are hard to find, and there are many claims based on circumstantial evidence and these cannot be relied upon. But in the year 1903, JWT Duvel, who was then an Assistant in the US Department of Agriculture, began an experiment to determine how long weed seeds last when buried in the soil. The observations covered the period 1903-1941 and should have continued much longer but the United States War Department took over the land and the experiment was destroyed. Some 22 % of the Stellaria media seeds were still capable of germinating after 9 years (other sources have claimed up to 60 years). You can view this historic publication, and check the longevity of several other common weeds, here.

Chickweed in winter (February 1st 2023), removed from the garden and displayed on a bench, illuminated by winter sunshine, showing green leaves, closed flowers and the sprawling mat-like growth form. Photo: John Grace.

Holm et al (1977) reviewed the Agricultural significance of this weed. They said it was one of the three most serious weeds of cereals in England, and of many crops in other European countries. But they were writing in the 1970s. Now, many weeds that were rampant in that period are controlled by application of herbicides: most of the crops grown in the UK are sprayed with herbicides several times per year to ensure plenty of food is produced by farmers and available to consumers at an affordable price. It is possible to view the official UK statistics on pesticides here (it’s a data base that takes a while to learn, but well worth the effort). But now, many ‘undesirable’ plant species are becoming resistant, and so new compounds must be developed in a never-ending arms race.

Aerial view of young chickweed plant in winter (February 1st 2023), growing on the pavement next to a wall, rooted in the crack between path and wall. Note a few open flowers in the lower portion of the image. Photo: John Grace.

Stellaria media is a variable species. European authors recognise subspecies and/or biotypes, and the BSBI data base has it as an aggregate species (Stellaria media agg.) but Stace (2019) has none of that. He has Stellaria media as a single species, quite distinct from the Lesser Chickweed (S. pallida) and the Greater Chickweed (S. neglecta). However, the chromosome counts are perplexing, having been variously reported as 2n = 20, 28, 36, 40, 42 and 44. One wonders, how is this possible? It is however thought that its globally wide distribution is a result of the genotypic variation.

So, is Stellaria media really one of the world’s worst weeds? As a gardener, I know many that are much worse. All those with rhizomes or tap roots that go deep into the soil are very hard to remove, like Equisetum arvense (Field Horsetail, called Mare’s tail in Scotland), Elymus repens (Common Couch-grass), Calystegia sepium (Hedge Bindweed), and Taraxacum (Dandelion). Chickweed does rapidly colonise my onion bed, but it can be pulled out by hand. For farmers, the situation has changed radically over the last 50 years. Chickweed was one of the worst weeds until development of a range of herbicides which keep it and other weeds suppressed. It may one day return as a ‘worst weed’ as it develops further resistance (its short life span implies that natural selection works fast). Views on what are the worst weeds may change with farming practices. My casual browsing of farming magazines suggests that the worst weed for cereal farmers in the UK is currently the annual grass Alopecurus myosuroides (Black-grass).

Global distribution of Stellaria media, from GBIF. It is one of the most cosmopolitan of all plant species.

As an ecologist I regard Stellaria media as a remarkably successful species with special traits that have enabled it to colonise much of the world. Herbalists of old including Gerard and Culpeper have lauded its efficacy in the treatment of diverse medical conditions but these authorities seemed to have conflated several species (Gerard shows 13 types of chickweed, some of them evidently not chickweed, and Culpeper believed ‘chickweed’ to be a plant of watery places). Stellaria media does however contain a range of biologically active chemicals, and several lines of research suggest that some of these may have value in the treatment of various conditions including obesity.

The plant is edible and valued by some as a salad plant, or boiled in a soup. Advice on using Chickweed for culinary purposes can be found here. It is included in a dish consumed in the Japanese spring-time Festival of Seven Herbs. As we may guess from its name, chickens and other birds love it, and it is often fed to caged birds.

The genus Stellaria has 9 species in Britain, but about 190 world-wide. I hope to return to some of the British species later in the year, when they are flowering.

References

Grime JP et al. (1988) Comparative Plant Ecology. Unwin Hyman.

Holm LG et al., (1977) The World’s Worst Weeds. University Press of Hawaii and Krieger (Florida).

Sobey DG (1981) Stellaria media (L.) Vill. Journal of Ecology 69, 311-335.

Sobey DG and Kenworthy JB (1979) The Relationship between Herring Gulls and the Vegetation of their Breeding Colonies. Journal of Ecology 67, 469-496.

Stace CA (2019) New Flora of the British Isles. C&M Floristics.

Turkington, R et al. (1980). The biology of Canadian weeds. 42. Stellaria media (L.)Vill. Canadian Journal of Plant Science 60, 981-992.