Plant of the Week – 29th September – Common ragweed Ambrosia artemisiifolia

A fast-growing plant recently appeared beneath our peanut feeder. We didn’t recognise it, and in the spirit of botanical inquiry we let it grow rather than putting it on the compost heap.

We couldn’t find it in our floras, and suspected it was an alien species, so we tested the Seek app (developed in the US) to identify it https://www.inaturalist.org/pages/seek_app. A couple of photos later and we had a suggestion which we cross-checked with our reference books. Once we knew what to look for, we found it in Stace’s (2011, 2019) floras, and in Streeter et al. (2016), but not in Blamey et al. (2013), Rose (2006) or Harrap (2013).

It is Ambrosia artemisiifolia, a member of the Asteraceae (Compositae), a ragweed (not a ragwort). We agree with Streeter et al. (2016) who say ‘it doesn’t look like a composite’. However, close examination by experts shows that it is in the Asteraceae, the family which contains familiar flowers such as dandelions, daisies, thistles, mayweeds and tansies, and includes Oxford ragwort and the Sonchuses which have already been subjects of the Botany in Scotland blog. After the grasses, the Asteraceae is the largest family of flowering plants in Britain.

In the Asteraceae, the flowers are usually arranged in a compact cluster of flowers which is called a capitulum. Ambrosia keys out very early in Stace’s key, alongside Xanthium (the cockleburs), with the couplet ‘Male and female capitula separate on the same plant, the male more apical and with several flowers, the female lower down and with 1-2 flowers. …… the male flowers are in dense terminal elongated racemes, the female just below in axils of leaf-like bracts, with only one flower’. This makes Ambrosia are rather unusual member of the Asteraceae. The plants are strongly self-incompatible, and daylength influences the sex-ratio of the flowers. Male flowers predominate in long days, while female flowers become more dominant as days shorten (Allard 1943).

The male capitula hang downwards, and their tiny flowers have a yellowish corolla.

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Female flowers with long yellow stigmas occur beneath the male flowers. They are wind pollinated. The capitula of the female flowers are upright.

The monograph by Essl et al. (2015) is a mine of information about this species, and contains some beautiful botanical drawings by Krisztina Biró which are well worth a look.

While looking closely at the male flowers, we were puzzled by this feathery structure which protrudes from them. It looks rather like a female stigma, but this species is described as ‘monoecious’ – it bears male and female flowers separately on the same plant. The drawings by Biró show something similar, but what is it?

This species is an annual herb, native to North America and was accidentally introduced to Europe in the 19^th century in imported seed. It is a ruderal species, preferring open habitats such as waste ground and field edges. In Europe it is a common pest in sunflower crops (which are also Asteraceae) where it is difficult to control (Lockton, A.J. & Crocker, J. 2020). It is spread by road networks (Lavoie et al. 2007), machinery and contaminated seed. The maps below show the distribution of the species in Europe before 1975 and from 1975 – 2020.

Looking closer at the records for Britain and Ireland, the Botanical Society of Britain and Ireland’s records show the acceleration of spread of the species in the British Isles (see below).

Ambrosia has become a rampant pest in many parts of Europe, but not yet so in Britain (Lockton and Crocker, 2020), although there are concerns that this may change with climate warming. Many exotic species colonise and spread, but what makes this one a particular cause for concern is that its pollen is extremely allergenic and causes problems for many hay fever and asthma suffers, particularly in late summer and autumn, and it is a serious weed of agricultural crops.  It is of such concern that several countries hold ‘International Ragweed Day’ (in 2020 this was on June 20^th, when – in Vetschau, Germany – residents were invited to pull out ragweed plants) (Observatoire des Ambroisies). Unfortunately, ragweed has evolved multiple resistance to herbicides http://weedscience.org/details/Case.aspx?ResistID=17109 , which makes its control more difficult.

In Europe, modellers predict that with warmer summers and later autumn frosts, the species is likely to move northwards and colonise increasing altitudes, while hotter and drier summers may cause its southern limit to retreat (Essl et al., 2015). Chapman et al. (2014) show that phenology can be a key determinant of limits to species distribution. Ambrosia flowers late and seeds must mature before autumn frosts. Their model predicts that in an average year ragweed can reach maturity and produce seed in lowland Europe south of northern Britain, Estonia and Fennoscandia. Most of Scotland is currently within the range where anthesis is reached, but seed does not mature, however predictions to 2050 suggest that parts of Scotland will come within the range where seed does mature, but although the life cycle may be completed, summers may be too cool for sustained long-term population growth, unless there are repeated re-introductions – such as in contaminated bird seed.

We mentioned our find to Barbara Sumner, who is the BSBI Vice-county Recorder for Midlothian, who informed us that it’s an unusual find.  Apparently, Miss Beattie recorded it on the embankment by the Union Canal at Sighthill from 1963 to 1968, but it was not there in 1969.  She thought it might have been introduced with hen food.  Then there was a gap in records until 1996, when it was recorded it as a garden weed at Baberton Crescent in western Edinburgh, where it was thought to have probably been introduced with wild bird seed.  Then there were no more records until 2010, when it was found beneath a bird table at Lennox Row, Trinity.

This link with bird feeders is interesting, as our two plants were beneath a peanut feeder and there are seed feeders elsewhere in the garden, and next door. This points the finger at bird food being a spreader of this species. Hanson and Mason (1985) and Hanson (2000) have extensively surveyed contaminants in bird seed. Their 1985 paper considered that 483 plant species were introduced to Britain as bird seed contaminants, and that ragweed was a common contaminant in millet imported from the USA. Many additional taxa were listed in their 2000 update. So, while we put out seed to help our birds in winter, at the same time we are inadvertently introducing alien plant species to our environment.

What is the law on seed contaminants? According to Essl et al. (2015), there have been strict controls on the maximum amount of Ambrosia artemisiifolia seeds in bird seed and animal feed since 2012 and 2013 respectively (European Union, 2012). The limit is 50 mg kg^-1, equivalent to about 10 -12 seeds per kg. This doesn’t sound much, but a large 12 kg sack of bird seed from your local garden centre could legally contain more than 100 ragweed seeds, plus other aliens. Perhaps this will spur me to purchase ‘no grow’ bird seed in the future. Repeated introductions as a seed contaminant, mean that even if seed does not mature in some years, a new supply is available through contaminated bird seed or other sources.

As part of our urban flora project we are interested in whether the growing conditions in towns and cities (including the heat island effect) influence the species which thrive in urban areas, whether they adapt to urban environments and whether the early appearance of alien species in cities is indicative of changes in overall species distribution which will occur in the future with climate warming. It turns out that this species has been used to study this. Gorton et al. (2018) conducted a reciprocal transplant experiment in the USA (Minneapolis area) using common ragweed to test whether there was local adaptation to the urban environment. Seeds were collected from rural and urban environments and planted together in ‘common gardens’ – two in urban and two in rural locations. Dates of the opening of male and female flowers were recorded, together with the number of male flowers and the number of fruits. They found plants which originated from urban populations flowered consistently earlier than those originating from rural populations when planted in both urban and rural locations. This might be an adaptation to drought, as city plants need to mature earlier to beat the onset of drought in built up areas with impervious surfaces. However, in contradiction to this, both rural and urban origin plants growing in urban common gardens flowered later than those growing in rural common gardens. This apparent contradiction was not resolved in this study, but might be related to the fact that all seeds were stratified in a standardised way, before seedlings were planted in the common gardens, so differences in germination conditions in the real world did not come into play in this study. Their results indicated adaptive divergence of urban and rural populations, but also indicated that there may be greater environmental heterogeneity in urban environments, which will also affect evolution in urban landscapes.

Meanwhile, we are watching our plants grow and wondering whether they will set viable seed.

References

Allard HA (1943) The North American ragweeds and their occurrence in other parts of the world. Science, 98, 292–294.

Chapman DS, Haynes T, Beal S, Essl F, and Bullock JM. (2014) Phenology predicts the native and invasive range limits of common ragweed. Global Change Biology (2014) 20, 192–202, doi: 10.1111/gcb.12380

Essl F, Biró K, Brandes D, Broennimann O, Bullock JM, Chapman DS, Chauvel B, Dullinger S, Fumanal B, Guisan A, Karrer G, Kazinczi G, Kueffer C, Laitung B, Lavoie C, Leitner M, Mang T, Moser D, Müller‐Schärer H, Petitpierre B, Richter R, Schaffner U, Smith M, Starfinger U, Vautard R, Vogl G, von der Lippe M, and Follak S. (2015), Biological Flora of the British Isles: Ambrosia artemisiifolia. J Ecol, 103: 1069-1098. doi:10.1111/1365-2745.12424

European Union (EU) (2012) Commission Regulation (EU) No 744/2012 of 16 August 2012 amending Annexes I and II to Directive 2002/32/EC of the European Parliament and of the Council as regards maximum levels for arsenic, fluorine, lead, mercury, endosulfan, dioxins, Ambrosia spp., diclazuril and lasalocid A sodium and action thresholds for dioxins. Official Journal of the European Union, L219, 5–12.

Ambrosia artemisiifolia L. in GBIF Secretariat (2019). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2020-09-24.

Blamey M, Fitter R, Fitter A. (2013) Wild flowers of Britain and Ireland. Bloomsbury.

GBIF Secretariat (2019) Ambrosia artemisiifolia L. in GBIF Secretariat (2019). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2020-09-24.

Gorton AJ, Moeller DA, Tiffin P. 2018 Little plant, big city: a test of adaptation to urban environments in common ragweed (Ambrosia artemisiifolia). Proc. R. Soc. B 285: 20180968. http://dx.doi.org/10.1098/rspb.2018.0968

Hanson CG and Mason JL (1985) Bird seed aliens in Britain.  Watsonia, 15 ,237-252. http://archive.bsbi.org.uk/Wats15p237.pdf

Hanson CG (2000) Notes: Update on birdseed aliens (1985-1998). Watsonia 23: 213-220 http://archive.bsbi.org.uk/Wats23p213.pdf

Harrap S. (2013) Harrap’s wild flowers. Bloomsbury Natural History. ISBN 9781408113608.

Lavoie C, Jodoin Y and Merlis A. (2007). How did common ragweed (Ambrosia artemisiifolia L.) spread in Québec? A historical analysis using herbarium records. Journal of Biogeography. 34. 1751 – 1761. 10.1111/j.1365-2699.2007.01730.x.

Lockton AJ and Crocker J. (accessed 23/09/2020). BSBI Species account: Ambrosia artemisiifolia. Botanical Society of the British Isles, www.bsbi.org.uk. http://sppaccounts.bsbi.org/content/ambrosia-artemisiifolia.html

Observatoire des Ambroisies https://solidarites-sante.gouv.fr/IMG/pdf/ragweed_observatory_letter_july2020.pdf

Rose F. (updated by O’Reilly C.) 2006 The Wild Flower Key (Revised Edition) – How to identify wild plants, trees and shrubs in Britain and Ireland. Warne; Rev Ed Edition (30 Mar. 2006). ISBN-13 : 978-0723251754

Stace C. (2011) New flora of the British Isles. Third edition. Cambridge University Press. ISBN 978-0521707725

Stace C. (2019) New flora of the British Isles. Fourth edition. C&M Floristics. ISBN 978-1-5272-26302

Streeter D, Hart-Davies C, Hardcastle A, Cole F, and Harper L. (2016) Collins wild flower guide. Second edition. William Collins. ISBN 978-0008156756

Text: Julia Wilson, photographs © Chris Jeffree