Ragweed: on the warpath
Ragweed – not to be confused with wormwood – is a wild plant responsible for many allergy problems. As yet, it is still uncommon in Belgium but following the work of the Biodiversity and Landscape Unit at Gembloux Agro-bio Tech, we now know that it wouldn’t take much for it to invade our countryside. Early detection systems are a must.
Common ragweed (Ambrosia artemisiifolia L.) is a plant belonging to the Asteraceae family. It grows to an average height of approximately 70 centimetres and in some areas of Europe, it is making life difficult for farmers and health professionals. Introduced during the 19th and 20th centuries through seeds imported from America for agricultural purposes, it is now very present in many spring crops: sunflower, corn, soya, etc. In the Rhône-Alpes region, for instance, it is considered the number one self-propagating plant, capable of invading fields over several hectares and compromising the crops. But it is also invading the north of Italy, extending right into Eastern Europe, where it is causing serious problems, particularly in Hungary. This ruderal (with a fondness for wasteland, embankments and excavated areas) also takes advantage of works that disrupt the natural vegetation. Sticking to the soles of workers’ shoes and the tyres of building site vehicles, and becoming buried in earth used for backfilling, its seeds are capable of remaining dormant in the soil for more than 10 years. It can therefore be displaced over short or long distances, although its presence and its effects may not be noticeable straight away.
And what an effect it can have! When it flowers, generally in August, ragweed releases millions of pollen grains into the atmosphere, which are capable of travelling several dozen kilometres or so. This pollen isn’t simply an allergen, causing problems as varied as rhinitis, conjunctivitis, tracheitis or asthma attacks in sensitive persons; it is also allergenic, which means that it is capable of triggering allergic reactions in people who don’t usually suffer from this sort of sensitivity. In some regions in France, ragweed is no joke: it has caused some trade union organisations to rebel against scientific experiments that are nevertheless necessary owing to its allergenic particularities. Between 6 and 12 % of the population seems to be allergic to the plant’s pollen. In the Rhône-Alpes region, in 2012, the effects of ragweed were estimated to have cost the health service some EUR 20 millions.
Up until now, this undesirable plant has remained very discreet in Belgium. And all the better! Its most northerly distribution in this part of Western Europe is restricted to Burgundy. Only a few stations have been observed in Belgium. These are most often industrial sites associated with food-processing where sunflower seeds are handled, and transfer docks along waterways. Ragweed seeds regularly slip into cargos of sunflower seeds. Botanists have also noted its recurring presence in private gardens, because it contaminates bird seed mixes. However, all these stations remain very limited for the moment both here and in the Netherlands.
But for how long? Characterised by the long ‘dormancy’ of its seed stocks, could ragweed migrate northwards one day and reach other regions of Europe? Could the promise of warmer and longer summers as a result of global warming, encourage this phenomenon? Would this possible dissemination be gradual or, as seen in other species, sudden and merciless? These are just some of the questions at the core of William Ortmans’ doctoral thesis in bioengineering, at Gembloux Agro-bio Tech (ULg). He began by studying the performance of adult plants growing in France and at the Belgian and Dutch stations. After having sown the seeds in an experimental garden, he realised that regardless of the region or latitude where they are collected, these plants are perfectly capable of completing their reproductive cycle and creating a considerable seed stock in a single generation. "Belgium’s colder climate therefore doesn’t explain why the species seems to be limited in our parts”, he initially concludes.