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 19^th and 20^th 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. 

Belgium condemned?  

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.

It takes all sorts

The young researcher therefore wanted to find out whether the characteristics of the seeds have an influence on the subsequent development of the plants. Ragweed seeds are indeed highly variable, as regards their size, mass, colour, density, etc. It therefore seemed like a good idea to check whether these characteristics had an impact on the development of the seedlings and on the plant’s viability in the long term. This approach is original because rather than just limiting himself to measuring seed mass, as is the case in the majority of the research conducted in this domain, William Ortmans also focused on their surface. "I worked on 900 seeds from nine different ragweed populations. I selected 10 mother plants in each population and I collected 10 seeds per plant. After weighing them, I photographed them in standardised conditions. A software program allowed me to precisely measure the colour of every seed and estimate the surface of each one based on the measurement of the largest ellipse present on the seed’s surface. Besides the originality of the photographic technique, I wanted, above all, to go beyond simply measuring mass, and thereby avoid overlooking other possibly decisive characteristics. If I had limited myself to mass, I wouldn’t have been able to estimate the true quantity of food reserves that exist for the future seedling, for instance". 

The seeds were then sown in two seed chambers that were identical except for the ambient temperature. One – the warm chamber – was supposed to provide the ideal growing conditions (simulating southern Europe), while the other one – the cold chamber – provided suboptimum conditions such as those in Belgium. At the age of two weeks, the seedlings were photographed and their foliage cover was estimated. After two months, the 900 plants were cut in order to estimate their aerial biomass. "Biomass is an excellent indicator of the plant’s performance, in particular the amount of pollen and seeds produced. For instance, I wasn’t surprised to see that fewer of the seeds sown in the cold chamber germinated and that germination was slower. But what interested me above all was understanding whether the characteristics of the seed, i.e. the colour, size or mass, influenced the seedling’s growth, which is the stage when the plant is most vulnerable”.

Ready for invasion

And the result? "We may well have expected all the seeds to more or less identical, regarding the traits taken into account, within each population (geographical area) considered. But this wasn’t the case. Variability exists at all levels: geographical areas, local populations and the plants themselves. The seed traits didn’t all have a very significant impact, but the big and heavy seeds produced seedlings that grew more quickly and had a greater biomass. The extensive variability of the seeds therefore allows the species to grow in a wide range of environments, including colder ones". This fundamental observation is worrying. It means that ragweed is perfectly capable of growing in temperatures found in Belgium; albeit more slowly than elsewhere, but with an excellent germination rate and a complete cycle, i.e. the fructification and dispersal of pollen in the environment.

"For the time being, the ragweed hotspots in Europe are still far from Belgium, separated by a few hundred kilometres”, says Arnaud Monty, senior research fellow at the BIOSE Department’s Biodiversity and Landscape Unit, and promoter for William Ortmans’ thesis. “But if human activities were to bring considerable seed stocks into our regions, it is clear that we would have to react very quickly. An early detection system is essential. Even if the authorities only wait two or three years before reacting, it will already be very difficult to be effective. If they wait 10 years before they adopt limitation or eradication measures, it will really be too late". Among the range of monitoring measures suggested by the expert from Gembloux, is a focus on the "fauna strips" developed within the framework of greening the Common Agricultural Policy (CAP). Indeed, some sowing provided for within the framework of the agri-environment measures includes the use of sunflowers, potentially contaminated by ragweed seeds.

Anti-ragweed smartphones 

Various monitoring and early reaction systems have already been invented, and even put into practice in Europe. In 2014, for instance, a smartphone application – which has now been abandoned due to its cost – allowed observers in the field who found a suspicious plant (ragweed can easily be confused, especially with wormwood) to quickly receive validation of its discovery by experienced botanists. In countries where ragweed already has the status of an invasive plant, brochures and leaflets have been handed out to farmers and naturalists. In Dijon, in France, a specialised observatory has even been set up (www.ambroisie.info). Preventing the needless transfer of seeds seems to be an effective preventive measure. Because the use of herbicides isn’t really recommended in farming or along waterways, above all, another of the species’ favourite dispersal routes. Mechanical cutting is also just as inefficient. Labour and equipment intensive, it often does nothing more than strengthen the plant in the long term, encouraging the dispersal of numerous seeds (up to 14,000 per individual in hot regions!). 

William Ortmans’ research, along with that of others, pleads in favour of a preventive policy, in places where it is still possible in Europe. This research will be presented to the scientific community in September at NEOBIOTA, a reference symposium on invasive plants. This will mark the official end of the Smarter(Sustainable Management of Ambrosia artemisiifolia in Europe) project, which is part of the European COST programme (bringing researchers together around a specific theme). If all goes well, this should lead to integrated ragweed management programmes on a European scale.