Ragweed, an invasive plant that is problematic for both health and agriculture

What is ragweed?

Ambrosia is a genus of plants that includes around 40 different species, mainly native to the American continent. The best known and most widespread is common ragweed (Ambrosia artemisiifolia L.), an invasive plant that has been introduced into many parts of the world, including France and much of Europe. Three other species have also been introduced into mainland France: giant ragweed (Ambrosia trifida L.), western ragweed (Ambrosia psilostachya DC.) and lacy ragweed (Ambrosia tenuifolia Spreng.). They are considered to be emerging invasive plants and are also covered by ANSES's recommendations for action.

What are the health effects of ragweed pollen?

In an expert appraisal published in 2014, ANSES demonstrated that ragweed pollen is among the most problematic in France. This is because it is highly allergenic: it can induce allergy symptoms but also causes individuals to develop new allergies.

Ragweed pollen causes the same symptoms as other types of pollen in allergic individuals: sneezing, blocked nose, conjunctivitis, redness, swelling of the eyelids, etc. These symptoms heavily impact the quality of life of sufferers, with consequences on their social and professional lives, for example.

Ragweed allergy seems particularly debilitating compared with other pollen allergies according to a national survey of healthcare professionals (ANSES, 2020 – ragweed).

In 2020, ANSES estimated that between 1,115,000 and 3,504,000 people were allergic to ragweed pollen in mainland France and Corsica.

When do pollen levels peak?

The pollen peak occurs between mid-August and mid-September. Allergies caused by ragweed pollen therefore appear later than other pollen allergies, particularly those caused by grasses.

Where is ragweed found?

Native to North America, ragweed was first identified in the wild in Europe in the mid-19th century.

Today, the main allergy centres in France are in the Rhône and Loire valleys and in the south-west, particularly from the Tarn-et-Garonne to Charentes départements. Ragweed populations seem to have continued to spread increasingly rapidly and become denser in mainland France. The plant is now found across the entire country, with varying levels of infestation. There are three main types of infestation areas:

• Areas with heavy infestation/establishment, including the Rhône, Isère and Drôme départements, but also Nièvre, Cher, Charentes and Tarn-et-Garonne;
• "Frontline" areas such as the Yonne, northern Côte-d'Or and western Gard départements, located on the edge of the heavily infested areas;
• Areas that are currently affected very little or not at all, such as Brittany, Normandy and Hauts-de-France.

Although it has been found in Guadeloupe and Martinique, ragweed is not considered invasive in these territories.

How does ragweed spread?

Its spread is favoured by certain human activities: the transport of ragweed-contaminated soil or seeds, agricultural and/or mowing machinery, animal feed, etc.

Common ragweed thrives in:

• certain agricultural crops such as sunflower, soya or maize, where it causes considerable yield losses;
• other environments such as riverbanks or roadsides.

What is the cost of the health impacts of ragweed?

In 2020, ANSES estimated the costs of the health impact associated with ragweed at a national level:

• The cost of medical care (e.g. medicines and consultations) may be between €59 million and €186 million each year;
• The cost of production losses based on absences from work may be between €10 million and €30 million per year.

These costs are expected to increase in the future, due to the expansion of ragweed-infested areas and an increase in pollen levels in ambient air, mainly as a result of climate change.

What are the effects of ragweed on agriculture?

The rapid growth of ragweed can have a major impact on the yield of certain crops due to competition for resources. This is particularly true of sunflowers, which are closely related to ragweed in the botanical classification (tribe Heliantheae). The proximity of the two species makes it difficult to eradicate the ragweed, either by chemical means or by sorting the seeds.

In 2011, economic losses in the European Union due to reduced agricultural production caused by ragweed were estimated to be €1846 million per year.

Does the spread of ragweed have an impact on biodiversity?

The presence of ragweed does not appear to have a significant effect on plant diversity in the colonised natural environments. For example, a 2014 study counted an average of 9.9 species in 4 m² plots on riverbanks invaded by ragweed and 10.6 species in neighbouring plots that had not been colonised. Several factors could explain ragweed’s lack of effect on biodiversity: it is an annual plant, which prevents it from forming dense, persistent colonies in the same place. It also prefers to colonise disturbed sites, where plant communities regrow each year from seeds stored in the soil.

What steps can be taken to keep it from spreading?

Individuals:

• In your garden: pull the weeds out before flowering (end of summer) and take care to uproot the entire plant while wearing gloves. Do not do this if you are allergic or sensitive to ragweed;
• If there are large amounts or for ragweed growing outside your property, send a report to signalement-ambroisie.fr.

Building & public works and agricultural sectors:

• Adopt good ragweed management practices such as cleaning machinery, limiting the amount of bare land and managing contaminated land.

Local authorities:

• Act locally in frontline areas and areas still relatively unaffected by ragweed (northern third of mainland France) by immediately implementing specific (prefectural order) and coordinated regulations, and appointing a ragweed expert responsible for control measures on the ground.