The Ragweed's case

Origin and Distribution

The American continent is ragweed's native habitat. It entered Europe starting in the nineteenth century and especially during World War I, probably by accident through imported seeds and contaminated horse feed, and has spread rapidly since the 1950s. In Europe, common ragweed (Ambrosia artemisiifolia) is primarily widespread, which happens to be the most allergenic among all ragweed species. There are about 30 species in total.

Currently, the species is widespread in southern parts of Eastern and Central Europe, with invasion hotspots in the Pannonian plains of Hungary, Croatia, Slovakia, Romania and Austria. Ragweed is found in Serbia, Italy, Ukraine and Russia. After the 1980s, Ambrosia spread increased rapidly and became a serious threat to human health in the infested countries. Hungary, Northern Italy, and the Rhône Valley in France are considered the most infested European regions, and intense research activities have been dedicated to this topic. The main factors favoring Ambrosia spread are considered to be inappropriate changes in agricultural practices, urbanization, and the international trade of seeds and grains.

Current List of Ragweed-Infested Countries:

Why Is Ragweed So Allergenic?

Let's start with the nature of flowering allergy—pollinosis. This disease is caused by proteins contained in plant pollen. Our immune system recognizes them as hostile and produces antibodies that ultimately trigger the allergic response mechanism.

Ragweed proteins enter the bodies of sensitive people through mucous membranes, primarily the nose, along with inhaled air. While pollen from other plants may quickly pass down into the respiratory tract, ragweed pollen grains catch on the mucous membrane and have time to swell on it, releasing their contents onto the mucosa.

This is because the surface of ragweed pollen has spines that facilitate the penetration of allergens into our body.

Another reason ragweed causes allergies lies in the properties of its proteins. Specifically, the first and most important allergen of this plant, Amb a 1, is a pectate lyase. Proteins of this type are responsible for pollen tube growth, through which sperm contained in pollen move to the egg cell of the flower's pistil.

When it lands on the stigma of the pistil—the female organ of the flower—pectate lyase causes the softening of plant cell walls, which ultimately allow sperm to pass further from the pistil surface deep into the egg cell. Scientists believe that pectate lyase acts similarly in our body.

After pollen swells on the mucosa, the Amb a 1 allergen released from it weakens the connection between our cells, allowing all the allergenic contents of the pollen to enter the bloodstream. There, immune system cells encounter it and synthesize IgE antibodies, which are later recognized by the same immune system as hostile.

Moreover, the amount of Amb a 1 in pollen increases as it matures. This is why ragweed pollen is more allergenic at the end of the season, when its main allergen has accumulated in large quantities and actively provokes IgE antibody synthesis.

How to Get Rid of Ragweed?

Currently, this is very difficult. The weed easily spreads by seed, and one plant produces up to a billion pollen grains per season. Inept tactics for its destruction, such as improper mowing, have only helped ragweed regrow and form seeds by the end of the season.

For example, chaotic random mowing of common ragweed leads to the plant forming lateral shoots in the lower part of the main shoot, which almost lie on the ground. Subsequent mowing won't affect them, seeds will form on these branches that will contaminate the soil, and pollen will ripen closer to the end of the season containing an increased dose of allergen.

Therefore, it's considered that ragweed can only be mowed to the root immediately before flowering, when panicle-like inflorescences form but haven't yet opened, which will solve several problems—the formation of allergenic pollen and seeds.

The plant can also be either pulled out by the roots (mechanical method) or destroyed with special chemicals. There are also attempts to combat ragweed with robots that can distinguish it well from sunflowers in fields where both plants grow. After recognition, the robot burns only the ragweed, leaving the sunflower undamaged.

Therefore, the most effective means of combating the weed is applying a comprehensive approach to solving the problem—using agrotechnical, mechanical, and chemical control methods.

The mechanical method should preferably be used in populated areas and on lands with high and very high sanitary-hygienic requirements (lands around residential buildings, public buildings, livestock farms, parks, stadiums, sports grounds, industrial territories, etc.), where herbicide application is not possible. Here, ragweed is destroyed by burning, manual removal, or mowing. On private plots and flower beds, it's most effective to weed out common ragweed plants. The smaller the plant, the easier it is to perform this procedure. Small plants, especially after rain, are well pulled out with roots.

However, there are two important details:

Fighting ragweed should never be conducted during flowering. The best time to destroy the weed is June: the stem has already grown, but flowers with pollen haven't yet formed. If you pull the plant from the ground or otherwise disturb the stem in July and later, this will only promote the dispersal of pollen and seeds.
Remember that fighting ragweed must be comprehensive. At minimum, within one city and adjacent territories, because pollen easily travels hundreds of kilometers. If your neighbors haven't destroyed ragweed, sooner or later you'll still suffer from allergies to it.