Ambrosia (Ambrosia artemisiifolia L.) is one of the rare plants that have no quality, of whatever nature, to highlight: neither taste, nor aesthetic, nor medicinal. On the contrary, it is the cause of many pathologies linked to the dissemination of its pollen, from August to October. This extremely allergenic pollen causes rhinitis, urticaria, cough, eczema, conjunctivitis, asthma, tracheitis, etc., which can be very disabling but also very expensive in terms of treatment.
Ambrosia, also called Saint John’s weed, ragweed, country wormwood, artemisia leaf ambrosia, was spotted in France in the middle of the 19th century; it has continued to develop, therefore, to colonize the places, more particularly the so-called “open” soils, taking advantage of all the upheavals and modifications of the ecosystem due to the mechanization of agriculture and road developments. The Rhône corridor is its historical melting pot but today, the entire southern half of France is affected by ambrosia.
Ambrosia has a tall straight stem covered in white hairs with finely cut bright green leaves and alternate reddish twigs. Its port is quite wide bushy, the yellow-green monoecious flowers turn yellow once opened, they form straight candles from July to September-October which produce pollen so allergenic. Thereafter, from September to December, it will be the seeds (achenes) that will appear to allow the rapid reproduction of ragweed.
- Family: Asteraceae
- Type: annual
- Origin: North America
- Color: yellow green then yellow flowers
- Sowing: yes
- Cutting: no
- Planting: spontaneous
- Flowering: July to September
- Height: up to 2 m
Ideal soil and exposure for ambrosia
Ambrosia is a ruderal plant, it will find a favorable ground for its development in soils left bare by weedkillers which have no effect on it, land upset by road or construction works, as well as on the edge of roads. Warm, dry, sandy soil is its ideal.
Multiplication of ambrosia
Ambrosia is a therophyte annual plant, that is to say that it dies after having produced its seeds, which themselves survive the winter and many years, and begin to germinate as soon as the conditions allow them. are conducive, both climatically and physically (ploughing, land management, etc.). So development can be rapid.
Curiously, ragweed seeds are often contained in bird food seed mixes, causing the plant to thrive around aviaries.
Obviously, ambrosia cannot be sown in the garden. Its multiplication is unfortunately done naturally by spontaneous sowing.
How to distinguish ambrosia from mugwort
Ambrosia looks a bit like themugwort (Artemisia vulgaris), but this one lacks its hairy stem, and its leaves are whitish below, while ambrosia has the same green on the reverse side. The flowers of ambrosia are yellow green then yellow once opened, while those of mugwort are gray green then whitish. While ragweed has no odor when the leaves are crumpled, mugwort gives off a very pleasant smell.
How to prevent the development of ragweed
It is before the flowering of the ambrosia that you must take action against it, either in spring or at the beginning of summer. It is better to protect yourself with a mask, gloves and glasses, especially if you are allergic in nature.
Since ragweed likes bare soil, the first thing to do to upset it is to mulch without leaving any soil in the air. When planting and landscaping your garden, immediately place a shredded material at the foot of shrubs and other plants (fruit trees, roses, hedges, etc.).
In addition to mulching, you should also avoid turning the soil as much as possible. The permaculture is then a cultural practice well adapted to counter the untimely appearance of ragweed.
Moreover, if your garden is small, you can pull out any ragweed that appears, without putting it in the compost.
On large spaces, sow clover: you will not have bare soil and it will not leave room for ambrosia to appear.
In driveways, rip or use a thermal weeder to remove ragweed.
The fight against ambrosia
Ragweed is fought on a global level: thus, the first Saturday of summer is International Ambrosia Day to raise awareness of the damage caused by ragweed.
There is an interdisciplinary network of European experts, SMARTER, involved in ambrosia monitoring.
In France, the law of January 26, 2016 on the modernization of our health system introduces a chapter relating to the fight against plant and animal species harmful to human health in the public health code (CSP) and a new national regulatory system specific to the fight against ragweed was adopted in 2017, the aim of which is to prevent their appearance or fight against their proliferation, and to prohibit their intentional introduction into the territory, their transport and their use. The prefect sets, by prefectural order, in each department, the measures to be implemented and their methods of application. Violators risk 1st class fines.
The 3rd National Health-Environment Plan integrates the fight against ragweed and ambrosia observatory coordinates the fight against this plant.
(photo credit 1 by SB Johnny — Own work, CC BY-SA 3.0 and photo 2 by Meneerke bloem — Own work, CC BY-SA 3.0)