It is a beautiful plant, Annual Ragweed (Ambrosia artemisiifolia). It comes from the Americas and it feels at home in France. A bit too much, you could possibly say. Some years ago it was nowhere to be seen, and now it is everywhere.
It grows upright like a small Xmas tree with many branches, and every branch carries a lot of flowers.
The red colour of those branches nicely contrasts with the tender green of the fern-like leaves and the tiny pale flowers.
It is a monoic plant, which means it has flowers with pollen and flowers with pistils but no flowers with both. Here above the male flowers just begin to loose the yellow pollen. Some female flowers with long white protruding pistils are also visible. The pollen is dispersed with the wind.
It is an annual plant that can form large colonies. Her grows Annual Ragweed in a cereal field after harvest. The seeds have germinated in spring and began to grow somewhat later than the cereals. When the combine harvester passed the little plants were too small to be damaged by it. After harvest there was no longer any competition from the cereals and they could grow fast and big.
In Septembre the field turned yellow, not because of the hot and dry weather but because of a herbicide treatment the farmer put in place to get rid of weeds. It worked very well, there are not many plants alive anymore on the field. With one exception: Annual Ragweed plants are still green and producing a lot of seeds.
If coming spring the circumstances are favorable, those grains will germinate. They need a well-worked soil with only little competition from other plants. The farmer will do what he can to achieve this. He already killed other plants, and with his next passage with tractor and harrow he will break up the soil. Thus a good seed bed will be prepared. Ar the same time he will scatter the seeds, not only in this field but also elsewhere in other fields. This is very effective because modern tractors do much more kilometers a day as those of our grand-parents. A good crop of Annuel Ragweed is almost guaranteed.
Di farmers really want to cultivate Annual Ragweed? Of course not, this plant is not useful for them and unedible for humans or animals. Its pollen is allergenic and a single plant can produce a lot of it. In a few years, Annual Ragweed can cover a large surface and thus prevent other, more desirable, plants to grow. It is not sensitive to frost nor to drought and it can survive under many circumstances. In short it is a nuisance for the farmer. Moreover, there is a legal obligation to destroy it and do do what one can to eradicate it.
Is it possible to eradicate it? Maybe, but then farmers really need to dedicate themselves to the task. Chemical destruction is difficult - Annual ragweed is resistant to many weedkillers - and only useful when done at the right moment, before the plant begins to produce seeds. The same for mechanical destruction by mowing or crushing that should be repeated often to avoid re-growth. To grow a perennial crop that covers the soil as alfalfa or clover can help but then there are no cereals or corn.
Is it advisable to eradicate it? In any case there will be a lot of collateral damage. While destroing Annual Ragweed you also destroy other species living in the fields. Plants, among which rare species that need a specific habitat, insects, soil species and all life that depends on them. There is a reason why the number of swallows is going down so fast! Also, it happens rarely we manage to eradicate an invasive species even if we really do our best. To mention a few well-known exemples: There are still rabbits in Australia and Asian hornets are still in expansion in France. Same story for Japanese Knotweed or Water Primrose, to mention some plants. There is no reason at all to assume it would be different for Annual Ragweed.
A real conundrum.
What to do (or not to do) with Annual Ragweed and other exotic and invasive plants?
Good question...
