Corn marigold

A very long day of survey down in Hertfordshire - leaving just after 7am and returning home about 7.30pm. Still, at least my leg coped with eight hours of steady walking! During our perambulation we visited a conservation area that has been sown with a mix of declining arable weeds, such as cornflower, corn chamomile and corn marigold, all of which were still flowering. Corn marigold was probably introduced into Britain with the introduction of agriculture and has certainly been here since the Iron Age. It  has a distinct preference for light and moderately acid soils and is most often found as an arable weed though it can also be found on other disturbed sites such as rubbish tips, road verges, waste ground and even over-grazed pasture. Once a serious agricultural weed, it is now much reduced due to a combination of better seed cleaning, herbicides, liming and a switch to autumn sown crops.

As well as the arable weeds, the area also contained species associated with bird seed, including proso millet, four-rowed barley, quinoa and rather worryingly, abundant ragweed. Ragweed pollen is a common allergen. A single plant may produce about a billion grains of pollen per season and the pollen is transported on the wind. It causes about half of all cases of pollen-associated allergic rhinitis in North America, where ragweeds are most abundant and diverse.

Ragweed pollen can remain airborne for days and travel great distances, affecting people hundreds of miles away. It can even be carried 300 to 400 miles (640 km) out to sea. Ragweeds native to the Americas have been introduced to Europe starting in the nineteenth century and especially during World War I, and have spread rapidly there since the 1950s. Eastern Europe, particularly Hungary, has been badly affected by ragweed since the early 1990s, when the dismantling of Communist collective agriculture led to large-scale abandonment of agricultural land, and new building projects also resulted in disturbed, un-landscaped acreage.

Ragweed allergy sufferers may show signs of oral allergy syndrome, a food allergy classified by a cluster of allergic reactions in the mouth in response to the consumption certain fruits, vegetables, and nuts. Signs of reaction can include itching, burning, and swelling of the mouth and throat, runny eyes and nose, hives, and, less commonly, vomiting, diarrhea, asthma, and anaphylaxis. After spending a short time in the field I had sore and itchy eyes, and I'm not a particularly allergy-prone person. This is definitely not a species we want to become widespread in the UK!