Not to be semantically confused with : Common Ragwort (Senecio jacobaea) or Oxford Ragwort (Senecio squalidus) [a plant of similar name which is in the same Daisy & Dandelion Family (Asteraceae) which have similar, but not identical, leaves]
Some similarities to : Ribbed Melilot (Melilotus officinalis) (jiz) and to Vervain (Verbena officinalis), but mostly to Mugworts or Wormwoods such as
Norwegian Mugwort (Artemesia norvegica) or Wormwood (Artemesia absinthium),
Field Wormwood (Artemesia campestris) and
Sea Wormwood (Artemesia maritimum) (both leaves and flowers).
Slight resemblance to : Monk's-hood (Aconitum napellus) (leaves only).
Ragweed has separate male and female flowers; the male flowers are much more prominent than the female ones which are tucked in the leaf axil, whereas the male flowers are clustered beneath a bract which acts as an umbrella (but protection from sun or rain or both your Author knows not).
Unusually, the flowers are held upside down within a cap-shaped bract. There are no ray florets, only disc florets, but unusually the disc florets have a yellow tubular skirt which hangs down, the skirt having five narrow yellow 'filaments' dangling from the periphery. In some ways the stringy 'filaments' might resemble Fringe Cups (Tellima grandiflora) which belongs to the Rose Family (Rosaceae), but there the 'filaments' are multiply bifurcated.
Norwegian Mugwort also has disc florets held upside down, but otherwise the two are quite dissimilar. Nodding Bur-Marigold is another with only disc florets held upside down, but in each case the florets are held within a cap consisting of many sepals, rather than the monocoque cap of Ragweed.
Ragweed differs from
Perennial Ragweed where the flowers are held facing sideways rather than downwards. They too are held within sepals rather than a monocoque cap. The flowers of Ragweed most resemble those of other Mugworts and Wormwoods.
Ragweed is an annual and is wind pollinated, the pollen being a potent allergen to those who suffer from hay fever. It is to be found most concentrated around the London area, but occurs as far north as in Scotland. It grows in dockyards, disturbed ground, arable fields, rubbish tips and other sorts of waste ground. Also in places where birds seed is scattered about.
It is monoecious with separate male and female flowers on the same plant. The male flowers are in upside-down heads between 3 and 5mm in diameter in long slender spikes of flowers. The female flowers are far less conspicuous being at the bases of the upper leaves (your Author has looked and cannot see any in these photographs - so he has borrowed a photo from wikipaedia).
As a Metallophyte which is capable of absorbing and hyperaccumulating certain metals from the soil, Ragweed can be employed as a
phytoremediator to decontaminate soils, providing that, once it has grown, it is cropped and disposed of safely! This procedure will have to be repeated over a number of growing seasons for it to have any noticeable effect on the contamination. Unless, of course, the only reason for growing something which will tolerate the lead contamination is to cover the otherwise bare ground where few other plants will grow. The latter has the benefit of stabilising the soil against being wind-blown elsewhere. In the case of Ragweed, it is capable of hyperaccumulating the metal lead. Your Author does not know if Ragweed is employed for its metallophyte properties in the UK, but it might be elsewhere in the World where it is more rampant. It is relatively scarce in the UK perhaps because our climate is not inducive to it becoming a rampant weed like it is in some European countries or the Northern Territories of the USA.
Ragweed contains the secondary metabolites called
Psilostachyin B and
Psilostachyin C (which are
sesquiterpene dilactones - well - two of them may be, the third seems to be a mono-lactone). But your Author gave up trying to draw these three chemical structures when the available drawings on the web contradicted each other, using differing chemical structures! Which is not unusual.