Actually two: Ambrosia artemisiifolia (common ragweed) and Ambrosia trifida (giant ragweed) are both herbaceous native annuals in the Asteraceae. Both begin growing in the spring and produce their flowers and pollen in late summer and into the fall. Both are the causes of lots of allergies and are invasive (Note some would not use this word since these are native but I find it used for these two species on the internet quite a bit and was used to describe these two species before native vs non-native was popular). Both are monoecious with male flowers in terminal racemes and female flowers in axillary clusters below the male. The flowers are green at first but turn yellow-green then brown at maturity. Now for the differences: Ambrosia artemisiifolia (common ragweed) is also called; American Wormwood, Bitterweed, Blackweed, Carrot Weed, Hay Fever Weed, Roman wormwood, short ragweed, stammerwort, stickweed, tassel weed, and Ragweed. The leaves are mostly alternate (but can be opposite) have pinnate major veins, are deeply pinnatifid (appearing to be compound), broadly lanceolate (in outline), and usually much wider at the base than the tip. Large plants are 5-6 feet tall. These are very very common on Allen Acres and seem to prefer sandy soils over the soils with more clay content. The pollen is collected by bees and seeds eaten by many wildlife species. Some reports of medicinal use by native Americans. Ambrosia trifida (giant ragweed) is also called great ragweed, Texan great ragweed, tall ragweed, blood ragweed, blood root, perennial ragweed, horseweed, buffaloweed, and kinghead. The leaves are opposite with palmate major veins and vary from entire to lobed but not deep lobed. The stems get very large and the plants very tall to 20 feet. Very few on Allen Acres and it seems to thrive in soils with more clay than sand. Broken plant tips will produce a red liquid hence the blood in the common names. Seeds were eaten by Native Americans and also wildlife. I have heard reports of this being a favorite plant to feed to horses. I wonder how these plants got the name Ambrosia since that refers to food of the Gods in Greek and often depicted as conferring longevity or immortality upon whoever consumed it. Ambrosia also refers to a salad that back in St Helena Parish was slices of oranges and coconut but today has other things added like pineapple, cool whip. nuts, cherries etc. Caterpillars of the border patch butterfly and these moths; Peridroma juncidella and saucia, Variegated Cutworm Moth, wavy-Lined Emerald, and Ragweed Plume Moth (Adaina ambrosiae) are reported to eat artemisiifolia. Caterpillars of ten moth species are reported to eat trifida. Caterpillars of the butterfly common sooty wing plus 14 moth species are reported to eat the genus Ambrosia. There are several other species of Ambrosia but these are the two most common. I have just a few pictures of Ambrosia trifida so here is a link to a picture to show its size (https://willowhousechronicles.files.wordpress.com/2010/09/giantragweed.jpg). If someone out there has or can create a picture to show the size of giant ragweed, I would appreciate one to add to my collection. Only a few small ones on Allen Acres.