Cockleburs

Moist sunny areas at UOBS and Fobb Bottom WMA (large areas) support abundant populations of cocklebur Xanthium strumarium. Native to US and to Europe, now a cosmopolitan invasive. Flowers have no scent and are wind-pollinated & compound. Plant is an annual but produces a woody tap root. Well-defended mechanically (leaves tough like sandpaper) and chemically (produces poisonous diterpene, carboxyatractyloside ). Each bur is produced from two flowers, with one seed per flower, so each bur contains two seeds. One is ready for immediate germination within a year. The other seed is delayed in germination - up to a decade. Delayed germination - 'bet-hedging'. Plant is poisonous to mammals and can sicken or kill livestock. Poison is most concentrated in prickles of bur. Bur is very hard. Many insects feed on the plant: the Ragweed Leaf Beetle (Ophraella communa), stem-boring larvae of the Sunflower Longhorn Beetle (Dectes texanus), stem-boring larvae of the Sunflower Stem Weevil (Cylindrocopturus adspersus), Five-spotted Billbug (Rhodobaenus quinquepunctatus),  larvae of leaf-miner flies (Calycomyza platyptera, Liriomyza trifolii), larvae of the Bur-Seed Fly (Euaresta aequalis), Spotted Green Plant Bug (Ilnacora stalii), a leaf-feeding aphid (Capitophorus xanthii), stem-boring larvae of the Ragweed Borer Moth (Epiblema strenuana), and seed-eating larvae of a moth, the Pale-headed Phaneta (Phaneta ochrocephala) [ source: https://www.illinoiswildflowers.info/weeds/plants/cocklebur.htm  ] 
The poison produced inhibits growth of other plants, including seedling cockleburs.