1. Sometimes by producing an enormous number of weeds. (A large plant of purslane, 1,250,000 seeds; a patch of daisy fleabane, 3,000 to a square inch.)
2. In other cases by the great vitality of their seeds. Shepherd's purse, mustard, purslane, pigeon-grass, pigweeds, pepper-grass, May weed, evening primrose, smart weed, narrow-leaved dock, two chick-weeds survive when buried in the soil thirty years at least, as I have found by actual test.
3. In each prickly fruit of a cocklebur there are two seeds, only one of which grows the first year, the other surviving to grow the second year.
4. Some are very succulent, and ripen seeds even when pulled. (Purslane.)
5. Often by ripening and scattering seeds before the cultivated crop is mature. (Red root, fleabane.)
6. Sometimes by ripening seeds at the time of harvesting a crop, when all are harvested together. (Chess, cockle.)
7. Some seeds are difficult to separate from seeds of the crop cultivated. (Sorrel, mustard, narrow-leaved plantain in seeds of red clover and alfalfa.)
8. Some are very small and escape notice. (Mullein, fleabane.)
9. Some plants go to seed long before suspected, as no showy flowers announce the time of bloom. (Pigweeds.)
10. In a few cases the plants break loose from the soil when mature and become tumble-weeds. (Some pigweeds, Russian thistle.)