Flowers can produce enormous quantities of pollen. Some American ragweeds çan produce 1,500 million pollen grains in an hour. Thus they can release 18,000 million grains of pollen in one day. The American ragweed is a major cause of hay-fever, which is bad news for sufferers: US estimates put ragweed pollen production at a staggering 8 tonnes a week over a single square mile!