Ragweed is the name of several weeds that grow in the United States and Canada. These weeds grow along roadsides, and in pastures, fields, and vacant lots. Many people are allergic to ragweed pollen. It is produced in great amounts and spread by the wind. People who are allergic to ragweed pollen may get symptoms of hay fever when there are about 25 grains of pollen per cubic yard (33 grains per cubic meter) of air. The air may contain more pollen than this when the plants bloom.
The common ragweed is a coarse annual plant with finely divided leaves. It usually grows 1 to 3 feet (30 to 91 centimeters) high. Its small, hard fruit has short, sharp spines near the end. Giant ragweed, sometimes called kinghead, is also an annual. It commonly grows 3 to 6 feet (0.9 to 1.8 meters) tall but may grow to 10 feet (3 meters). Its leaves usually are divided into three broad parts. Perennial ragweed grows from long, spreading roots. It looks something like the common ragweed but its fruit has blunt tubercles (small projections) instead of spines.
Ragweed grows quickly in any untended spot. It is so ordinary looking and its flowers are so inconspicuous that efforts to eliminate it have failed.
