Abstract: Field experiments show that the plant height of ragweed untreated reaches to 223.3 cm, while the average plant height is respectively 212.1 and 184.0cm after single and multiple treatment with defoliation. Plants with six times defoliation died when grew to 130cm in height. In control, the number of fruit-bearing branches is 14.9. With defoliation at early vegetative growing stages, the formation of branches is obviously restricted; whereas defoliation at later, the growth of lateral branches is stimulated. The influence of cutting leaves on the numbers of floret and branch is basically consistent. For the plants untreated, the number of seeds is 1,873.2, and that of treated plants is obviously decreased. When treated in vegetative growing stages, the number of seeds reduces to 40—70%, and after treated for six times, the seed reduction rate is 100%.

Key words: Ragweed, Defoliation, Weed control