In the United States there are no hemp diseases of economic importance, and hemp has not been seriously attacked by insects. The European corn borer and similar stem-boring insects occasionally kill a hemp stem. However, they have not proved important, perhaps because hemp has not been grown to any extent in the sections of the United States where the European corn borer is a serious pest. Seedling plants are frequently attacked by cutworms and white grubs after spring plowing of sod land.

Broom rape is a small weed 6 to 15 inches high that is parasitic on the roots of hemp, tobacco, and tomatoes, it usually grows in clumps and has purple flowers, which produce many very small seeds. These adhere to the waxy flower parts surrounding the hempseed and are distributed in this manner. Broom rape can be very serious on hemp if proper control measures are not followed. Only well-cleaned hempseed and seed from fields containing no broom rape should be sown.

Hemp has been recommended as a weed-control crop. Its dense, tall growth helps to kill out many common weeds. The noxious bindweed, a member of the morning-glory family, is checked to some extent by hemp. Unfortunately, bindweed and several other species of morning-glory have seeds so near the same size and weight of hempseed that mixtures obtained in producing hempseed are carried to the field planted for fiber production. In growing hemp for seed all vine weeds of this type found on the hemp stalks should be removed before the hemp plants begin to produce seed.