Relevant laws include The Plant Quarantine Act of 1912 (as amended), The Organic Act of 1944 (as amended), The Federal Plant Pest Act of 1957 (as amended), and the Federal Noxious Weed Act of 1974 (as amended). Regulations promulgated under these laws are found in the Code of Federal Regulations, specifically in CFR 319.
To determine whether the crop or plant part that you want to import is prohibited, you can check the Plants for Planting Manual
Prohibited plants include such valuable crops and natural flora as apples, bamboo, citrus, elms, grapes, grasses, maples, peaches, potatoes, rice, sweet potato, and sugarcane. When any plant part capable of vegetative propagation is excised from a foreign plant and then shipped to the United States, these propagules usually carry most, if not all, of the pests present in or on their original foreign plants. Therefore, in order to protect U.S. agriculture from the introduction of foreign pests, small quantities of these plants must be inspected during growth and tested for pathogens while in quarantine at an APHIS approved containment facility. A network of quarantine programs across the United States ensures that all prohibited plant genera can be imported through quarantine. After release from quarantine, these plants or plant parts can be propagated or distributed without federal restrictions. Special permits must be obtained by the importer if these plants are genetically modified (genetically engineered), if they are listed on the Federal Noxious Weed list, or if they are regulated by the importer’s state department of agriculture.