In case you're wondering, here is the response I got from USDA:
If you bring back 12 or fewer articles of admissible plants that have no special restrictions, you do not need a permit. However, you do need a phytosanitary certificate from the country of origin.
For help in determining admissibility, refer to the Plant Protection and Quarantine Plants for Planting Manual's reference section. Click on “Plants for Planting Manual” and then find the list of Regulated Propagative Material. Plants that are not listed in this section are generally admissible. Check for entries at the family, genus and species levels.
Although does not mention from what entity to obtain the certificate exactly.
