In my experience, it doesn't matter if it is a full plant, a cutting, or a seed. The concern is not the plant itself, but any microbial plant organisms that might inadvertently be present. I know of a lady who was arrested (never charged, but she was arrested) for having tomato seeds in her purse. They were seeds from a tomato she had eaten at a restaurant in France and wanted to keep, and the dog at the airport detected it. She had no idea what the dog was doing, as she thought it was looking for drugs. It's kind of weird how specific the process can be. She forfeited the seeds to the USDA, they were destroyed, and she was released without issue.
I had animals imported from Germany once and there was a bit of dried hay in the crate. This caused me to be detained overnight (though not arrested) and questioned by the USDA, Fish and Wildlife Service, and FDA. I had to agree to having all plant matter destroyed by incineration and the shipping crate destroyed as well. Pretty hardcore.