This question can be important from a law enforcement perspective. In some states, such as New Hampshire, possession of any amount of marijuana, even for personal use, can result in serious legal trouble.
Marijuana plants with no detectable Delta-9 THC, or a concentration lower than .3% are now characterized and defined by federal law as “hemp.” Hemp is not a controlled substance. So, possessing hemp is not the same thing as having an illegal pot stash. But, a zip lock bag or mason jar containing industrial hemp flower looks and smells the same as the version that gets people high. How is a police officer simply trying to do his or her job supposed to know the difference? Are they expected to take someone’s word for it? Also what is the legal definition of “hemp” – and is there a consistent definition for the whole country, or does the meaning vary state by state?
There is a constitutional law doctrine called “preemption.” This doctrine says that federal law trumps state law. So by application of the Farm Bill Act 0f 2018, also known as the Agriculture Improvement Act of 2018 (cited as Pub. L. 115-334) a judge would have to dismiss a prosecution for criminal possession — provided that the defendant could show that the Delta-9 THC content of what they had was not above .3%. This could be done through a certificate of analysis from the company that sold the hemp flower, or independent laboratory testing.
A link to the Agriculture Improvement Act of 2018 is here: https://www.govinfo.gov/content/pkg/PLAW-115publ334/pdf/PLAW-115publ334.pdf.