are you still in South Korea? If so it is very difficult to port and export and plant material (seeds included) in or out of the US without first filling out a ton of govt paper work. A Phytosanitary certificate approved be the FDA is required for each item. All items are additionally subject to search and possible confiscation by the deparemt of agriculture. The penalties for not following these rules is pretty stiff.
If you are currently on the US than hopefully someone else can help. Good luck
I don't tell my rhubabrb go to seed. So the only way would be after the harvest, and you would get a huge piece of root. I don't know if the US would let it out of the country because of it being a root. Your not suppose to take anything out of Canada thats a root. Disease I suppose. Good Luck