Did he bring the flower from Japan?
He brought a flower into his house when he returns home which I assume was the Mr Miyagi flower. I didn't think US Customs would allow this but it's apparently okay. According to U.S. Customs and Border Protection:
Can I bring cut flowers for myself or significant other?
Most cut flowers are able to enter the United States. However, they must be declared and able to be inspected by an Agriculture Specialist. If the plant material is found to be propagatable or bug and disease agents are present the plant material will be seized and denied entry.