Your son is at the age where he may easily be influenced by peer pressure and simply just be curious by acquiring the seeds. Before you explain to your son what the consequences will be, allow him to tell his side of the story and offer an explanation for having the seeds and what his intentions were. You first want to ensure that the lines of communication are open and honest. Then state your feelings with regards XXXXX XXXXX situation.
Since this event seems to be the first time that your son has not made good judgment, then the consequences do not need to be too drastic but should have a meaningful and lasting impact. The consequence must affect your son in a way so that he feels like he is being deprived of something. A month is too long to be grounded. Implement the consequences for a solid week. You can ground your son, take away the phone, and disallow your son to use the computer or play video games (if he likes those things) . Those consequences can be utilized all at the same time.
I would not bother with the hair cutting. That "punishment" is usually used for younger children.
If he does not comply, extend the consequences for another week and add on the delaying of his driving permit. This should get the message across that you mean business for his lack of judgment.
I hope that this has helped. If so, please ACCEPT so that I may be paid for my efforts. Positive feedback is appreciated as well. Have a great day.