At least a guide on how because I spent about half an hour researching what it was like with the seeds. This is not a topic, it's just a comment to see if someone explains to me how it works.
I don't know what a NPS is. I can tell you a bit about NPCs though. I haven't played too much, but watching videos from folks such as ExpansionFan can give ya a rough idea where the opportunities are.
Many are randomly implemented and will request for them. These folks are noted to ask. If it is a shop owner, it'll ask "Hey do you wish to [buy/rent] [xxx]." A menu will pop up saying "Yes" or "No." Select no. It may ask "Do you wish to give them a seed?" Select "Yes." Sometimes it's immediate. Sometimes it is a couple of days. Either way, check back on them and watch em grow.
There are some that you cannot do without the second person (I forget her name, but I think if Flora). If you have her, certain events can be played out.
Overall, keep an eye out for sparkley areas that after going to says "place seed?" Select yes and an NPC will be having a kid.
Some are scripted to be so after completing a quest, so just ask around and look around for side quests. The more you complete, the better. Also, you'll get some bonus stats while you're at it.
Some are at night and won't be available unless you sleep at a local inn. If there is a scripted event, one of your three characters (Leia, Flora, or Kitty) will wake up in the middle of the night and you can walk around doing stuff. So check around the towns and see what is available.
I should note you need to get a good farm up if you want to get enough for all available NPCs. On average, estimate three seeds if it is a "normal" NPC. If it is a fairy, expect about 10 per, unless specified otherwise.
I haven't tested it yet, but you might be able to sell them a seed and see what happens. If it does work, sell them a seed or two.
I hope this helps Pucca.