The persuasion mentioned in the Journal refers to the fact that if you poke him several times, he'll throw seeds. You can also try talking to him but if you do that he will only throw so many (from 2-4 seeds). Don't talk to him while he's throwing a seed however, as there is a game glitch that can occur if you do so.
If you hit Seedos multiple times, his teeth will break down to 3 or 4 and his glasses will be damaged. He will laugh madly, go away and then he will come back to your garden later and start planting Venus Piñata Trap plants or weeds. Which can ill (sicken) your piñatas.
To get new kinds of seeds from Seedos and Costolot's Store, you need to level up to unlock the ability to have the seed.
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