Could you post a picture? It should be pretty easy to see how old the plants are and whether they could be second generation or not. Although your description says of ‘ a handful’ of seeds sounds like quite a lot of seeds.

Thanks, Floral. I'll add a photo. Maybe 'handful' was the wrong word. ;-) I probably put maybe 10 seeds in there?..... yet I am seeing dozens of individual seedlings...

And I'm not sure what you mean by 'second generation'? If something is grown from seed, then isn't it by default first generation? ;-)

By second generation I meant if your plants had flowered and produce seed which then self sowed. But that is clearly not the case. Looks to me exactly like what you'll get from a pinch of seed. I see about 24 seedlings. I don’t see anything untoward. You just scattered more seeds than you realised.

Wow, ok. I didn't think I would have dropped 24 seeds in there. And to clarify/confirm, so one seed of shiso should theoretically only create one new seedling, yes? In other words, as a seedling, shiso wouldn't then spread underground via rhizomes or anything.... and is incapable of self-sowing/re-seeding as a seedling, correct?

If so, then I guess I put 24 or so seeds in there.....

Guess I'll have to share the seedlings then. Hate to waste. ;-)

Yes, 1 seed = 1 seedling. And as a young seedling it won’t produce seed or spread. The only way to have multiple seedlings is to sow multiple seeds. Presumably you sowed to eat so just eat the extras.