I got 3 tomato seedlings from planting two seeds in one pod, but I may have accidentally put in a third one.

You may have had a double seed and not noticed it. I've had it happen on occasion. Sometimes it looks like one fat round nast seed and it's actually two seeds stuck together.

Though goodness, if you are in zone 5, not sure why you started nast seeds a few weeks ago unless you are growing them for some indoor greens before the winter is over. They pop as fast and easy as beans and peas.

I grow a lot of nasts, they are one of my favorite annuals and edibles. Sometimes I'll start some seeds indoors a couple weeks early for hanging baskets.. but mostly they do really well direct seeded.

Yep, planted 'em indoors just to get a bit of a head start. :)

I'd think that if it were a double seed, it would've come apart when I nicked 'em and soaked 'em overnight... but there were definitely only six elements when I planted them!

From some more research and consultation with someone who's grown a lot of nasturtiums, it sounds like it's possible that the seed just had multiple embryos.

If one thinks one has inadvertently planted two seed then this is quite likely the best explanation. However I don't see it as completely out of the question that one has gotten a rare mutant double chambered seed.

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Yeah. Double chambered or multiple embryos reads way better technically than two seeds stuck together. But I've definitely seen it a time or few with nasts.

Yes, double embryos are not uncommon with many plant species.

Turns out right after I posted this, yet another sprout came up from that cell. I'm going with multiple embryos as the explanation -- thanks all!