Look! Why Are My Nasturtiums Stringy?

Remember the Rosso’s Biodegradable Pots we got and recently started it's Nasturtium seeds? Pretty quickly it started to sprout, but now 2 months later, it's, well, really stringy...

The first month and a half we had the pot indoors, but now that the weather has warmed up we've been keeping the pot outside. We thought this would help, but really it hasn't. Through a quick Google search we've found that Nasturtiums are supposed to be very easy to grow... So, what's the deal?

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Typically we start our plants from seedlings, not from seeds, so maybe we're just not used to this process - is this normal? Are we watering it too much, or is it not getting enough sun? Suggestions anyone?