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?

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?