When the light source is too dim or far away, the seedlings kick into survival mode and grow quickly in height to try to get closer to that light.
As soon as the seeds germinate, they respond to the heat by putting up tall, skinny stems before leaf production has a chance to catch up. This results in unbalanced seedlings that are “all legs.”
Continued lack of moisture will turn them spindly and eventually kill them as they’re unable to access the nutrients they need from the soil.
With tiny seeds, it’s tempting to simply scatter them in one large tray and thin the seedlings as they grow, but proper spacing helps prevents leggy seedlings as well.
Leggy seedlings can sometimes be saved by gently brushing your fingers back and forth along the tops of the plants every day. This simple motion simulates an outdoor breeze and tricks the seedlings into thinking they need to grow thicker stems to hold up against windy conditions.