Leggy seedlings can happen for a variety reasons. Uneven moisture, high temperatures, and insufficient light. More often than not it is due to insufficient light. When starting seeds they need a lot of light, close to where they are growing. 2 -5 inches above them is recommended. 16 hours of light is great for seedlings, which is why it’s best to have a specific spot to start them. The leggy-ness is because the seedling is trying to get enough light so instead of growing in girth for healthy stems they grow tall and thin. This can result in seedlings that will be hard to harden off, can be damaged in the rain or strong winds.
When you have uneven watering or you don’t keep your soil mix damp your seedlings don’t have a chance to develop good roots. They need to maintain moisture without drowning them. Remember seeds are small and the amount of water they need is small, but it’s important to not let them completely dry out or they could die. To help this start by making sure your seed starting mix is damp enough before planting seeds. Afterwards cover with a clear dome, monitoring the temperature and humidity, remove dome completely after a few days of your seedlings sprouting. Water from below by adding water to the bottom tray instead of the actual seed tray and keep the soil moist and avoid making it water logged.
High temperatures can cause seedlings to have a growth spurt quicker than it has time to grow the girthy stem it needs to be a healthy plant outdoors. Yes seeds like heat to germinate, but afterwards they no longer need to be on heat mats or under the dome.
Overcrowding your seedlings can also cause leggy seedlings as they struggle to find the light. The best thing to do is plant less seeds. Some say plant 3 seeds per pod, others say just one. I prefer doing just one and if they don’t germinate I can quickly start more. This is better to me than having to thin seedlings where you can accidentally hurt the main seedling your are trying to keep for transfer to the garden.
Have you ever grown leggy seedlings? I feel like it’s a just step on the path to learning how to start your own seeds. Everyone goes through it at some point, so don’t feel bad when it happens to you. Ask yourself is the moisture right? Is the light sufficient? What is the temperature? Did I plant too many seeds in one pod?