If they grow spindly it is more likely a light issue than a fertiliser issue. Fertilising etiolated plants will not help them. They need more light and, probably, less heat.

Agree that spindly seedlings, aka leggy seedlings, isn't related to fertilization. It is caused by insufficient light and/or excessive warm air temps in the growing area.

The cotyledons, the first fake leaves, contain all the plant needs to survive until after true leaves develop. Fertilizing seedlings only stresses the plant and forces leaf growth, often sacrificing the root development needed to support that top growth. So while they make look better above ground they are actually weaker below ground.

That is why the standard recommendation is no feeding of seedlings until after true leaves develop and then only with very diluted mixtures.

If you have spindly plants then you aren't providing sufficient supplemental lighting and it is likely too warm too.

Dave

These replies are a big help. Thanks.

How many hours of light are required? We have florescent bulbs over seedlings for 12-14 hours a day.

It isn't just the number of the hours. It is also the lumens and the type of the bulbs you are using. 4' T12 bulbs with 3200 lumens are usually recommended for 16 on and 8 off. But how close to the plants are they? That is even more important than the time. No more than 1-1 1/2" above the tops of the plants.

If you are using T5 bulbs the hours are the same but they can be 2-3" above.

And don't ignore the heat factor too. Assuming they were removed from any heat mat as soon as they germinated as is required then even with sufficient light, warm air temps in the area add to the problems. 60-65 degrees is considered ideal. If they are in an area where the air temps are over 70 they may get leggy. Above 75 they definitely will.

Dave

I'm thinking that heat may be a problem I'm having too. After seeds sprout...would it be fine for them to grow at temps below 55 or 60? My seedlings that stay cool do not seem to grow at all...but they do stay alive....which is more than I can say for the ones I'm trying to keep warmer. The problem with my MT spring weather is too cold, or too hot...never just right. On over-cast days the temp can stay around freezing, so I use supplemental heat...but when the sun peaks out....even on cold days....my greenhouse quickly surpasses the 100 degree mark. I wonder if it's because of elevation? It's a problem I haven't been able to solve.