Your HID lights - are they sodium vapor? Those are not recommended for seedlings for this reason.
I would transplant into larger cells or pots, burying the stem up to the first true leaf.
The heating mats are off, right?

This post was edited by Slimy_Okra on Fri, May 9, 14 at 12:09

I concur - heat mat off, and deep transplanting. Those big plastic Solo cups are ideal for that. Just make sure to punch some holes in the bottoms for drainage. Also, do you have a fan on them at all? It's kind of like Resistance Training for the stems.

Edie

the leaves are looking a bit yellow

From overwatering - capillary matting is notorious for keeping plants too wet.

The root system is busting out the bottom of the cells.

Then they are way past the need for transplanting so yes, ASAP.

I also watered from the top with a dilute solution of MiracleGrow Tomato food (1 tablespoon in 1 gal of water).

More over-watering and too strong fertilizing for plants in such small containers that are already root-bound. Cut it to 1/4 strength even after transplanting to larger containers.

Like the other have said I sure hope they were removed from the heat mat as soon as they germinated!!

They will need to be transplanted as deep as possible burying all that bare stem in a good growing mix.

All of this is covered in much more detail on the Growing from Seed Forum here and in the FAQs on the Growing Tomatoes forum here.

Dave

Here is a link that might be useful: Growing from Seed forum

Wow, lots of great help. Thank you! Yes, the mat is off. Lights are fluorescent. I will transplant tomorrow, but will need another light.

Interesting idea about the fan. I do not, but will apply one.

Finally, I had not found the Growing from Seed forum on quick inspection. Thanks for the referral.

In aggregate, it doesn't sound like anyone is concerned about the light cycle....

I see you have more than one seeding in some of those cells. Way past time to pull the extras, you'll need to pinch them off now.

Nope, I wouldn't worry about the light cycle. I like to go with 16-18 hour "days" but have gone with 24 hours continually a few years when things were started late. All grew and produced fine when placed outside. I might be concerned if it were lettuce, spinach, and some other daylight sensitive veggies but I usually direct sow them anyway.

With your tomatoes......keep the tops of the plants within an inch or two of the fluorescent lights. Raise or lower the plants OR the lights as the tomatoes grow. Air temps between 65 and 70 F promote more stocky growth than higher temps as do the close lights and light breeze/fan.

I was wondering about needing to pinch to one plant per cell. If I move to large solo cups is that still necessary?

Will pinching alone be sufficient rather than trying to extract the seedling with roots? I realize that the roots of the two adjacent seedlings are likely intertwined.

Thanks for the encouragement on the lights. It is tough to keep my garage below 80 degrees at this time of year in NC.

The longer you keep the two seedlings together, no matter the size of the pot, the more their roots will entangle. What would be your aim in keeping the extra? To plant it separately? If you're going to try this, moving to larger pots would be the time to separate them, not keep them.

My own thinking is that this would weaken both plants, particularly if they're not so healthy to begin with.

I was wondering about needing to pinch to one plant per cell. If I move to large solo cups is that still necessary?

If you can still separate them without damage fine. If not, you will have to sacrifice one of them so the other can survive. But 1 plant per container.

Dave