8-10 inch tomato "seedlings"? Grown under lights? Good heavens, get those things in the ground! You've done very well, but they should have gone in the ground when they were 3-4 inches tall. Yes, plant them deep. In the ground or in a bigger pot. They'll do great. When my tomatoes in the ground are that tall, they usually need some support.

Did you start them too early? Your plant out date isn't until when? middle or end of May?

Either way its too much fertilizer and not enough light. If the stem can't support the top growth the plants are called 'leggy'. Leggy plants are caused by insufficient light and too warm growing conditions. So much fertilizer gets you top growth but minimal root development and that is what is the goal for seedlings - lots of roots.

If I'm right and you still have 5-6 weeks before planting out I'd clone those plants and use the clones to plant out at the proper time.

Dave

I'm not quite sure what is happening. Is the plant top-heavy, so that the pot falls on its side? Or is it that the lower part of the stem collapses and the plant suddenly dies?

If the latter, I think you have a fungal disease called damping off, which you can read about here:

http://www.tomatodirt.com/damping-off.html

[Fans are used not so much to cool the plant as to move the air to prevent disease (including damping off). Fans also provide some resistance and stimulation which helps grow a stronger stem -- you can get much the same effect by running your fingers along the top of the plant, but fans take less effort!]

In addition to preventing diseases like damping off, using fans promotes the development of sturdy, stocky stems through thigmomorphogenesis.

Thigmomorphogenesis refers to the widespread ability of sessile organisms to modify their morphology in response to a variety of mechanical stimulations, from direct contact with the stem by insects or other plants to flexure caused by wind, water, or snow.

Plants subjected to thigmomorphogenesis, or physical disturbances such as wind and touch, generally respond through reduction in the rate of stem elongation and shoot height, and they increase in stem diameter. All of these features result in the formation of short, stocky plants.

This response is purely adaptive and allows individual plants to compensate for the different levels of stress that occur in their natural environment. The advantage of this is that shorter and stronger plants are less easily damaged by natural mechanical stresses (especially wind) than their taller, more slender counterparts.

I hope that helps.

Betsy

You should buy your tomatoes from me.....I never tip.

Brett: it sounds to me as if most of what you are doing is right. The only exception is that it sounds like you started too soon. My plants get to about 12 inches tall in six weeks. Some of the giant heirloom types can be growing almost an inch a day at that point. They get very floppy when they get that tall. Each year I force myself to start later than the previous year. Our last frost date is around April 30, but I don't put my plants into the garden until two weeks after that. I started my plants April 1, and they are already four inches tall. I'm nervous that I started too early once again. In spite of all the talk on GW about putting out plants earlier because of mild temps, our 10-day forecast calls for lows in the low 30s on most days.
In your situation, I would transplant to larger, deeper pots and bury at least 3 inches of the stem. I would also increase the hours of light to 18 and add a small fan. I really think you need to wait until the soil temperatures are over 50 and night time lows are reliably above that before you start a week of hardening off.

Last year, my tomatoes got very leggy because the garden kept flooding when it was time to plant them. When it did dry out, I planted a good long section of the lower stems and they did fine.

It's worth it to try to prevent them from getting leggy when you can because they're awkward to handle and are more easily damaged when they flop around, but it's certainly not the worst problem they could have. It will probably be a lot easier to safely harden them off, though, if they get planted deep into larger pots.