I'm not sure what kind of light you have. It looks like a flourescent but I don't know of any that come as 75 watts. A common flourescent for growing seedlings would be a four foot shop light with two forty watt tubes. You can grow tomato seedlings under that if they are about two inches under the lights. You also need to water from the top and keep the temperatures closer to 70 degrees, at least in the first couple weeks. There is a good description of how to grow tomatoes from seeds in the FAQ I'll link to below.

Here is a link that might be useful: How to grow tomatoes from seeds

Re Fill your seeding tray full of potting mix around your plants They are growing but need more /better light Can you put the seedling tray in a window that gets a lot of sunlight ? Sunlight is much better for growing ..go figure ,,, your light is really a POS . If you can put them so they get more sunlight great if not buy a four foot florescent fixture that holds two bulbs and use that 1-2 inches from plant tops

Your light looks like it's a fluorescent. If it actually is, it should be just fine, and better then a window, as tomatoes tend to stretch way to much in windows. If it's some kind of halogen or something, get rid of it and get a cheap 4 foot flourescent fixture with 6500k bulbs, or even better a couple T8's or T5HO's.

My suspicion is that your slow growth and purple undersides are due to low temperatures at the roots. Plants tend to have problems uptaking phosphorous when temperatures get too low giving the purple undersides. Get a heat mat and controller (do not use a heat mat without one). It doesn't need to be set very high, just 70-72F for tomatoes. Also make sure you aren't overwatering, as this can also cause this as well.

As you can see you get lots of different advice, much of it confusing. So rather than add more advice since much of it would conflict with that given above, I'll just give you a link to all the previous discussions on 'slow growing'. You can read though it and then decide how you want to correct your problems.

I'd also suggest you drop in over on the Growing from Seed forum here and check out all the FAQs posted there for step-by-step instructions.

Good luck.

Dave

Here is a link that might be useful: Discussions on slow growing tomato seedlings

@Ohiofem
I'll try raising the temp, thanks. I don't have control of my thermostat since I'm in a dorm and with the insane weather we've been having the temp in my room has been swinging from low 60s to nearly 90, depending on the weather and sunlight. The last few days the temps in my room were about 60-65 average, but now with warmer weather they're hanging at 70.

@dickiefickle
I know my light is a POS, but I was hoping it would be sufficient for the purpose. My room faces south and has lots of ambient sunlight, but it's cloudy most days here and so not strong enough for seedlings, based on what I've read.

@the0ry
Well, since I can't get a good price reselling them, may as well make them useful!

@Capoman
It's a fluorescent. Model 7020 E GL, some googling of forum posts says it has about 1200 lumens. Is that enough for roughly 2 square feet of plants? If not, would it be better to buy an additional fixture or install a better bulb?

@Dave
I've read most of those posts already, and was hoping someone had an idea of what part of my setup was causing the most issues, since it's hard to diagnose without experience. Thanks for the link though, there were a few new reads.

I use 2x 4foot T5H0s, but I can put as many as four flats under that. That's a total of 10,000 lumens, and they are boxed in four side with white paint with a fan blowing through. You probably don't need that much and can probably make what you have work, but 2x2 foot bulbs would be better for a single flat so that the outside plants don't get so much drop off, and they'll grow faster. This is a popular aquarium size fixture.

For heat, it's the roots that are important for your issue, not air temperature. If you don't have a heat mat and controller available, how about another warm place to sit it on such as on top of an old fridge, or cable/satellite box or something similar. I think really, root temperature, not light is your real issue here.

"another warm place to sit it on such as on top of an old fridge, or cable/satellite box".

The refrigerator is ok but on top of electronics are not the place to keep seedlings. There is risk of water seeping through the containers. Also electronics need to dissipate heat. Covering them with anything and the heat gets trapped and premature failure will result.

3 years ago I started seeds in seed starting mix and never transplanted into potting soil. They remained tiny until planted outside. Since then I grow in potting soil and they grow like weeds.

gjcore, you are correct. If you are putting on top of electronics make sure you have a good tray and also make sure you stand off with pieces of wood or something so that it's raised above allowing ventilation and preventing getting too hot. Heat mat with controller is still the best though if that's an option. Many newer fridges also won't work due to efficiency and often don't give off much heat. The curling up can also be caused by overwatering and high humidity, an early sign of edema.

If it's cold downstairs I wouldn't worry about the slow top growth. That will catch up quickly once the seedlings are outside. In cold temps what you get is root growth, and that is a wonderful thing for seedlings.

For bottom heat, see if you can find a string of old Christmas lights, or better, a rope light, and coil it under the tray. Cheap and easy.

Light above is harder. More is better, and on the floor is not really light. I know because I'm having the same issues. I know it's tough on a student budget, but using the window and the light would be better.

I've read about epsom salts for purple leaves. But yours don't look all that purple to me. There is so much information around. I'm sure you'll be fine. 8)

Since you're in a dorm, how about just taking the tray off the floor and letting it live in a higher place? The floor can be 5-10 degrees cooler than up near the ceiling. Also, it could be that your seed starting mix doesn't have much fertility, since often seedlings are moved into bigger and better environs once they are up and running. If you can't afford some plant food, maybe make friends with someone in the bio department, especailly their plant person, and maybe they will help you out. Or make a request for help from the homefront? Good luck!