I really appreciate it. So you think keeping it cool will be my biggest problem? It’s kind of a toss up here some years it’s very warm then some like this year we get real cold spells that last until end of March. What size trays should I get? I was thinking at least 2x3” cells or maybe 3x3” because I only want to transplant once to their final home, but don’t want the seedlings to become root bound

Even in my zone keeping it cool enough is the main expense - and that's with 50% shade cloth on. In your zone it will likely even be more. I get maybe 10-12 nights where I have to run the heater to keep it above 50 but even in Feb and March I get days when just the sun heats it up to over 100 degrees. By early May I have to have everything outside or it cooks.

Since you are already heating your house it would be less expensive to use that if you have the space. The upfront costs of the lights, bulbs, fan, etc. is steep but it is a one time thing. Check out all the posts and FAQs over on the Growing from Seed forum here as many have included pics of their indoors seed starting set ups.

Dave

You basically need to keep it from freezing, and if it stays above freezing outside, it will certainly be more than that inside. But if temps are warm outside, and the sun is beating down on a greenhouse, it can get awfully hot inside, unless you've got it all opened up.

Compared to direct seeding outside, ANY cell size would be easier.

With 3 inch seedling pots, I don't think you'll have a problem with anyone being seriously root-bound until they are a few inches tall. If you're going for 400 plants, you sure don't want to transplant more than once!

I'm in zone 8 in Central Texas, and the Pacific Northwest is in zone 8. So zone 8 doesn't mean much. Where are you, a little more precisely?

But why not outside? Just put seedling trays outside when you would ordinarily have direct-seeded outdoors. Plenty of sun. If it gets cold overnight, just take them in. If they miss a day outside in the sun because it is cold, no sweat. The advantage is, you're using seedling mix that doesn't have big interfering clods, and the soil temperature is at least that of the air. Probably warmer than the soil in the ground. The plants grow up already hardened off. They'll grow more slowly than they would if they were in a really warm place, but they'll get there. Again, at least as fast as if they were direct-seeded.

Digdirt yeah I didn’t think about that I guess I would be spending money to keep them warm and I definitely don’t want to have to go out there every day almost to open it up to keep them alive.

daninthedirt I’m in southeastern coastal N.C. and yeah exactly it’d be a lot of work... also that’s a pretty good idea, but I guess it might be more work than I think. Also I don’t live out where my farm house is, it’s 30min away, so I can kept close eyes on it every day and don’t make it out there every day like I try. And that’s kind of why I want to do it inside something because of the pests and disease, I want to keep them at bay as long as possible especially after my catastrophe this year I’m not doing that again

Well, you might think about whether you can do them where you live, and then when they're ready to be planted out, bring them out to the farm. Disease? I don't think you're going to have any troubles with trays of seedlings planted in ideally sterilized mix. As to pests, if you just get the trays off the ground, they'll probably be safe.

I think you should grow them in a greenhouse on heat (propagation) mats. You can get heat mats with or without a thermostat. This will warm the soil. You will need very little supplemental heating unless the temperatures dip down below freezing. As long as the roots are warm your tomato seedlings will grow. As far as cooling, there are many different ways to cool a greenhouse automatically and even without electricity. For sure your greenhouse would need roof vents with solar powered openers.

A second option (and the way I start my tomato seeds every year) is to grow them inside. I have a indoor plant stand with fluorescent lights. I put the lights on a timer. I put my seedling trays on a heat mat. Works every year! But, I only start about 25 plants. You would need some room to do as many as you grow.

As far as solar powered heaters, I don't know of any. But, there are greenhouse propane or natural gas heaters that require no electricity. You could have a propane tank put on your property to supply it.

I am in zone 8 in Louisiana.

If you're worried about electric bills, I wouldn't advise building a greenhouse with heating mats or an indoor setup with lights and fans. Those would be VASTLY more expensive than any electric bill. If price isn't an object, sure, you could put together an awesome setup. But a table outdoors in the sun is dirt cheap. Almost as cheap as direct-seeding. Seedlings would mature more slowly, but you'd get where you want to go with some confidence. Of course, in that case you need a rescue option, in case of freezing temps.

I grow a few tomato plants over the winter in my greenhouse in Raleigh and they don't perform very well during Dec, Jan, and Feb - there just isn't enough light (I don't supplement with lights). The tomatoes I go get aren't that special flavor-wise either. To me, it makes more sense to start early (indoors or in a greenhouse) but only about a month early. Do whatever you can to keep the roots warm and they will grow faster. Use sterile potting media to avoid fungal diseases. Don't worry about feeding them until they are in the ground and starting to bloom. I start mine in my basement (which seems to stay about 65 degrees in the winter) in 3 inch cells and just leave them in them until the ground outside is warm enough. They do sometimes get very leggy and stretched (just bury them deep when you plant them). My first tom's are usually 'Stupice' because they are not as sensitive to cool soil. The flavor is decent and they do the best outside in wall-o-water protectors during cold snaps.