Personally I'd prefer 6 to 8" . But the weather doesn't always cooperate. Plus the greenhouse temperature will affect the growth rate.

I'm sure Dawn will have more information on the subject.

One thing you could do is stagger your starting times.
I think if you can get them transplanted befor you get blossoms then there is less stress.

Im doing this from my phone so ill try and check back tonight.

I thought for sure there would be more posts by now.

Not know what kind of experience you have it's hard to give any tips.

A greenhouse gives you an early start but also has it's own drawbacks. One of them is starting early and then trying to get the plants hardened off to cooler temps for transplanting.

I think the smaller greenhouse are more challenging since the temperature swings can be much faster . Usually people think of it being cheaper to heat, but they also cool down just as fast.

And on warm days they heat up faster and it's a challenge to get enough air exchange to regulate the temperature.

I know this isn't what you asked but I try to give people a heads up on some things that may be overlooked.

If you do peppers, you'll want to start them a little earlier then tomatoes as they tend to grow slower and may not be as large if you started them at the same time.

Squash and cucumbers grow a lot faster and are usually only started a couple of weeks before your last frost date. At least that is how I try to plan it.

Eggplant needs to be started with the peppers as they tend to grow slower also.

hope this helps.

I start my main crop of tomatoe in the starting bench--with a heating cable buried in sand in mid-January, but start 6 varietes in the house, because I don't wnt to heat the starting bench for so few plants, in early December. Start about 6 plants of peppers inside too. I put the seeds in a warm place, usually on top of a hot pad set on low, then move them to room temperature after they germinate in a south or west window. Then move them to the planting bench on a shelf without bottom heat when I start the main crop. I don't let the plants get root bound, but keep potting them up into bigger pots and end up with at least quart pots by the time to transplant. The main crop gets potted up only once to pint pots.

Thanks,
I have garden for 60 yrs but never in a green house It is small and I as going to use it to finish off plants in March on pretty days and bring at nite. I was going to use my grandmothers method which was like mulberryknob stated sprout in warm place, south window, and keep repoting. I may only do this with some early girls for early tomatoes they were some of my best producers this year.

I like to put my home-grown tomato seedlings in the ground while they are somewhere between 8-15" tall. The perfect size is 10-12" tall, but sometimes they get taller because the cold weather is hanging on too long. To get them to that size normally requires 6-8 weeks of growing time, so just pick a target planting date and count backwards 6 to 8 weeks to determine when to start your seeds. Occasionally I have put the plants in the ground when they were only 6" tall because the weather warmed up very early (our last freeze in 2011 was in February) and I wanted to take advantage of that to get the plants in the ground as early as possible. Occasionally I have had to wait a couple of extra weeks for the cold weather to go away and the plants have gotten significantly larger during that extra couple of weeks. (To a certain degree, you can slow down their growth by keeping them cooler.)

For years and years I planted my tomato seeds in flats on a lightshelf in our spare room on Super Bowl Sunday. Using that method, I would have plants ready to go into the ground in mid-March, weather permitting, or late March. My average last frost date is March 28th, but the last few years we have continued to have freezing temperatures about 1 night a week through early May, so I usually have planted a bit later and then had to protect the plants from an occasional cold night once they are in the ground. Because I didn't have a greenhouse back then, I'd carry the plants outside on warm days as soon as they were 2 or 3" tall, gradually exposing them to larger doses of sunlight to harden them off, but would carry them back inside at night because the temperatures were too cold for them. I would pot them up from the starter flat to individual pots once they had a couple of leaves, and I sometimes moved them up to successively larger pots if I had to keep them inside for a couple more weeks due to persistently cold weather.

Not much has changed since we built the greenhouse a few years back. I still start the plants inside on the light shelf, but as soon as they are a few inches tall, I now move them to the unheated greenhouse and leave them there. I can throw floating row cover material over them at night if I expect the temperatures in the greenhouse to drop below freezing. Because my greenhouse has shade cloth on it that blocks 50% of the sunlight, I still have to harden off the plants by gradually exposing them to the stronger light outdoors, but it is a pretty easy process since they're already getting 50% of the sunlight in the greenhouse.

Beginning in 2011, I have been starting the seeds a week to 10 days before Super Bowl Sunday. That way, in a warm year like 2011, I have slightly larger seedlings that I can transplant into the ground earlier if weather permits. Sometimes I start half of them a week before the Super Bowl, and the other half either on Super Bowl Sunday or within the next couple of days after the Super Bowl. The downside to starting earlier is that if the cold nights hang on for too long, the plants can start getting too big. When they get too big, you need to keep potting them up to ever-larger pots so they don't become rootbound.

I agree with everything Robert said about greenhouse size and the struggle to keep the greenhouse from warming up too much on sunny winter days. Even with 50% shade cloth on my greenhouse, if I forget to open both doors and all 4 vents first thing in the morning, even on coldish winter days with sunlight, the greenhouse can hit 115 degrees by about 9 a.m. It drives me crazy! I thought the big issue would be keeping the plants warm enough at night without running a heater, but the bigger issue is to keep the greenhouse from heating up too much during the daytime.

Last year I filled large molasses feed tubs with water to serve as solar collectors to keep the greenhouse warm at night. I had a lot of containers of water in there--about 28 to 30 molasses feed tubs and about 40 cat litter buckets filled with water, and my greenhouse stayed above freezing almost every night. One night when the overnight low dipped into the teens, the greenhouse did go down to 31 or 32 degrees, but I had thrown a heavy-weight frost blanket over the veggie, flower and herb seedlings and none of them suffered freeze damage.

Before we built the greenhouse, I thought the biggest challenge would be to keep it warm in the winter. Surprisingly, that is not the biggest challenge, even though I prefer not to drive up the electric bill by running a heater in there at night. Instead, the biggest challenge is to keep the greenhouse from heating up too much during the day. Before I bought the shade cloth and put it over the greenhouse, the temperature could hit 145 degrees if I left all the doors and windows closed on sunny days. Since I added the shadecloth it only hits 115 degrees. I deliberately choose Aluminet shade cloth because it was supposed to not only keep the greenhouse cooler during the day, but also was supposed to help it stay a little warmer at night. I believe it does exactly that.

Obviously, the smart thing is for me just to remember to open the doors and vents around sunrise every morning, but we are members of our local VFD and sometimes we go on fire calls during the night and don't make it back home early enough for me to open up the greenhouse to keep it from overheating. I haven't lost any plants to overheating, but I've had them wilt and look miserable a couple of times.

I do run a large fan (it is one of those big round fans on wheels that you can buy at big box stores for barns, workshops, etc.) in the spring. I usually place it at the west door of the greenhouse and have the air blowing east. Having the great air flow helps reduce the prospect of overheating being a problem, and also helps prevent fungal and bacterial infections from developing by promoting good air flow.

With a small greenhouse, you likely could use a box fan or small oscillating fan, depending on the size of the greenhouse. My greenhouse is not too large and not too small--it is 12' wide by 20' long, and the shade cloth covers the 20'-long south and north walls. We left the east and west walls unshaded to allow good light penetration.

I generally raise several hundred tomato plants from seed every year, which gives me plenty to put in the ground, and plenty of backup plants to have in the greenhouse for a few more weeks in case a late freeze kills everything in the ground. (That hasn't happened since either 2007 or 2008, not because we don't have late freezes and late frosts, but because I've become more consistent about covering the plants with frost blankets on potentially cold nights after the plants are in the ground.) I usually give away the backup plants at some point, but generally not until late April or early May. The extra plants in the greenhouse are a nice security blanket to have. I had to experiment for several years to figure out the best seed-starting dates for my specific location because I am in cold microclimate and need to be careful to not start the seeds too early. I like the concept of starting some seeds early and then waiting a couple of weeks and starting more, but it doesn't always work out well for me. Sometimes I get too busy and don't get the second batch started on time and those plants are a little late.

I do stagger tomato transplanting from late February through late April. Then, I often plant a few fresh plants (sometimes purchased, sometimes from seeds sown in flats in May) in the last week of June or first week of July for fall plants so that we'll have tomatoes all season long. I usually purchase tomato plants the minute they hit the stores down here in mid- to late-February. I put the purchased plants in large containers either in the greenhouse (where they stay 24/7) or near the greenhouse so I can carry them inside on cold nights but have them outside in full sun all day. From those February-purchased plants, we usually start harvesting ripe fruit in late April. If I get the home-grown plants into the ground by late March or earliest April, we often are harvesting from them by late May or, in a bad year when the weather stays cold too long, by early June.

Here it is early November, and we still have tomato plants going in molasses feed tubs. I have 8 plants, all of them have tomatoes in varying sizes, and if I move those containers into the greenhouse before frost gets the plants, we'll be harvesting from those containers into December or maybe even January. So, we have exactly the long tomato season I was dreaming of when we built the greenhouse, but I have to fight the weather pretty hard to get that long tomato season.

I love my greenhouse, but it can be a surprisingly difficult task to keep it warm enough at night and cool enough during the day in the winter and spring. It is simply wonderful, though, to be out there working in the greenhouse on a sunny, cold winter day....almost like taking a vacation to a warmer climate. The tomato plants were the main reason we built the greenhouse, but I use it for a lot of flowers, herbs and veggies as well, and overwinter my citrus trees and brugmansias in there as well. The main reason I wanted the greenhouse in the first place was so I could stop carrying many flats of plants back inside the house and upstairs to the spare room at night during the hardening off process, but I have found an even bigger benefit is that the plants are better-protected from the cold and often strong north winds in late winter and spring in the greenhouse. Strong winds can be just as hard on young plants as cold temperatures, so with the greenhouse I don't even have to worry about windburn being an issue every day.

Dawn