Hi, I'm in zone 7 in E. Tennessee.
I have gardened for many years, but just last year, I took the greenhouse plunge. I have a RION Green Giant 8x20. It has 4 manual vents in the roof, an automatic exhaust fan, and is heated only when the temps drop below freezing. We use a kerosene heater and it works really well.
I start seedling for myself as well as for many customers. I mostly do custom orders. Kind of what they want, how many they want and when they want it.
Right now I have broccoli that will be ready in 2-3 weeks, cabbages in about 4-5 weeks (seed shipment delay) and my first round of tomatoes and peppers (600 & about 400 respectively) are beginning to pop up. In the next few days, I'm gonna do a second round of tomatoes.
For the tomatoes and peppers, I have seedling heat mats. They like to germinate at about 55-70 degrees. I have also found that an inexpensive, plastic covered 4 shelf greenhouse within my greenhouse is really heat efficient. It gets and stays very toasty in there.
Your greenhouse should receive 6-8 hours of sunlight during the time your seedlings start emerging. Mine gets sun from about 10:00 am till 4:00 pm during the last 2-3 weeks of winter. As the seedlings grow, they get more sunshine.
As far as when to start seeds, read the back of the packages. If they need 8 weeks before they should be set out in the garden, count back 8 weeks before your average last frost date. That should give you some idea about scheduling for outside planting.
You will be suprised by just how much you can get out of your space. Last year, I raised over 1,000 tomatoes in my 8x20. This year, I'm putting in a 12x30 hoop house for the older plants and hardening off time.
Enjoy!! Your greenhouse can be your own little tropical vacation paradise.
